From Inoue@tpf.co.jp Tue Jan 18 19:08:30 2000 Received: from sd.tpf.co.jp (sd.tpf.co.jp [210.161.239.34]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id UAA10148 for ; Tue, 18 Jan 2000 20:08:27 -0500 (EST) Received: from cadzone ([126.0.1.40] (may be forged)) by sd.tpf.co.jp (2.5 Build 2640 (Berkeley 8.8.6)/8.8.4) with SMTP id KAA02790; Wed, 19 Jan 2000 10:08:02 +0900 From: "Hiroshi Inoue" To: "Bruce Momjian" Cc: "pgsql-hackers" Subject: RE: [HACKERS] Index recreation in vacuum Date: Wed, 19 Jan 2000 10:13:40 +0900 Message-ID: <000201bf621a$6b9baf20$2801007e@tpf.co.jp> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook 8.5, Build 4.71.2173.0 X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2314.1300 Importance: Normal In-Reply-To: <200001181821.NAA02988@candle.pha.pa.us> Status: ROr > -----Original Message----- > From: Bruce Momjian [mailto:pgman@candle.pha.pa.us] > > [Charset iso-8859-1 unsupported, filtering to ASCII...] > > Hi all, > > > > I'm trying to implement REINDEX command. > > > > REINDEX operation itself is available everywhere and > > I've thought about applying it to VACUUM. > > That is a good idea. Vacuuming of indexes can be very slow. > > > . > > My plan is as follows. > > > > Add a new option to force index recreation in vacuum > > and if index recreation is specified. > > Couldn't we auto-recreate indexes based on the number of tuples moved by > vacuum, Yes,we could probably do it. But I'm not sure the availability of new vacuum. New vacuum would give us a big advantage that 1) Much faster than current if vacuum remove/moves many tuples. 2) Does shrink index files But in case of abort/crash 1) couldn't choose index scan for the table 2) unique constraints of the table would be lost I don't know how people estimate this disadvantage. > > > Now I'm inclined to use relhasindex of pg_class to > > validate/invalidate indexes of a table at once. > > There are a few calls to CatalogIndexInsert() that know the > system table they > are using and know it has indexes, so it does not check that field. You > could add cases for that. > I think there aren't so many places to check. I would examine it if my idea is OK. Regards. Hiroshi Inoue Inoue@tpf.co.jp From owner-pgsql-hackers@hub.org Tue Jan 18 19:15:27 2000 Received: from hub.org (hub.org [216.126.84.1]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id UAA10454 for ; Tue, 18 Jan 2000 20:15:26 -0500 (EST) Received: from localhost (majordom@localhost) by hub.org (8.9.3/8.9.3) with SMTP id UAA42280; Tue, 18 Jan 2000 20:10:35 -0500 (EST) (envelope-from owner-pgsql-hackers) Received: by hub.org (bulk_mailer v1.5); Tue, 18 Jan 2000 20:10:30 -0500 Received: (from majordom@localhost) by hub.org (8.9.3/8.9.3) id UAA42081 for pgsql-hackers-outgoing; Tue, 18 Jan 2000 20:09:31 -0500 (EST) (envelope-from owner-pgsql-hackers@postgreSQL.org) Received: from sd.tpf.co.jp (sd.tpf.co.jp [210.161.239.34]) by hub.org (8.9.3/8.9.3) with ESMTP id UAA41943 for ; Tue, 18 Jan 2000 20:08:39 -0500 (EST) (envelope-from Inoue@tpf.co.jp) Received: from cadzone ([126.0.1.40] (may be forged)) by sd.tpf.co.jp (2.5 Build 2640 (Berkeley 8.8.6)/8.8.4) with SMTP id KAA02790; Wed, 19 Jan 2000 10:08:02 +0900 From: "Hiroshi Inoue" To: "Bruce Momjian" Cc: "pgsql-hackers" Subject: RE: [HACKERS] Index recreation in vacuum Date: Wed, 19 Jan 2000 10:13:40 +0900 Message-ID: <000201bf621a$6b9baf20$2801007e@tpf.co.jp> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook 8.5, Build 4.71.2173.0 X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2314.1300 Importance: Normal In-Reply-To: <200001181821.NAA02988@candle.pha.pa.us> Sender: owner-pgsql-hackers@postgreSQL.org Status: RO > -----Original Message----- > From: Bruce Momjian [mailto:pgman@candle.pha.pa.us] > > [Charset iso-8859-1 unsupported, filtering to ASCII...] > > Hi all, > > > > I'm trying to implement REINDEX command. > > > > REINDEX operation itself is available everywhere and > > I've thought about applying it to VACUUM. > > That is a good idea. Vacuuming of indexes can be very slow. > > > . > > My plan is as follows. > > > > Add a new option to force index recreation in vacuum > > and if index recreation is specified. > > Couldn't we auto-recreate indexes based on the number of tuples moved by > vacuum, Yes,we could probably do it. But I'm not sure the availability of new vacuum. New vacuum would give us a big advantage that 1) Much faster than current if vacuum remove/moves many tuples. 2) Does shrink index files But in case of abort/crash 1) couldn't choose index scan for the table 2) unique constraints of the table would be lost I don't know how people estimate this disadvantage. > > > Now I'm inclined to use relhasindex of pg_class to > > validate/invalidate indexes of a table at once. > > There are a few calls to CatalogIndexInsert() that know the > system table they > are using and know it has indexes, so it does not check that field. You > could add cases for that. > I think there aren't so many places to check. I would examine it if my idea is OK. Regards. Hiroshi Inoue Inoue@tpf.co.jp ************ From owner-pgsql-hackers@hub.org Tue Jan 18 19:57:21 2000 Received: from hub.org (hub.org [216.126.84.1]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id UAA11764 for ; Tue, 18 Jan 2000 20:57:19 -0500 (EST) Received: from localhost (majordom@localhost) by hub.org (8.9.3/8.9.3) with SMTP id UAA50653; Tue, 18 Jan 2000 20:52:38 -0500 (EST) (envelope-from owner-pgsql-hackers) Received: by hub.org (bulk_mailer v1.5); Tue, 18 Jan 2000 20:52:30 -0500 Received: (from majordom@localhost) by hub.org (8.9.3/8.9.3) id UAA50513 for pgsql-hackers-outgoing; Tue, 18 Jan 2000 20:51:32 -0500 (EST) (envelope-from owner-pgsql-hackers@postgreSQL.org) Received: from candle.pha.pa.us (pgman@s5-03.ppp.op.net [209.152.195.67]) by hub.org (8.9.3/8.9.3) with ESMTP id UAA50462 for ; Tue, 18 Jan 2000 20:51:06 -0500 (EST) (envelope-from pgman@candle.pha.pa.us) Received: (from pgman@localhost) by candle.pha.pa.us (8.9.0/8.9.0) id UAA11421; Tue, 18 Jan 2000 20:50:50 -0500 (EST) From: Bruce Momjian Message-Id: <200001190150.UAA11421@candle.pha.pa.us> Subject: Re: [HACKERS] Index recreation in vacuum In-Reply-To: <000201bf621a$6b9baf20$2801007e@tpf.co.jp> from Hiroshi Inoue at "Jan 19, 2000 10:13:40 am" To: Hiroshi Inoue Date: Tue, 18 Jan 2000 20:50:50 -0500 (EST) CC: pgsql-hackers X-Mailer: ELM [version 2.4ME+ PL66 (25)] MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-pgsql-hackers@postgreSQL.org Status: ROr > > > Add a new option to force index recreation in vacuum > > > and if index recreation is specified. > > > > Couldn't we auto-recreate indexes based on the number of tuples moved by > > vacuum, > > Yes,we could probably do it. But I'm not sure the availability of new > vacuum. > > New vacuum would give us a big advantage that > 1) Much faster than current if vacuum remove/moves many tuples. > 2) Does shrink index files > > But in case of abort/crash > 1) couldn't choose index scan for the table > 2) unique constraints of the table would be lost > > I don't know how people estimate this disadvantage. That's why I was recommending rename(). The actual window of vunerability goes from perhaps hours to fractions of a second. In fact, if I understand this right, you could make the vulerability zero by just performing the rename as one operation. In fact, for REINDEX cases where you don't have a lock on the entire table as you do in vacuum, you could reindex the table with a simple read-lock on the base table and index, and move the new index into place with the users seeing no change. Only people traversing the index during the change would have a problem. You just need an exclusive access on the index for the duration of the rename() so no one is traversing the index during the rename(). Destroying the index and recreating opens a large time span that there is no index, and you have to jury-rig something so people don't try to use the index. With rename() you just put the new index in place with one operation. Just don't let people traverse the index during the change. The pointers to the heap tuples is the same in both indexes. In fact, with WAL, we will allow multiple physical files for the same table by appending the table oid to the file name. In this case, the old index could be deleted by rename, and people would continue to use the old index until they deleted the open file pointers. Not sure how this works in practice because new tuples would not be inserted into the old copy of the index. -- Bruce Momjian | http://www.op.net/~candle pgman@candle.pha.pa.us | (610) 853-3000 + If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill, Pennsylvania 19026 ************ From pgman Tue Jan 18 20:04:11 2000 Received: (from pgman@localhost) by candle.pha.pa.us (8.9.0/8.9.0) id VAA11990; Tue, 18 Jan 2000 21:04:11 -0500 (EST) From: Bruce Momjian Message-Id: <200001190204.VAA11990@candle.pha.pa.us> Subject: Re: [HACKERS] Index recreation in vacuum In-Reply-To: <200001190150.UAA11421@candle.pha.pa.us> from Bruce Momjian at "Jan 18, 2000 08:50:50 pm" To: Bruce Momjian Date: Tue, 18 Jan 2000 21:04:11 -0500 (EST) CC: Hiroshi Inoue , pgsql-hackers X-Mailer: ELM [version 2.4ME+ PL66 (25)] MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Status: RO > > I don't know how people estimate this disadvantage. > > That's why I was recommending rename(). The actual window of > vunerability goes from perhaps hours to fractions of a second. > > In fact, if I understand this right, you could make the vulerability > zero by just performing the rename as one operation. > > In fact, for REINDEX cases where you don't have a lock on the entire > table as you do in vacuum, you could reindex the table with a simple > read-lock on the base table and index, and move the new index into place > with the users seeing no change. Only people traversing the index > during the change would have a problem. You just need an exclusive > access on the index for the duration of the rename() so no one is > traversing the index during the rename(). > > Destroying the index and recreating opens a large time span that there > is no index, and you have to jury-rig something so people don't try to > use the index. With rename() you just put the new index in place with > one operation. Just don't let people traverse the index during the > change. The pointers to the heap tuples is the same in both indexes. > > In fact, with WAL, we will allow multiple physical files for the same > table by appending the table oid to the file name. In this case, the > old index could be deleted by rename, and people would continue to use > the old index until they deleted the open file pointers. Not sure how > this works in practice because new tuples would not be inserted into the > old copy of the index. Maybe I am all wrong here. Maybe most of the advantage of rename() are meaningless with reindex using during vacuum, which is the most important use of reindex. Let's look at index using during vacuum. Right now, how does vacuum handle indexes when it moves a tuple? Does it do each index update as it moves a tuple? Is that why it is so slow? If we don't do that and vacuum fails, what state is the table left in? If we don't update the index for every tuple, the index is invalid in a vacuum failure. rename() is not going to help us here. It keeps the old index around, but the index is invalid anyway, right? -- Bruce Momjian | http://www.op.net/~candle pgman@candle.pha.pa.us | (610) 853-3000 + If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill, Pennsylvania 19026 From Inoue@tpf.co.jp Tue Jan 18 20:18:48 2000 Received: from sd.tpf.co.jp (sd.tpf.co.jp [210.161.239.34]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id VAA12437 for ; Tue, 18 Jan 2000 21:18:46 -0500 (EST) Received: from cadzone ([126.0.1.40] (may be forged)) by sd.tpf.co.jp (2.5 Build 2640 (Berkeley 8.8.6)/8.8.4) with SMTP id LAA02845; Wed, 19 Jan 2000 11:18:18 +0900 From: "Hiroshi Inoue" To: "Bruce Momjian" Cc: "pgsql-hackers" Subject: RE: [HACKERS] Index recreation in vacuum Date: Wed, 19 Jan 2000 11:23:55 +0900 Message-ID: <000801bf6224$3bfdd9a0$2801007e@tpf.co.jp> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook 8.5, Build 4.71.2173.0 X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2314.1300 Importance: Normal In-Reply-To: <200001190204.VAA11990@candle.pha.pa.us> Status: ROr > -----Original Message----- > From: Bruce Momjian [mailto:pgman@candle.pha.pa.us] > > > > I don't know how people estimate this disadvantage. > > > > That's why I was recommending rename(). The actual window of > > vunerability goes from perhaps hours to fractions of a second. > > > > In fact, if I understand this right, you could make the vulerability > > zero by just performing the rename as one operation. > > > > In fact, for REINDEX cases where you don't have a lock on the entire > > table as you do in vacuum, you could reindex the table with a simple > > read-lock on the base table and index, and move the new index into place > > with the users seeing no change. Only people traversing the index > > during the change would have a problem. You just need an exclusive > > access on the index for the duration of the rename() so no one is > > traversing the index during the rename(). > > > > Destroying the index and recreating opens a large time span that there > > is no index, and you have to jury-rig something so people don't try to > > use the index. With rename() you just put the new index in place with > > one operation. Just don't let people traverse the index during the > > change. The pointers to the heap tuples is the same in both indexes. > > > > In fact, with WAL, we will allow multiple physical files for the same > > table by appending the table oid to the file name. In this case, the > > old index could be deleted by rename, and people would continue to use > > the old index until they deleted the open file pointers. Not sure how > > this works in practice because new tuples would not be inserted into the > > old copy of the index. > > Maybe I am all wrong here. Maybe most of the advantage of rename() are > meaningless with reindex using during vacuum, which is the most > important use of reindex. > > Let's look at index using during vacuum. Right now, how does vacuum > handle indexes when it moves a tuple? Does it do each index update as > it moves a tuple? Is that why it is so slow? > Yes,I believe so. It's necessary to keep consistency between heap table and indexes even in case of abort/crash. As far as I see,it has been a big charge for vacuum. Regards. Hiroshi Inoue Inoue@tpf.co.jp From owner-pgsql-hackers@hub.org Tue Jan 18 20:53:49 2000 Received: from hub.org (hub.org [216.126.84.1]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id VAA13285 for ; Tue, 18 Jan 2000 21:53:47 -0500 (EST) Received: from localhost (majordom@localhost) by hub.org (8.9.3/8.9.3) with SMTP id VAA65183; Tue, 18 Jan 2000 21:47:47 -0500 (EST) (envelope-from owner-pgsql-hackers) Received: by hub.org (bulk_mailer v1.5); Tue, 18 Jan 2000 21:47:33 -0500 Received: (from majordom@localhost) by hub.org (8.9.3/8.9.3) id VAA65091 for pgsql-hackers-outgoing; Tue, 18 Jan 2000 21:46:33 -0500 (EST) (envelope-from owner-pgsql-hackers@postgreSQL.org) Received: from candle.pha.pa.us (pgman@s5-03.ppp.op.net [209.152.195.67]) by hub.org (8.9.3/8.9.3) with ESMTP id VAA65034 for ; Tue, 18 Jan 2000 21:46:12 -0500 (EST) (envelope-from pgman@candle.pha.pa.us) Received: (from pgman@localhost) by candle.pha.pa.us (8.9.0/8.9.0) id VAA13040; Tue, 18 Jan 2000 21:45:27 -0500 (EST) From: Bruce Momjian Message-Id: <200001190245.VAA13040@candle.pha.pa.us> Subject: Re: [HACKERS] Index recreation in vacuum In-Reply-To: <000801bf6224$3bfdd9a0$2801007e@tpf.co.jp> from Hiroshi Inoue at "Jan 19, 2000 11:23:55 am" To: Hiroshi Inoue Date: Tue, 18 Jan 2000 21:45:27 -0500 (EST) CC: pgsql-hackers X-Mailer: ELM [version 2.4ME+ PL66 (25)] MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-pgsql-hackers@postgreSQL.org Status: RO > > > In fact, for REINDEX cases where you don't have a lock on the entire > > > table as you do in vacuum, you could reindex the table with a simple > > > read-lock on the base table and index, and move the new index into place > > > with the users seeing no change. Only people traversing the index > > > during the change would have a problem. You just need an exclusive > > > access on the index for the duration of the rename() so no one is > > > traversing the index during the rename(). > > > > > > Destroying the index and recreating opens a large time span that there > > > is no index, and you have to jury-rig something so people don't try to > > > use the index. With rename() you just put the new index in place with > > > one operation. Just don't let people traverse the index during the > > > change. The pointers to the heap tuples is the same in both indexes. > > > > > > In fact, with WAL, we will allow multiple physical files for the same > > > table by appending the table oid to the file name. In this case, the > > > old index could be deleted by rename, and people would continue to use > > > the old index until they deleted the open file pointers. Not sure how > > > this works in practice because new tuples would not be inserted into the > > > old copy of the index. > > > > Maybe I am all wrong here. Maybe most of the advantage of rename() are > > meaningless with reindex using during vacuum, which is the most > > important use of reindex. > > > > Let's look at index using during vacuum. Right now, how does vacuum > > handle indexes when it moves a tuple? Does it do each index update as > > it moves a tuple? Is that why it is so slow? > > > > Yes,I believe so. It's necessary to keep consistency between heap > table and indexes even in case of abort/crash. > As far as I see,it has been a big charge for vacuum. OK, how about making a copy of the heap table before starting vacuum, moving all the tuples in that copy, create new index, and then move the new heap and indexes over the old version. We already have an exclusive lock on the table. That would be 100% reliable, with the disadvantage of using 2x the disk space. Seems like a big win. -- Bruce Momjian | http://www.op.net/~candle pgman@candle.pha.pa.us | (610) 853-3000 + If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill, Pennsylvania 19026 ************ From owner-pgsql-hackers@hub.org Tue Jan 18 21:15:24 2000 Received: from hub.org (hub.org [216.126.84.1]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id WAA14115 for ; Tue, 18 Jan 2000 22:15:23 -0500 (EST) Received: from localhost (majordom@localhost) by hub.org (8.9.3/8.9.3) with SMTP id WAA72950; Tue, 18 Jan 2000 22:10:40 -0500 (EST) (envelope-from owner-pgsql-hackers) Received: by hub.org (bulk_mailer v1.5); Tue, 18 Jan 2000 22:10:32 -0500 Received: (from majordom@localhost) by hub.org (8.9.3/8.9.3) id WAA72644 for pgsql-hackers-outgoing; Tue, 18 Jan 2000 22:09:36 -0500 (EST) (envelope-from owner-pgsql-hackers@postgreSQL.org) Received: from candle.pha.pa.us (pgman@s5-03.ppp.op.net [209.152.195.67]) by hub.org (8.9.3/8.9.3) with ESMTP id WAA72504 for ; Tue, 18 Jan 2000 22:08:40 -0500 (EST) (envelope-from pgman@candle.pha.pa.us) Received: (from pgman@localhost) by candle.pha.pa.us (8.9.0/8.9.0) id WAA13965; Tue, 18 Jan 2000 22:08:25 -0500 (EST) From: Bruce Momjian Message-Id: <200001190308.WAA13965@candle.pha.pa.us> Subject: Re: [HACKERS] Index recreation in vacuum In-Reply-To: <000f01bf622a$bf423940$2801007e@tpf.co.jp> from Hiroshi Inoue at "Jan 19, 2000 12:10:32 pm" To: Hiroshi Inoue Date: Tue, 18 Jan 2000 22:08:25 -0500 (EST) CC: pgsql-hackers X-Mailer: ELM [version 2.4ME+ PL66 (25)] MIME-Version: 1.0 Content-Type: text/plain; charset=UNKNOWN-8BIT Content-Transfer-Encoding: 8bit Sender: owner-pgsql-hackers@postgreSQL.org Status: RO > I heard from someone that old vacuum had been like so. > Probably 2x disk space for big tables was a big disadvantage. That's interesting. > > In addition,rename(),unlink(),mv aren't preferable for transaction > control as far as I see. We couldn't avoid inconsistency using > those OS functions. I disagree. Vacuum can't be rolled back anyway in the sense you can bring back expire tuples, though I have no idea why you would want to. You have an exclusive lock on the table. Putting new heap/indexes in place that match and have no expired tuples seems like it can not fail in any situation. Of course, the buffers of the old table have to be marked as invalid, but with an exclusive lock, that is not a problem. I am sure we do that anyway in vacuum. > We have to wait the change of relation file naming if copying > vacuum is needed. > Under the spec we need not rename(),mv etc. Sorry, I don't agree, yet... -- Bruce Momjian | http://www.op.net/~candle pgman@candle.pha.pa.us | (610) 853-3000 + If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill, Pennsylvania 19026 ************ From Inoue@tpf.co.jp Tue Jan 18 21:05:23 2000 Received: from sd.tpf.co.jp (sd.tpf.co.jp [210.161.239.34]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id WAA13858 for ; Tue, 18 Jan 2000 22:05:21 -0500 (EST) Received: from cadzone ([126.0.1.40] (may be forged)) by sd.tpf.co.jp (2.5 Build 2640 (Berkeley 8.8.6)/8.8.4) with SMTP id MAA02870; Wed, 19 Jan 2000 12:04:55 +0900 From: "Hiroshi Inoue" To: "Bruce Momjian" Cc: "pgsql-hackers" Subject: RE: [HACKERS] Index recreation in vacuum Date: Wed, 19 Jan 2000 12:10:32 +0900 Message-ID: <000f01bf622a$bf423940$2801007e@tpf.co.jp> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook 8.5, Build 4.71.2173.0 X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2314.1300 Importance: Normal In-Reply-To: <200001190245.VAA13040@candle.pha.pa.us> Status: ROr > -----Original Message----- > From: Bruce Momjian [mailto:pgman@candle.pha.pa.us] > > > > > > Maybe I am all wrong here. Maybe most of the advantage of > rename() are > > > meaningless with reindex using during vacuum, which is the most > > > important use of reindex. > > > > > > Let's look at index using during vacuum. Right now, how does vacuum > > > handle indexes when it moves a tuple? Does it do each index update as > > > it moves a tuple? Is that why it is so slow? > > > > > > > Yes,I believe so. It's necessary to keep consistency between heap > > table and indexes even in case of abort/crash. > > As far as I see,it has been a big charge for vacuum. > > OK, how about making a copy of the heap table before starting vacuum, > moving all the tuples in that copy, create new index, and then move the > new heap and indexes over the old version. We already have an exclusive > lock on the table. That would be 100% reliable, with the disadvantage > of using 2x the disk space. Seems like a big win. > I heard from someone that old vacuum had been like so. Probably 2x disk space for big tables was a big disadvantage. In addition,rename(),unlink(),mv aren't preferable for transaction control as far as I see. We couldn't avoid inconsistency using those OS functions. We have to wait the change of relation file naming if copying vacuum is needed. Under the spec we need not rename(),mv etc. Regards. Hiroshi Inoue Inoue@tpf.co.jp From dms@wplus.net Wed Jan 19 15:30:40 2000 Received: from relay.wplus.net (relay.wplus.net [195.131.52.179]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id QAA25919 for ; Wed, 19 Jan 2000 16:30:38 -0500 (EST) X-Real-To: pgman@candle.pha.pa.us Received: from wplus.net (ppdms.dialup.wplus.net [195.131.52.71]) by relay.wplus.net (8.9.1/8.9.1/wplus.2) with ESMTP id AAA64218; Thu, 20 Jan 2000 00:26:37 +0300 (MSK) Message-ID: <38862C9D.C2151E4E@wplus.net> Date: Thu, 20 Jan 2000 00:29:01 +0300 From: Dmitry Samersoff X-Mailer: Mozilla 4.61 [en] (WinNT; I) X-Accept-Language: ru,en MIME-Version: 1.0 To: Hiroshi Inoue CC: Bruce Momjian , pgsql-hackers Subject: Re: [HACKERS] Index recreation in vacuum References: <000f01bf622a$bf423940$2801007e@tpf.co.jp> Content-Type: text/plain; charset=koi8-r Content-Transfer-Encoding: 7bit Status: ROr Hiroshi Inoue wrote: > > > Yes,I believe so. It's necessary to keep consistency between heap > > > table and indexes even in case of abort/crash. > > > As far as I see,it has been a big charge for vacuum. > > > > OK, how about making a copy of the heap table before starting vacuum, > > moving all the tuples in that copy, create new index, and then move the > > new heap and indexes over the old version. We already have an exclusive > > lock on the table. That would be 100% reliable, with the disadvantage > > of using 2x the disk space. Seems like a big win. > > > > I heard from someone that old vacuum had been like so. > Probably 2x disk space for big tables was a big disadvantage. Yes, It is critical. How about sequence like this: * Drop indices (keeping somewhere index descriptions) * vacuuming table * recreate indices If something crash, user have been noticed to re-run vacuum or recreate indices by hand when system restarts. I use script like described above for vacuuming - it really increase vacuum performance for large table. -- Dmitry Samersoff, DM\S dms@wplus.net http://devnull.wplus.net * there will come soft rains From dms@wplus.net Wed Jan 19 15:42:49 2000 Received: from relay.wplus.net (relay.wplus.net [195.131.52.179]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id QAA26645 for ; Wed, 19 Jan 2000 16:42:47 -0500 (EST) X-Real-To: pgman@candle.pha.pa.us Received: from wplus.net (ppdms.dialup.wplus.net [195.131.52.71]) by relay.wplus.net (8.9.1/8.9.1/wplus.2) with ESMTP id AAA65264; Thu, 20 Jan 2000 00:39:02 +0300 (MSK) Message-ID: <38862F86.20328BD3@wplus.net> Date: Thu, 20 Jan 2000 00:41:26 +0300 From: Dmitry Samersoff X-Mailer: Mozilla 4.61 [en] (WinNT; I) X-Accept-Language: ru,en MIME-Version: 1.0 To: Bruce Momjian CC: Hiroshi Inoue , pgsql-hackers Subject: Re: [HACKERS] Index recreation in vacuum References: <200001192132.QAA26048@candle.pha.pa.us> Content-Type: text/plain; charset=koi8-r Content-Transfer-Encoding: 7bit Status: ROr Bruce Momjian wrote: > > We need two things: > > auto-create index on startup IMHO, It have to be controlled by user, because creating large index can take a number of hours. Sometimes it's better to live without indices at all, and then build it by hand after workday end. -- Dmitry Samersoff, DM\S dms@wplus.net http://devnull.wplus.net * there will come soft rains From owner-pgsql-hackers@hub.org Thu Jan 20 23:51:34 2000 Received: from hub.org (hub.org [216.126.84.1]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id AAA13891 for ; Fri, 21 Jan 2000 00:51:31 -0500 (EST) Received: from localhost (majordom@localhost) by hub.org (8.9.3/8.9.3) with SMTP id AAA91784; Fri, 21 Jan 2000 00:47:07 -0500 (EST) (envelope-from owner-pgsql-hackers) Received: by hub.org (bulk_mailer v1.5); Fri, 21 Jan 2000 00:45:38 -0500 Received: (from majordom@localhost) by hub.org (8.9.3/8.9.3) id AAA91495 for pgsql-hackers-outgoing; Fri, 21 Jan 2000 00:44:40 -0500 (EST) (envelope-from owner-pgsql-hackers@postgreSQL.org) Received: from candle.pha.pa.us (pgman@s5-03.ppp.op.net [209.152.195.67]) by hub.org (8.9.3/8.9.3) with ESMTP id AAA91378 for ; Fri, 21 Jan 2000 00:44:04 -0500 (EST) (envelope-from pgman@candle.pha.pa.us) Received: (from pgman@localhost) by candle.pha.pa.us (8.9.0/8.9.0) id AAA13592; Fri, 21 Jan 2000 00:43:49 -0500 (EST) From: Bruce Momjian Message-Id: <200001210543.AAA13592@candle.pha.pa.us> Subject: [HACKERS] vacuum timings To: Tom Lane Date: Fri, 21 Jan 2000 00:43:49 -0500 (EST) CC: PostgreSQL-development X-Mailer: ELM [version 2.4ME+ PL66 (25)] MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-pgsql-hackers@postgreSQL.org Status: RO I loaded 10,000,000 rows into CREATE TABLE test (x INTEGER); Table is 400MB and index is 160MB. With index on the single in4 column, I got: 78 seconds for a vacuum 121 seconds for vacuum after deleting a single row 662 seconds for vacuum after deleting the entire table With no index, I got: 43 seconds for a vacuum 43 seconds for vacuum after deleting a single row 43 seconds for vacuum after deleting the entire table I find this quite interesting. -- Bruce Momjian | http://www.op.net/~candle pgman@candle.pha.pa.us | (610) 853-3000 + If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill, Pennsylvania 19026 ************ From owner-pgsql-hackers@hub.org Fri Jan 21 00:34:56 2000 Received: from hub.org (hub.org [216.126.84.1]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id BAA15559 for ; Fri, 21 Jan 2000 01:34:55 -0500 (EST) Received: from localhost (majordom@localhost) by hub.org (8.9.3/8.9.3) with SMTP id BAA06108; Fri, 21 Jan 2000 01:32:23 -0500 (EST) (envelope-from owner-pgsql-hackers) Received: by hub.org (bulk_mailer v1.5); Fri, 21 Jan 2000 01:30:38 -0500 Received: (from majordom@localhost) by hub.org (8.9.3/8.9.3) id BAA03704 for pgsql-hackers-outgoing; Fri, 21 Jan 2000 01:27:53 -0500 (EST) (envelope-from owner-pgsql-hackers@postgreSQL.org) Received: from sunpine.krs.ru (SunPine.krs.ru [195.161.16.37]) by hub.org (8.9.3/8.9.3) with ESMTP id BAA01710 for ; Fri, 21 Jan 2000 01:26:44 -0500 (EST) (envelope-from vadim@krs.ru) Received: from krs.ru (dune.krs.ru [195.161.16.38]) by sunpine.krs.ru (8.8.8/8.8.8) with ESMTP id NAA01685; Fri, 21 Jan 2000 13:26:33 +0700 (KRS) Message-ID: <3887FC19.80305217@krs.ru> Date: Fri, 21 Jan 2000 13:26:33 +0700 From: Vadim Mikheev Organization: OJSC Rostelecom (Krasnoyarsk) X-Mailer: Mozilla 4.5 [en] (X11; I; FreeBSD 3.0-RELEASE i386) X-Accept-Language: ru, en MIME-Version: 1.0 To: Bruce Momjian CC: Tom Lane , PostgreSQL-development Subject: Re: [HACKERS] vacuum timings References: <200001210543.AAA13592@candle.pha.pa.us> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Sender: owner-pgsql-hackers@postgreSQL.org Status: RO Bruce Momjian wrote: > > I loaded 10,000,000 rows into CREATE TABLE test (x INTEGER); Table is > 400MB and index is 160MB. > > With index on the single in4 column, I got: > 78 seconds for a vacuum > 121 seconds for vacuum after deleting a single row > 662 seconds for vacuum after deleting the entire table > > With no index, I got: > 43 seconds for a vacuum > 43 seconds for vacuum after deleting a single row > 43 seconds for vacuum after deleting the entire table Wi/wo -F ? Vadim ************ From vadim@krs.ru Fri Jan 21 00:26:33 2000 Received: from sunpine.krs.ru (SunPine.krs.ru [195.161.16.37]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id BAA15239 for ; Fri, 21 Jan 2000 01:26:31 -0500 (EST) Received: from krs.ru (dune.krs.ru [195.161.16.38]) by sunpine.krs.ru (8.8.8/8.8.8) with ESMTP id NAA01685; Fri, 21 Jan 2000 13:26:33 +0700 (KRS) Sender: root@sunpine.krs.ru Message-ID: <3887FC19.80305217@krs.ru> Date: Fri, 21 Jan 2000 13:26:33 +0700 From: Vadim Mikheev Organization: OJSC Rostelecom (Krasnoyarsk) X-Mailer: Mozilla 4.5 [en] (X11; I; FreeBSD 3.0-RELEASE i386) X-Accept-Language: ru, en MIME-Version: 1.0 To: Bruce Momjian CC: Tom Lane , PostgreSQL-development Subject: Re: [HACKERS] vacuum timings References: <200001210543.AAA13592@candle.pha.pa.us> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Status: ROr Bruce Momjian wrote: > > I loaded 10,000,000 rows into CREATE TABLE test (x INTEGER); Table is > 400MB and index is 160MB. > > With index on the single in4 column, I got: > 78 seconds for a vacuum > 121 seconds for vacuum after deleting a single row > 662 seconds for vacuum after deleting the entire table > > With no index, I got: > 43 seconds for a vacuum > 43 seconds for vacuum after deleting a single row > 43 seconds for vacuum after deleting the entire table Wi/wo -F ? Vadim From Inoue@tpf.co.jp Fri Jan 21 00:40:35 2000 Received: from sd.tpf.co.jp (sd.tpf.co.jp [210.161.239.34]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id BAA15684 for ; Fri, 21 Jan 2000 01:40:33 -0500 (EST) Received: from cadzone ([126.0.1.40] (may be forged)) by sd.tpf.co.jp (2.5 Build 2640 (Berkeley 8.8.6)/8.8.4) with SMTP id PAA04316; Fri, 21 Jan 2000 15:40:35 +0900 From: "Hiroshi Inoue" To: "Bruce Momjian" Cc: "PostgreSQL-development" , "Tom Lane" Subject: RE: [HACKERS] vacuum timings Date: Fri, 21 Jan 2000 15:46:15 +0900 Message-ID: <000201bf63db$36cdae20$2801007e@tpf.co.jp> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook 8.5, Build 4.71.2173.0 Importance: Normal X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2314.1300 In-Reply-To: <200001210543.AAA13592@candle.pha.pa.us> Status: RO > -----Original Message----- > From: owner-pgsql-hackers@postgreSQL.org > [mailto:owner-pgsql-hackers@postgreSQL.org]On Behalf Of Bruce Momjian > > I loaded 10,000,000 rows into CREATE TABLE test (x INTEGER); Table is > 400MB and index is 160MB. > > With index on the single in4 column, I got: > 78 seconds for a vacuum vc_vaconeind() is called once > 121 seconds for vacuum after deleting a single row vc_vaconeind() is called twice Hmmm,vc_vaconeind() takes pretty long time even if it does little. > 662 seconds for vacuum after deleting the entire table > How about half of the rows deleted case ? It would take longer time. Regards. Hiroshi Inoue Inoue@tpf.co.jp From owner-pgsql-hackers@hub.org Fri Jan 21 12:00:49 2000 Received: from hub.org (hub.org [216.126.84.1]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id NAA13329 for ; Fri, 21 Jan 2000 13:00:47 -0500 (EST) Received: from localhost (majordom@localhost) by hub.org (8.9.3/8.9.3) with SMTP id MAA96106; Fri, 21 Jan 2000 12:55:34 -0500 (EST) (envelope-from owner-pgsql-hackers) Received: by hub.org (bulk_mailer v1.5); Fri, 21 Jan 2000 12:53:53 -0500 Received: (from majordom@localhost) by hub.org (8.9.3/8.9.3) id MAA95775 for pgsql-hackers-outgoing; Fri, 21 Jan 2000 12:52:54 -0500 (EST) (envelope-from owner-pgsql-hackers@postgreSQL.org) Received: from candle.pha.pa.us (root@s5-03.ppp.op.net [209.152.195.67]) by hub.org (8.9.3/8.9.3) with ESMTP id MAA95720 for ; Fri, 21 Jan 2000 12:52:39 -0500 (EST) (envelope-from pgman@candle.pha.pa.us) Received: (from pgman@localhost) by candle.pha.pa.us (8.9.0/8.9.0) id MAA12106; Fri, 21 Jan 2000 12:51:53 -0500 (EST) From: Bruce Momjian Message-Id: <200001211751.MAA12106@candle.pha.pa.us> Subject: [HACKERS] Re: vacuum timings In-Reply-To: <3641.948433911@sss.pgh.pa.us> from Tom Lane at "Jan 21, 2000 00:51:51 am" To: Tom Lane Date: Fri, 21 Jan 2000 12:51:53 -0500 (EST) CC: PostgreSQL-development X-Mailer: ELM [version 2.4ME+ PL66 (25)] MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-pgsql-hackers@postgreSQL.org Status: RO > Bruce Momjian writes: > > I loaded 10,000,000 rows into CREATE TABLE test (x INTEGER); Table is > > 400MB and index is 160MB. > > > With index on the single in4 column, I got: > > 78 seconds for a vacuum > > 121 seconds for vacuum after deleting a single row > > 662 seconds for vacuum after deleting the entire table > > > With no index, I got: > > 43 seconds for a vacuum > > 43 seconds for vacuum after deleting a single row > > 43 seconds for vacuum after deleting the entire table > > > I find this quite interesting. > > How long does it take to create the index on your setup --- ie, > if vacuum did a drop/create index, would it be competitive? OK, new timings with -F enabled: index no index 519 same load 247 " first vacuum 40 " other vacuums 1222 X index creation 90 X first vacuum 80 X other vacuums <1 90 delete one row 121 38 vacuum after delete 1 row 346 344 delete all rows 440 44 first vacuum 20 <1 other vacuums(index is still same size) Conclusions: o indexes never get smaller o drop/recreate index is slower than vacuum of indexes What other conclusions can be made? -- Bruce Momjian | http://www.op.net/~candle pgman@candle.pha.pa.us | (610) 853-3000 + If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill, Pennsylvania 19026 ************ From scrappy@hub.org Fri Jan 21 12:45:38 2000 Received: from thelab.hub.org (nat200.60.mpoweredpc.net [142.177.200.60]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id NAA14380 for ; Fri, 21 Jan 2000 13:45:29 -0500 (EST) Received: from localhost (scrappy@localhost) by thelab.hub.org (8.9.3/8.9.1) with ESMTP id OAA68289; Fri, 21 Jan 2000 14:45:35 -0400 (AST) (envelope-from scrappy@hub.org) X-Authentication-Warning: thelab.hub.org: scrappy owned process doing -bs Date: Fri, 21 Jan 2000 14:45:34 -0400 (AST) From: The Hermit Hacker To: Bruce Momjian cc: Tom Lane , PostgreSQL-development Subject: Re: [HACKERS] Re: vacuum timings In-Reply-To: <200001211751.MAA12106@candle.pha.pa.us> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Status: RO On Fri, 21 Jan 2000, Bruce Momjian wrote: > OK, new timings with -F enabled: > > index no index > 519 same load > 247 " first vacuum > 40 " other vacuums > > 1222 X index creation > 90 X first vacuum > 80 X other vacuums > > <1 90 delete one row > 121 38 vacuum after delete 1 row > > 346 344 delete all rows > 440 44 first vacuum > 20 <1 other vacuums(index is still same size) > > Conclusions: > > o indexes never get smaller this one, I thought, was a known? if I remember right, Vadim changed it so that space was reused, but index never shrunk in size ... no? Marc G. Fournier ICQ#7615664 IRC Nick: Scrappy Systems Administrator @ hub.org primary: scrappy@hub.org secondary: scrappy@{freebsd|postgresql}.org From tgl@sss.pgh.pa.us Fri Jan 21 13:06:35 2000 Received: from sss2.sss.pgh.pa.us (sss.pgh.pa.us [209.114.166.2]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id OAA14618 for ; Fri, 21 Jan 2000 14:06:33 -0500 (EST) Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1]) by sss2.sss.pgh.pa.us (8.9.3/8.9.3) with ESMTP id OAA16501; Fri, 21 Jan 2000 14:06:31 -0500 (EST) To: Bruce Momjian cc: PostgreSQL-development Subject: Re: vacuum timings In-reply-to: <200001211751.MAA12106@candle.pha.pa.us> References: <200001211751.MAA12106@candle.pha.pa.us> Comments: In-reply-to Bruce Momjian message dated "Fri, 21 Jan 2000 12:51:53 -0500" Date: Fri, 21 Jan 2000 14:06:31 -0500 Message-ID: <16498.948481591@sss.pgh.pa.us> From: Tom Lane Status: RO Bruce Momjian writes: > Conclusions: > o indexes never get smaller Which we knew... > o drop/recreate index is slower than vacuum of indexes Quite a few people have reported finding the opposite in practice. You should probably try vacuuming after deleting or updating some fraction of the rows, rather than just the all or none cases. regards, tom lane From dms@wplus.net Fri Jan 21 13:51:27 2000 Received: from relay.wplus.net (relay.wplus.net [195.131.52.179]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id OAA15623 for ; Fri, 21 Jan 2000 14:51:24 -0500 (EST) X-Real-To: pgman@candle.pha.pa.us Received: from wplus.net (ppdms.dialup.wplus.net [195.131.52.71]) by relay.wplus.net (8.9.1/8.9.1/wplus.2) with ESMTP id WAA89451; Fri, 21 Jan 2000 22:46:19 +0300 (MSK) Message-ID: <3888B822.28F79A1F@wplus.net> Date: Fri, 21 Jan 2000 22:48:50 +0300 From: Dmitry Samersoff X-Mailer: Mozilla 4.7 [en] (WinNT; I) X-Accept-Language: ru,en MIME-Version: 1.0 To: Tom Lane CC: Bruce Momjian , PostgreSQL-development Subject: Re: [HACKERS] Re: vacuum timings References: <200001211751.MAA12106@candle.pha.pa.us> <16498.948481591@sss.pgh.pa.us> Content-Type: text/plain; charset=koi8-r Content-Transfer-Encoding: 7bit Status: ROr Tom Lane wrote: > > Bruce Momjian writes: > > Conclusions: > > o indexes never get smaller > > Which we knew... > > > o drop/recreate index is slower than vacuum of indexes > > Quite a few people have reported finding the opposite in practice. I'm one of them. On 1,5 GB table with three indices it about twice slowly. Probably becouse vacuuming indices brakes system cache policy. (FreeBSD 3.3) -- Dmitry Samersoff, DM\S dms@wplus.net http://devnull.wplus.net * there will come soft rains From owner-pgsql-hackers@hub.org Fri Jan 21 14:04:08 2000 Received: from hub.org (hub.org [216.126.84.1]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id PAA16140 for ; Fri, 21 Jan 2000 15:04:06 -0500 (EST) Received: from localhost (majordom@localhost) by hub.org (8.9.3/8.9.3) with SMTP id OAA34808; Fri, 21 Jan 2000 14:59:30 -0500 (EST) (envelope-from owner-pgsql-hackers) Received: by hub.org (bulk_mailer v1.5); Fri, 21 Jan 2000 14:57:48 -0500 Received: (from majordom@localhost) by hub.org (8.9.3/8.9.3) id OAA34320 for pgsql-hackers-outgoing; Fri, 21 Jan 2000 14:56:50 -0500 (EST) (envelope-from owner-pgsql-hackers@postgreSQL.org) Received: from candle.pha.pa.us (pgman@s5-03.ppp.op.net [209.152.195.67]) by hub.org (8.9.3/8.9.3) with ESMTP id OAA34255 for ; Fri, 21 Jan 2000 14:56:18 -0500 (EST) (envelope-from pgman@candle.pha.pa.us) Received: (from pgman@localhost) by candle.pha.pa.us (8.9.0/8.9.0) id OAA15772; Fri, 21 Jan 2000 14:54:22 -0500 (EST) From: Bruce Momjian Message-Id: <200001211954.OAA15772@candle.pha.pa.us> Subject: Re: [HACKERS] Re: vacuum timings In-Reply-To: <3888B822.28F79A1F@wplus.net> from Dmitry Samersoff at "Jan 21, 2000 10:48:50 pm" To: Dmitry Samersoff Date: Fri, 21 Jan 2000 14:54:21 -0500 (EST) CC: Tom Lane , PostgreSQL-development X-Mailer: ELM [version 2.4ME+ PL66 (25)] MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-pgsql-hackers@postgreSQL.org Status: RO [Charset koi8-r unsupported, filtering to ASCII...] > Tom Lane wrote: > > > > Bruce Momjian writes: > > > Conclusions: > > > o indexes never get smaller > > > > Which we knew... > > > > > o drop/recreate index is slower than vacuum of indexes > > > > Quite a few people have reported finding the opposite in practice. > > I'm one of them. On 1,5 GB table with three indices it about twice > slowly. > Probably becouse vacuuming indices brakes system cache policy. > (FreeBSD 3.3) OK, we are researching what things can be done to improve this. We are toying with: lock table for less duration, or read lock creating another copy of heap/indexes, and rename() over old files improving heap vacuum speed improving index vacuum speed moving analyze out of vacuum -- Bruce Momjian | http://www.op.net/~candle pgman@candle.pha.pa.us | (610) 853-3000 + If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill, Pennsylvania 19026 ************ From scrappy@hub.org Fri Jan 21 14:12:16 2000 Received: from thelab.hub.org (nat200.60.mpoweredpc.net [142.177.200.60]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id PAA16521 for ; Fri, 21 Jan 2000 15:12:13 -0500 (EST) Received: from localhost (scrappy@localhost) by thelab.hub.org (8.9.3/8.9.1) with ESMTP id QAA69039; Fri, 21 Jan 2000 16:12:25 -0400 (AST) (envelope-from scrappy@hub.org) X-Authentication-Warning: thelab.hub.org: scrappy owned process doing -bs Date: Fri, 21 Jan 2000 16:12:25 -0400 (AST) From: The Hermit Hacker To: Bruce Momjian cc: Dmitry Samersoff , Tom Lane , PostgreSQL-development Subject: Re: [HACKERS] Re: vacuum timings In-Reply-To: <200001211954.OAA15772@candle.pha.pa.us> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Status: RO On Fri, 21 Jan 2000, Bruce Momjian wrote: > [Charset koi8-r unsupported, filtering to ASCII...] > > Tom Lane wrote: > > > > > > Bruce Momjian writes: > > > > Conclusions: > > > > o indexes never get smaller > > > > > > Which we knew... > > > > > > > o drop/recreate index is slower than vacuum of indexes > > > > > > Quite a few people have reported finding the opposite in practice. > > > > I'm one of them. On 1,5 GB table with three indices it about twice > > slowly. > > Probably becouse vacuuming indices brakes system cache policy. > > (FreeBSD 3.3) > > OK, we are researching what things can be done to improve this. We are > toying with: > > lock table for less duration, or read lock if there is some way that we can work around the bug that I believe Tom found with removing the lock altogether (ie. makig use of MVCC), I think that would be the best option ... if not possible, at least get things down to a table lock vs the whole database? a good example is the udmsearch that we are using on the site ... it uses multiple tables to store the dictionary, each representing words of X size ... if I'm searching on a 4 letter word, and the whole database is locked while it is working on the dictionary with 8 letter words, I'm sitting there idle ... at least if we only locked the 8 letter table, everyone not doing 8 letter searches can go on their merry way ... Slightly longer vacuum's, IMHO, are acceptable if, to the end users, its as transparent as possible ... locking per table would be slightly slower, I think, because once a table is finished, the next table would need to have an exclusive lock put on it before starting, so you'd have to possibly wait for that...? > creating another copy of heap/indexes, and rename() over old files sounds to me like introducing a large potential for error here ... > moving analyze out of vacuum I think that should be done anyway ... if we ever get to the point that we're able to re-use rows in tables, then that would eliminate the immediate requirement for vacuum, but still retain a requirement for a periodic analyze ... no? Marc G. Fournier ICQ#7615664 IRC Nick: Scrappy Systems Administrator @ hub.org primary: scrappy@hub.org secondary: scrappy@{freebsd|postgresql}.org From tgl@sss.pgh.pa.us Fri Jan 21 16:02:07 2000 Received: from sss2.sss.pgh.pa.us (sss.pgh.pa.us [209.114.166.2]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id RAA20290 for ; Fri, 21 Jan 2000 17:02:06 -0500 (EST) Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1]) by sss2.sss.pgh.pa.us (8.9.3/8.9.3) with ESMTP id RAA09697; Fri, 21 Jan 2000 17:02:06 -0500 (EST) To: The Hermit Hacker cc: Bruce Momjian , PostgreSQL-development Subject: Re: [HACKERS] Re: vacuum timings In-reply-to: References: Comments: In-reply-to The Hermit Hacker message dated "Fri, 21 Jan 2000 16:12:25 -0400" Date: Fri, 21 Jan 2000 17:02:06 -0500 Message-ID: <9694.948492126@sss.pgh.pa.us> From: Tom Lane Status: RO The Hermit Hacker writes: >> lock table for less duration, or read lock > if there is some way that we can work around the bug that I believe Tom > found with removing the lock altogether (ie. makig use of MVCC), I think > that would be the best option ... if not possible, at least get things > down to a table lock vs the whole database? Huh? VACUUM only requires an exclusive lock on the table it is currently vacuuming; there's no database-wide lock. Even a single-table exclusive lock is bad, of course, if it's a large table that's critical to a 24x7 application. Bruce was talking about the possibility of having VACUUM get just a write lock on the table; other backends could still read it, but not write it, during the vacuum process. That'd be a considerable step forward for 24x7 applications, I think. It looks like that could be done if we rewrote the table as a new file (instead of compacting-in-place), but there's a problem when it comes time to rename the new files into place. At that point you'd need to get an exclusive lock to ensure all the readers are out of the table too --- and upgrading from a plain lock to an exclusive lock is a well-known recipe for deadlocks. Not sure if this can be solved. regards, tom lane From tgl@sss.pgh.pa.us Fri Jan 21 22:50:34 2000 Received: from sss2.sss.pgh.pa.us (sss.pgh.pa.us [209.114.166.2]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id XAA01657 for ; Fri, 21 Jan 2000 23:50:28 -0500 (EST) Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1]) by sss2.sss.pgh.pa.us (8.9.3/8.9.3) with ESMTP id XAA19681; Fri, 21 Jan 2000 23:50:13 -0500 (EST) To: Bruce Momjian cc: PostgreSQL-development Subject: Re: vacuum timings In-reply-to: <200001211751.MAA12106@candle.pha.pa.us> References: <200001211751.MAA12106@candle.pha.pa.us> Comments: In-reply-to Bruce Momjian message dated "Fri, 21 Jan 2000 12:51:53 -0500" Date: Fri, 21 Jan 2000 23:50:13 -0500 Message-ID: <19678.948516613@sss.pgh.pa.us> From: Tom Lane Status: ROr Bruce Momjian writes: > Conclusions: > o drop/recreate index is slower than vacuum of indexes BTW, I did some profiling of CREATE INDEX this evening (quite unintentionally actually; I was interested in COPY IN, but the pg_dump script I used as driver happened to create some indexes too). I was startled to discover that 60% of the runtime of CREATE INDEX is spent in _bt_invokestrat (which is called from tuplesort.c's comparetup_index, and exists only to figure out which specific comparison routine to call). Of this, a whopping 4% was spent in the useful subroutine, int4gt. All the rest went into lookup and validation checks that by rights should be done once per index creation, not once per comparison. In short: a fairly straightforward bit of optimization will eliminate circa 50% of the CPU time consumed by CREATE INDEX. All we need is to figure out where to cache the lookup results. The optimization would improve insertions and lookups in indexes, as well, if we can cache the lookup results in those scenarios. This was for a table small enough that tuplesort.c could do the sort entirely in memory, so I'm sure the gains would be smaller for a large table that requires a disk-based sort. Still, it seems worth looking into... regards, tom lane From owner-pgsql-hackers@hub.org Sat Jan 22 02:31:03 2000 Received: from renoir.op.net (root@renoir.op.net [207.29.195.4]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id DAA06743 for ; Sat, 22 Jan 2000 03:31:02 -0500 (EST) Received: from hub.org (hub.org [216.126.84.1]) by renoir.op.net (o1/$Revision: 1.6 $) with ESMTP id DAA07529 for ; Sat, 22 Jan 2000 03:25:13 -0500 (EST) Received: from localhost (majordom@localhost) by hub.org (8.9.3/8.9.3) with SMTP id DAA31900; Sat, 22 Jan 2000 03:19:53 -0500 (EST) (envelope-from owner-pgsql-hackers) Received: by hub.org (bulk_mailer v1.5); Sat, 22 Jan 2000 03:17:56 -0500 Received: (from majordom@localhost) by hub.org (8.9.3/8.9.3) id DAA31715 for pgsql-hackers-outgoing; Sat, 22 Jan 2000 03:16:58 -0500 (EST) (envelope-from owner-pgsql-hackers@postgreSQL.org) Received: from sd.tpf.co.jp (sd.tpf.co.jp [210.161.239.34]) by hub.org (8.9.3/8.9.3) with ESMTP id DAA31647 for ; Sat, 22 Jan 2000 03:16:26 -0500 (EST) (envelope-from Inoue@tpf.co.jp) Received: from mcadnote1 (ppm114.noc.fukui.nsk.ne.jp [210.161.188.33]) by sd.tpf.co.jp (2.5 Build 2640 (Berkeley 8.8.6)/8.8.4) with SMTP id RAA04754; Sat, 22 Jan 2000 17:14:43 +0900 From: "Hiroshi Inoue" To: "Tom Lane" , "Bruce Momjian" Cc: "PostgreSQL-development" Subject: RE: [HACKERS] Re: vacuum timings Date: Sat, 22 Jan 2000 17:15:37 +0900 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset="iso-2022-jp" Content-Transfer-Encoding: 7bit X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook IMO, Build 9.0.2416 (9.0.2910.0) X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2314.1300 In-Reply-To: <16498.948481591@sss.pgh.pa.us> Importance: Normal Sender: owner-pgsql-hackers@postgresql.org Status: RO > -----Original Message----- > From: owner-pgsql-hackers@postgresql.org > [mailto:owner-pgsql-hackers@postgresql.org]On Behalf Of Tom Lane > > Bruce Momjian writes: > > Conclusions: > > o indexes never get smaller > > Which we knew... > > > o drop/recreate index is slower than vacuum of indexes > > Quite a few people have reported finding the opposite in practice. > You should probably try vacuuming after deleting or updating some > fraction of the rows, rather than just the all or none cases. > Vacuum after delelting all rows isn't a worst case. There's no moving in that case and vacuum doesn't need to call index_insert() corresponding to the moving of heap tuples. Vacuum after deleting half of rows may be one of the worst case. In this case,index_delete() is called as many times as 'delete all' case and expensive index_insert() is called for moved_in tuples. Regards. Hiroshi Inoue Inoue@tpf.co.jp ************ From tgl@sss.pgh.pa.us Sat Jan 22 10:31:02 2000 Received: from renoir.op.net (root@renoir.op.net [207.29.195.4]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id LAA20882 for ; Sat, 22 Jan 2000 11:31:00 -0500 (EST) Received: from sss2.sss.pgh.pa.us (sss.pgh.pa.us [209.114.166.2]) by renoir.op.net (o1/$Revision: 1.6 $) with ESMTP id LAA26612 for ; Sat, 22 Jan 2000 11:12:44 -0500 (EST) Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1]) by sss2.sss.pgh.pa.us (8.9.3/8.9.3) with ESMTP id LAA20569; Sat, 22 Jan 2000 11:11:26 -0500 (EST) To: "Hiroshi Inoue" cc: "Bruce Momjian" , "PostgreSQL-development" Subject: Re: [HACKERS] Re: vacuum timings In-reply-to: References: Comments: In-reply-to "Hiroshi Inoue" message dated "Sat, 22 Jan 2000 17:15:37 +0900" Date: Sat, 22 Jan 2000 11:11:25 -0500 Message-ID: <20566.948557485@sss.pgh.pa.us> From: Tom Lane Status: RO "Hiroshi Inoue" writes: > Vacuum after deleting half of rows may be one of the worst case. Or equivalently, vacuum after updating all the rows. regards, tom lane From tgl@sss.pgh.pa.us Thu Jan 20 23:51:49 2000 Received: from sss2.sss.pgh.pa.us (sss.pgh.pa.us [209.114.166.2]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id AAA13919 for ; Fri, 21 Jan 2000 00:51:47 -0500 (EST) Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1]) by sss2.sss.pgh.pa.us (8.9.3/8.9.3) with ESMTP id AAA03644; Fri, 21 Jan 2000 00:51:51 -0500 (EST) To: Bruce Momjian cc: PostgreSQL-development Subject: Re: vacuum timings In-reply-to: <200001210543.AAA13592@candle.pha.pa.us> References: <200001210543.AAA13592@candle.pha.pa.us> Comments: In-reply-to Bruce Momjian message dated "Fri, 21 Jan 2000 00:43:49 -0500" Date: Fri, 21 Jan 2000 00:51:51 -0500 Message-ID: <3641.948433911@sss.pgh.pa.us> From: Tom Lane Status: ROr Bruce Momjian writes: > I loaded 10,000,000 rows into CREATE TABLE test (x INTEGER); Table is > 400MB and index is 160MB. > With index on the single in4 column, I got: > 78 seconds for a vacuum > 121 seconds for vacuum after deleting a single row > 662 seconds for vacuum after deleting the entire table > With no index, I got: > 43 seconds for a vacuum > 43 seconds for vacuum after deleting a single row > 43 seconds for vacuum after deleting the entire table > I find this quite interesting. How long does it take to create the index on your setup --- ie, if vacuum did a drop/create index, would it be competitive? regards, tom lane From pgsql-hackers-owner+M5909@hub.org Thu Aug 17 20:15:33 2000 Received: from hub.org (root@hub.org [216.126.84.1]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id UAA00644 for ; Thu, 17 Aug 2000 20:15:32 -0400 (EDT) Received: from hub.org (majordom@localhost [127.0.0.1]) by hub.org (8.10.1/8.10.1) with SMTP id e7I0APm69660; Thu, 17 Aug 2000 20:10:25 -0400 (EDT) Received: from fw.wintelcom.net (bright@ns1.wintelcom.net [209.1.153.20]) by hub.org (8.10.1/8.10.1) with ESMTP id e7I01Jm68072 for ; Thu, 17 Aug 2000 20:01:19 -0400 (EDT) Received: (from bright@localhost) by fw.wintelcom.net (8.10.0/8.10.0) id e7I01IA20820 for pgsql-hackers@postgresql.org; Thu, 17 Aug 2000 17:01:18 -0700 (PDT) Date: Thu, 17 Aug 2000 17:01:18 -0700 From: Alfred Perlstein To: pgsql-hackers@postgresql.org Subject: [HACKERS] VACUUM optimization ideas. Message-ID: <20000817170118.K4854@fw.wintelcom.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.2.4i X-Mailing-List: pgsql-hackers@postgresql.org Precedence: bulk Sender: pgsql-hackers-owner@hub.org Status: ROr Here's two ideas I had for optimizing vacuum, I apologize in advance if the ideas presented here are niave and don't take into account the actual code that makes up postgresql. ================ #1 Reducing the time vacuum must hold an exlusive lock on a table: The idea is that since rows are marked deleted it's ok for the vacuum to fill them with data from the tail of the table as long as no transaction is in progress that has started before the row was deleted. This may allow the vacuum process to copyback all the data without a lock, when all the copying is done it then aquires an exlusive lock and does this: Aquire an exclusive lock. Walk all the deleted data marking it as current. Truncate the table. Release the lock. Since the data is still marked invalid (right?) even if valid data is copied into the space it should be ignored as long as there's no transaction occurring that started before the data was invalidated. ================ #2 Reducing the amount of scanning a vaccum must do: It would make sense that if a value of the earliest deleted chunk was kept in a table then vacuum would not have to scan the entire table in order to work, it would only need to start at the 'earliest' invalidated row. The utility of this (at least for us) is that we have several tables that will grow to hundreds of megabytes, however changes will only happen at the tail end (recently added rows). If we could reduce the amount of time spent in a vacuum state it would help us a lot. ================ I'm wondering if these ideas make sense and may help at all. thanks, -- -Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org] From pgsql-hackers-owner+M5912@hub.org Fri Aug 18 01:36:14 2000 Received: from hub.org (root@hub.org [216.126.84.1]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id BAA07787 for ; Fri, 18 Aug 2000 01:36:12 -0400 (EDT) Received: from hub.org (majordom@localhost [127.0.0.1]) by hub.org (8.10.1/8.10.1) with SMTP id e7I5Q2m38759; Fri, 18 Aug 2000 01:26:04 -0400 (EDT) Received: from courier02.adinet.com.uy (courier02.adinet.com.uy [206.99.44.245]) by hub.org (8.10.1/8.10.1) with ESMTP id e7I5Bam35785 for ; Fri, 18 Aug 2000 01:11:37 -0400 (EDT) Received: from adinet.com.uy (haroldo@r207-50-240-116.adinet.com.uy [207.50.240.116]) by courier02.adinet.com.uy (8.9.3/8.9.3) with ESMTP id CAA17259; Fri, 18 Aug 2000 02:10:49 -0300 (GMT) Message-ID: <399CC739.B9B13D18@adinet.com.uy> Date: Fri, 18 Aug 2000 02:18:49 -0300 From: hstenger@adinet.com.uy Reply-To: hstenger@ieee.org Organization: PRISMA, Servicio y Desarrollo X-Mailer: Mozilla 4.72 [en] (X11; I; Linux 2.2.14 i586) X-Accept-Language: en MIME-Version: 1.0 To: Alfred Perlstein , pgsql-hackers@postgresql.org Subject: Re: [HACKERS] VACUUM optimization ideas. References: <20000817170118.K4854@fw.wintelcom.net> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Mailing-List: pgsql-hackers@postgresql.org Precedence: bulk Sender: pgsql-hackers-owner@hub.org Status: ROr Alfred Perlstein wrote: > #1 > > Reducing the time vacuum must hold an exlusive lock on a table: > > The idea is that since rows are marked deleted it's ok for the > vacuum to fill them with data from the tail of the table as > long as no transaction is in progress that has started before > the row was deleted. > > This may allow the vacuum process to copyback all the data without > a lock, when all the copying is done it then aquires an exlusive lock > and does this: > > Aquire an exclusive lock. > Walk all the deleted data marking it as current. > Truncate the table. > Release the lock. > > Since the data is still marked invalid (right?) even if valid data > is copied into the space it should be ignored as long as there's no > transaction occurring that started before the data was invalidated. Yes, but nothing prevents newer transactions from modifying the _origin_ side of the copied data _after_ it was copied, but before the Lock-Walk-Truncate-Unlock cycle takes place, and so it seems unsafe. Maybe locking each record before copying it up ... Regards, Haroldo. -- ----------------------+------------------------ Haroldo Stenger | hstenger@ieee.org Montevideo, Uruguay. | hstenger@adinet.com.uy ----------------------+------------------------ Visit UYLUG Web Site: http://www.linux.org.uy ----------------------------------------------- From pgsql-hackers-owner+M5917@hub.org Fri Aug 18 09:41:33 2000 Received: from hub.org (root@hub.org [216.126.84.1]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id JAA05170 for ; Fri, 18 Aug 2000 09:41:33 -0400 (EDT) Received: from hub.org (majordom@localhost [127.0.0.1]) by hub.org (8.10.1/8.10.1) with SMTP id e7IDVjm75143; Fri, 18 Aug 2000 09:31:46 -0400 (EDT) Received: from andie.ip23.net (andie.ip23.net [212.83.32.23]) by hub.org (8.10.1/8.10.1) with ESMTP id e7IDPIm73296 for ; Fri, 18 Aug 2000 09:25:18 -0400 (EDT) Received: from imap1.ip23.net (imap1.ip23.net [212.83.32.35]) by andie.ip23.net (8.9.3/8.9.3) with ESMTP id PAA58387; Fri, 18 Aug 2000 15:25:12 +0200 (CEST) Received: from ip23.net (spc.ip23.net [212.83.32.122]) by imap1.ip23.net (8.9.3/8.9.3) with ESMTP id PAA59177; Fri, 18 Aug 2000 15:41:28 +0200 (CEST) Message-ID: <399D3938.582FDB49@ip23.net> Date: Fri, 18 Aug 2000 15:25:12 +0200 From: Sevo Stille Organization: IP23 X-Mailer: Mozilla 4.61 [en] (X11; I; Linux 2.2.10 i686) X-Accept-Language: en, de MIME-Version: 1.0 To: Alfred Perlstein CC: pgsql-hackers@postgresql.org Subject: Re: [HACKERS] VACUUM optimization ideas. References: <20000817170118.K4854@fw.wintelcom.net> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Mailing-List: pgsql-hackers@postgresql.org Precedence: bulk Sender: pgsql-hackers-owner@hub.org Status: RO Alfred Perlstein wrote: > The idea is that since rows are marked deleted it's ok for the > vacuum to fill them with data from the tail of the table as > long as no transaction is in progress that has started before > the row was deleted. Well, isn't one of the advantages of vacuuming in the reordering it does? With a "fill deleted chunks" logic, we'd have far less order in the databases. > This may allow the vacuum process to copyback all the data without > a lock, Nope. Another process might update the values in between move and mark, if the record is not locked. We'd either have to write-lock the entire table for that period, write lock every item as it is moved, or lock, move and mark on a per-record base. The latter would be slow, but it could be done in a permanent low priority background process, utilizing empty CPU cycles. Besides, it probably could not only be done simply filling from the tail, but also moving up the records in a sorted fashion. > #2 > > Reducing the amount of scanning a vaccum must do: > > It would make sense that if a value of the earliest deleted chunk > was kept in a table then vacuum would not have to scan the entire > table in order to work, it would only need to start at the 'earliest' > invalidated row. Trivial to do. But of course #1 may imply that the physical ordering is even less likely to be related to the logical ordering in a way where this helps. > The utility of this (at least for us) is that we have several tables > that will grow to hundreds of megabytes, however changes will only > happen at the tail end (recently added rows). The tail is a relative position - except for the case where you add temporary records to a constant default set, everything in the tail will move, at least relatively, to the head after some time. > If we could reduce the > amount of time spent in a vacuum state it would help us a lot. Rather: If we can reduce the time spent in a locked state while vacuuming, it would help a lot. Being in a vacuum is not the issue - even permanent vacuuming need not be an issue, if the locks it uses are suitably short-time. Sevo -- sevo@ip23.net From pgsql-hackers-owner+M5911@hub.org Thu Aug 17 21:11:20 2000 Received: from hub.org (root@hub.org [216.126.84.1]) by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id VAA01882 for ; Thu, 17 Aug 2000 21:11:20 -0400 (EDT) Received: from hub.org (majordom@localhost [127.0.0.1]) by hub.org (8.10.1/8.10.1) with SMTP id e7I119m80626; Thu, 17 Aug 2000 21:01:09 -0400 (EDT) Received: from acheron.rime.com.au (root@albatr.lnk.telstra.net [139.130.54.222]) by hub.org (8.10.1/8.10.1) with ESMTP id e7I0wMm79870 for ; Thu, 17 Aug 2000 20:58:22 -0400 (EDT) Received: from oberon (Oberon.rime.com.au [203.8.195.100]) by acheron.rime.com.au (8.9.3/8.9.3) with SMTP id KAA03215; Fri, 18 Aug 2000 10:58:25 +1000 Message-Id: <3.0.5.32.20000818105835.0280ade0@mail.rhyme.com.au> X-Sender: pjw@mail.rhyme.com.au X-Mailer: QUALCOMM Windows Eudora Pro Version 3.0.5 (32) Date: Fri, 18 Aug 2000 10:58:35 +1000 To: Chris Bitmead , Ben Adida From: Philip Warner Subject: Re: [HACKERS] Inserting a select statement result into another table Cc: Andrew Selle , pgsql-hackers@postgresql.org In-Reply-To: <399C7689.2DDDAD1D@nimrod.itg.telecom.com.au> References: <20000817130517.A10909@upl.cs.wisc.edu> <399BF555.43FB70C8@openforce.net> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" X-Mailing-List: pgsql-hackers@postgresql.org Precedence: bulk Sender: pgsql-hackers-owner@hub.org Status: O At 09:34 18/08/00 +1000, Chris Bitmead wrote: > >He does ask a legitimate question though. If you are going to have a >LIMIT feature (which of course is not pure SQL), there seems no reason >you shouldn't be able to insert the result into a table. This feature is supported by two commercial DBs: Dec/RDB and SQL/Server. I have no idea if Oracle supports it, but it is such a *useful* feature that I would be very surprised if it didn't. >Ben Adida wrote: >> >> What is the purpose you're trying to accomplish with this order by? No matter what, all the >> rows where done='f' will be inserted, and you will not be left with any indication of that >> order once the rows are in the todolist table. I don't know what his *purpose* was, but the query should only insert the first two rows from the select bacause of the limit). >> Andrew Selle wrote: >> >> > Alright. My situation is this. I have a list of things that need to be done >> > in a table called tasks. I have a list of users who will complete these tasks. >> > I want these users to be able to come in and "claim" the top 2 most recent tasks >> > that have been added. These tasks then get stored in a table called todolist >> > which stores who claimed the task, the taskid, and when the task was claimed. >> > For each time someone wants to claim some number of tasks, I want to do something >> > like >> > >> > INSERT INTO todolist >> > SELECT taskid,'1',now() >> > FROM tasks >> > WHERE done='f' >> > ORDER BY submit DESC >> > LIMIT 2; ---------------------------------------------------------------- Philip Warner | __---_____ Albatross Consulting Pty. Ltd. |----/ - \ (A.B.N. 75 008 659 498) | /(@) ______---_ Tel: (+61) 0500 83 82 81 | _________ \ Fax: (+61) 0500 83 82 82 | ___________ | Http://www.rhyme.com.au | / \| | --________-- PGP key available upon request, | / and from pgp5.ai.mit.edu:11371 |/