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Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Tue Jan 28 20:08:25 EST 1997
Version: 6.0
Current maintainer: Bruce Momjian (maillist@candle.pha.pa.us)
The most recent version of this document can be viewed at the postgreSQL Web
site, http://postgreSQL.org.
Linux-specific questions are answered in
http://postgreSQL.org/docs/FAQ-Linux.phtml.
Irix-specific questions are answered in
http://postgreSQL.org/docs/FAQ-Irix.phtml.
Changes in this version (* = modified, + = new):
* *1.1) What is PostgreSQL?
* *1.5) Support for PostgreSQL
* *1.6) Latest release of PostgreSQL
* *1.10 Does PostgreSQL work with databases from earlier versions of
postgres?
* *3.1) How do I specify a KEY or other constraints on a column?
* *3.3) How do I define a unique indices?
* *3.11) Why doesn't the != operator work?
* *3.26) Why are my table files not getting any smaller after a delete?
* *3.28) I get the error 'default index class unsupported' when creating
an index. How do I do it?
* *3.37) What is the time-warp feature and how does it relate to vacuum?
* *5.1) How do I make a bug report?
----------------------------------------------------------------------------
Questions answered:
1) General questions
1.1) What is PostgreSQL?
1.2) What does PostgreSQL run on?
1.3) Where can I get PostgreSQL?
1.4) What's the copyright on PostgreSQL?
1.5) Support for PostgreSQL
1.6) Latest release of PostgreSQL
1.7) Is there a commercial version of PostgreSQL?
1.9) What version of SQL does PostgreSQL use?
1.10) Does PostgreSQL work with databases from earlier versions of postgres?
1.11) How many people use PostgreSQL?
2) Installation questions
2.1) initdb doesn't run
2.2) when I start up the postmaster, I get "FindBackend: could not find a
backend to execute..." "postmaster: could not find backend to execute..."
2.3) The system seems to be confused about commas, decimal points, and date
formats.
2.4) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
2.5) When I run postmaster, I get a Bad System Call core dumped message.
2.6) When I try to start the postmaster, I get IpcMemoryCreate errors.
2.7) I have changed a source file, but a recompile does not see the change.
2.8) I have changed a source file, but a recompile does not see the change?
3.1) How do I specify a KEY or other constraints on a column?
3.2) Does PostgreSQL support nested subqueries?
3.3) How do I define a unique indices?
3.4) I've having a lot of problems using rules.
3.5) I can't seem to write into the middle of large objects reliably.
3.6) Does PostgreSQL have a graphical user interface? A report generator? A
embedded query language interface?
3.7) How can I write client applications to PostgreSQL?
3.8) How do I prevent other hosts from accessing my PostgreSQL
3.9) How do I set up a pg_group?
3.10) What is the exact difference between binary cursors and normal
cursors?
3.11) Why doesn't the != operator work?
3.12) What is a R-tree index and what is it used for?
3.13) What is the maximum size for a tuple?
3.14) I defined indices but my queries don't seem to make use of them. Why?
3.15) Are there ODBC drivers for PostgreSQL?
3.16) How do I use postgres for multi-dimensional indexing (> 2 dimensions)?
3.17) How do I do regular expression searches? case-insensitive regexp
searching?
3.18) I can't access the database as the 'root' user.
3.19) I experienced a server crash during a vacuum. How do I remove the lock
file?
3.20) What is the difference between the various character types?
3.21) In a query, how do I detect if a field is NULL?
3.22) How do I see how the query optimizer is evaluating my query?
3.23) How do I create a serial field?
3.24) How do I create a multi-column index?
3.25) What are the temp_XXX files in my database directory?
3.26) Why are my table files not getting any smaller after a delete?
3.27) Why can't I connect to my database from another machine?
3.28) I get the error 'default index class unsupported' when creating an
index. How do I do it?
3.29) Why does creating an index crash the backend server?
3.30) How do I specify a decimal constant as a float8, or a string as a
text? Why am I getting poor precision?
3.31) How do I find out what indexes or operations are defined in the
database?
3.32) My database is corrupt. I can't do anything. What should I do?
3.33) Createdb, destroydb, createuser, destroyuser don't run. Why?
3.34) Why does 'createuser' return 'unexpected last match in input()'?
3.35) All my servers crash under concurrent table access. Why?
3.36) What tools are available for hooking postgres to Web pages?
3.37) What is the time-warp feature and how does it relate to vacuum?
3.38) How do I tune the database engine for better performance?
3.39) What debugging features are available in PostgreSQL?
3.40) What is an oid? What is a tid?
3.41) What is the meaning of some of the terms used in Postgres?
4) Questions about extending PostgreSQL
4.1) I wrote a user-defined function and when I run it in psql, it dumps
core.
4.2) I get messages of the type NOTICE:PortalHeapMemoryFree: 0x402251d0
4.3) I've written some nifty new types and functions for PostgreSQL.
4.4) How do I write a C function to return a tuple?
5) Bugs
5.1) How do I make a bug report?
----------------------------------------------------------------------------
Section 1: General Questions
1.1) What is PostgreSQL?
PostgreSQL is an enhancement of the POSTGRES database management system, a
next-generation DBMS research prototype. While PostgreSQL retains the
powerful data model and rich data types of POSTGRES, it replaces the
PostQuel query language with an extended subset of SQL. PostgreSQL is free
and the complete source is available.
PostgreSQL development is being performed by a team of Internet developers
who all subscribe to the PostgreSQL development mailing list. The current
coordinator is Marc G. Fournier (scrappy@postgreSQL.org). (See below on how
to join). This team is now responsible for all current and future
development of PostgreSQL.
The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen. Many others
have contributed to the porting, testing, debugging and enhancement of the
code. The original Postgres code, from which PostgreSQL is derived, was the
effort of many graduate students, undergraduate students, and staff
programmers working under the direction of Professor Michael Stonebraker at
the University of California, Berkeley.
The original name of the software at Berkeley was Postgres. When SQL
functionality was added in 1995, its name was changed to Postgres95. The
name was changed at the end of 1996 to PostgreSQL.
1.2) What does PostgreSQL run on?
The authors have compiled and tested PostgreSQL on the following
platforms(some of these compiles require gcc 2.7.0):
* aix - IBM on AIX 3.2.5
* alpha - DEC Alpha AXP on OSF/1 2.0
* BSD44_derived - OSs derived from 4.4-lite BSD (NetBSD, FreeBSD)
* bsdi - BSD/OS 2.0, 2.01, 2.1
* dgux - DG/UX 5.4R3.10
* hpux - HP PA-RISC on HP-UX 9.0
* i386_solaris - i386 Solaris
* irix5 - SGI MIPS on IRIX 5.3
* linux - Intel x86 on Linux 1.2 and Linux ELF (For non-ELF Linux, see
LINUX_ELF below).
* next - Motorola MC68K or Intel x86 on NeXTSTEP 3.2
* sparc_solaris - SUN SPARC on Solaris 2.4
* sunos4 - SUN SPARC on SunOS 4.1.3
* svr4 - Intel x86 on Intel SVR4
* ultrix4 - DEC MIPS on Ultrix 4.4
1.3) Where can I get PostgreSQL?
The primary anonymous ftp site for PostgreSQL is:
* ftp://ftp.postgreSQL.org/pub
A mirror site exists at:
* ftp://postgres95.vnet.net/pub/postgres95
* ftp://ftp.luga.or.at/pub/postgres95
* ftp://cal011111.student.utwente.nl/pub/postgres95
* ftp://ftp.uni-trier.de/pub/database/rdbms/postgres/postgres95
* ftp://rocker.sch.bme.hu
1.4) What's the copyright on PostgreSQL?
PostgreSQL is subject to the following COPYRIGHT.
PostgreSQL Data Base Management System
Copyright (c) 1994-6 Regents of the University of California
Permission to use, copy, modify, and distribute this software and its
documentation for any purpose, without fee, and without a written agreement
is hereby granted, provided that the above copyright notice and this
paragraph and the following two paragraphs appear in all copies.
IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY PARTY FOR
DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, INCLUDING
LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION,
EVEN IF THE UNIVERSITY OF CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.
THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY WARRANTIES,
INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
FITNESS FOR A PARTICULAR PURPOSE. THE SOFTWARE PROVIDED HEREUNDER IS ON AN
"AS IS" BASIS, AND THE UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO
PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
1.5) Support for PostgreSQL
There is no official support for PostgreSQL from the original maintainers or
from University of California, Berkeley. It is maintained through volunteer
effort only.
The main mailing list is: questions@postgreSQL.org. It is available for
discussion o f matters pertaining to PostgreSQL, including but not limited
to bug reports and fixes. For info on how to subscribe, send a mail with the
lines in the body (not the subject line)
subscribe
end
to questions-request@postgreSQL.org.
There is also a digest list available. To subscribe to this list, send email
to: questions-digest-request@postgreSQL.org with a BODY of:
subscribe
end
Digests are sent out to members of this list whenever the main list has
received around 30k of messages.
There is a bugs mailing list available. To subscribe to this list, send
email to bugs-request@postgreSQL.org with a BODY of:
There is also a developers discussion mailing list available. To subscribe
to this list, send email to hackers-request@postgreSQL.org with a BODY of:
subscribe
end
Additional information about PostgreSQL can be found via the PostgreSQL WWW
home page at:
http://postgreSQL.org
1.6) Latest release of PostgreSQL
The latest release of PostgreSQL is version 1.09. The next release will be
numbered 6.0 for historical reasons. This release is in beta and is
available at our ftp site. We expect the beta period to be complete during
the week of January 31, 1997. For information about what is new in 6.0, see
our TODO list on our WWW page.
We expect a 7.0 release in several months that will remove time-travel and
reduce by 50% the size of on-disk system columns maintained for each row in
a table. This release will also require a dump and restore.
1.7) Is there a commercial version of PostgreSQL?
Illustra Information Technology (a wholly owned subsidiary of Informix
Software, Inc.) sells an object-relational DBMS called Illustra that was
originally based on postgres. Illustra has cosmetic similarities to
PostgreSQL but has more features, is more robust, performs better, and
offers real documentation and support. On the flip side, it costs money. For
more information, contact sales@illustra.com
1.8) What documentation is available for PostgreSQL?
A user manual, manual pages, and some small test examples are included in
the distribution. The sql and built-in manual pages are particularly
important.
The www page contains pointers to an implementation guide and five papers
written about postgres design concepts and features.
1.9) What version of SQL does PostgreSQL use?
PostgreSQL supports a subset of SQL-92. It has most of the important
constructs but lacks some of the functionality. The most visible differences
are:
* no support for nested subqueries
* no HAVING clause under a GROUP BY
On the other hand, you get to create user-defined types, functions,
inheritance etc. If you're willing to help with PostgreSQL coding,
eventually we can also add the missing features listed above.
1.10) Does PostgreSQL work with databases from earlier versions of postgres?
PostgreSQL v1.09 is compatible with databases created with v1.01. Those
upgrading from 1.0 should read the directions in the MIGRATION_1.0_TO_1.02
directory.
Upgrading to 6.0 requires a dump and restore.
1.11) How many people use PostgreSQL?
Since we don't have any licensing or registration scheme, it's impossible to
tell. We do know hundreds copies of PostgreSQL v1.* have been downloaded,
and that there many hundreds of subscribers to the mailing lists.
----------------------------------------------------------------------------
Section 2: Installation Questions
2.1) initdb doesn't run
* check to see that you have the proper paths set
* check that the 'postgres' user owns all the right files
* ensure that there are files in $PGDATA/files, and that they are
non-empty. If they aren't, then "gmake install" failed for some reason
2.2) when I start up the postmaster, I get "FindBackend: could not find a
backend to execute..." "postmaster: could not find backend to execute..."
You probably do not have the right path set up. The 'postgres' executable
needs to be in your path.
2.3) The system seems to be confused about commas, decimal points, and date
formats.
Check your locale configuration. PostgreSQL uses the locale settings of the
user that ran the postmaster process. Set those accordingly for your
operating environment.
2.4) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
You need to edit Makefile.global and change POSTGRESDIR accordingly, or
create a Makefile.custom and define POSTGRESDIR there.
2.5) When I run postmaster, I get a Bad System Call core dumped message.
It could be a variety of problems, but first check to see that you have
system V extensions installed on your kernel. PostgreSQL requires kernel
support for shared memory.
2.6) I get the error message "obj/fmgr.h: No such file or directory"
This indicates that you did not generate the file fmgr.h properly. Something
failed in the running of the src/backend/utils/Gen_fmgrtab.sh script. Check
to see the paths used in that script is appropriate to your system.
2.7) When I try to start the postmaster, I get IpcMemoryCreate errors.
You either do not have shared memory configured properly in kernel or you
need to enlarge the shared memory available in the kernel. The exact amount
you need depends on your architecture and how many buffers you configure
postmaster to run with. For most systems, with default buffer sizes, you
need a minimum of ~760K.
2.8) I have changed a source file, but a recompile does not see the change?
The Makefiles do not have the proper dependencies for include files. You
have to do a 'make clean' and then another 'make'.
----------------------------------------------------------------------------
Section 3: PostgreSQL Features
3.1) How do I specify a KEY or other constraints on a column?
Column constraints are not supported in PostgreSQL. As a consequence, the
system does not check for duplicates.
Under 6.0, create a unique index on the column. Attempts to create duplicate
of that column will report an error.
3.2) Does PostgreSQL support nested subqueries?
Subqueries are not implemented, but they can be simulated using sql
functions.
3.3) How do I define a unique indices?
PostgreSQL 6.0 supports unique indices.
3.4) I've having a lot of problems using rules.
Currently, the rule system in PostgreSQL is mostly broken. It works enough
to support the view mechanism, but that's about it. Use PostgreSQL rules at
your own peril.
3.5) I can't seem to write into the middle of large objects reliably.
The Inversion large object system in PostgreSQL is also mostly broken. It
works well enough for storing large wads of data and reading them back out,
but the implementation has some underlying problems. Use PostgreSQL large
objects at your own peril.
3.6) Does PostgreSQL have a graphical user interface? A report generator? A
embedded query language interface?
No. No. No. Not in the official distribution at least. Some users have
reported some success at using 'pgbrowse' and 'onyx' as frontends to
PostgreSQL. Several contributions are working on tk based frontend tools.
Ask on the mailing list.
3.7) How can I write client applications to PostgreSQL?
PostgreSQL supports a C-callable library interface called libpq as well as a
Tcl-based library interface called libtcl.
Others have contributed a perl interface and a WWW gateway to PostgreSQL.
See the PostgreSQL home pages for more details.
3.8) How do I prevent other hosts from accessing my PostgreSQL backend?
Use host-based authentication by modifying the file $PGDATA/pg_hba
accordingly.
3.9) How do I set up a pg_group?
Currently, there is no easy interface to set up user groups. You have to
explicitly insert/update the pg_group table. For example:
jolly=> insert into pg_group (groname, grosysid, grolist)
jolly=> values ('posthackers', '1234', '{5443, 8261}');
INSERT 548224
jolly=> grant insert on foo to group posthackers;
CHANGE
jolly=>
The fields in pg_group are:
* groname: the group name. This a char16 and should be purely
alphanumeric. Do not include underscores or other punctuation.
* grosysid: the group id. This is an int4. This should be unique for each
group.
* grolist: the list of pg_user id's that belong in the group. This is an
int4[].
3.10) What is the exact difference between binary cursors and normal
cursors?
Normal cursors return data back in ASCII format. Since data is stored
natively in binary format, the system must do a conversion to produce the
ASCII format. In addition, ASCII formats are often large in size than binary
format. Once the attributes come back in ASCII, often the client application
then has to convert it to a binary format to manipulate it anyway.
Binary cursors give you back the data in the native binary representation.
Thus, binary cursors will tend to be a little faster since there's less
overhead of conversion.
However, ASCII is architectural neutral whereas binary representation can
differ between different machine architecture. Thus, if your client machine
uses a different representation than you server machine, getting back
attributes in binary format is probably not what you want. Also, if your
main purpose is displaying the data in ASCII, then getting it back in ASCII
will save you some effort on the client side.
3.11) Why doesn't the != operator work?
SQL specifies <> as the inequality operator, and that is what we have
defined for the built-in types.
In 6.0, != is equivalent to <>.
3.12) What is a R-tree index and what is it used for?
An r-tree index is used for indexing spatial data. A hash index can't handle
range searches. A B-tree index only handles range searches in a single
dimension. R-tree's can handle multi-dimensional data. For example, if a
R-tree index can be built on an attribute of type 'point', the system can
more efficient answer queries like select all points within a bounding
rectangle.
The canonical paper that describes the original R-Tree design is:
Guttman, A. "R-Trees: A Dynamic Index Structure for Spatial Searching." Proc
of the 1984 ACM SIGMOD Int'l Conf on Mgmt of Data, 45-57.
You can also find this paper in Stonebraker's "Readings in Database Systems"
3.13) What is the maximum size for a tuple?
Tuples are limited to 8K bytes. Taking into account system attributes and
other overhead, one should stay well shy of 8,000 bytes to be on the safe
side. To use attributes larger than 8K, try using the large objects
interface.
Tuples do not cross 8k boundaries so a 5k tuple will require 8k of storage.
3.14) I defined indices but my queries don't seem to make use of them. Why?
PostgreSQL does not automatically maintain statistics. One has to make an
explicit 'vacuum' call to update the statistics. After statistics are
updated, the optimizer has a better shot at using indices. Note that the
optimizer is limited and does not use indices in some circumstances (such as
OR clauses).
If the system still does not see the index, it is probably because you have
created an index on a field with the improper *_ops type. For example, you
have created a CHAR(4) field, but have specified a char_ops index
type_class.
See the create_index manual page for information on what type classes are
available. It must match the field type.
Postgres does not warn the user when the improper index is created.
Indexes not used for ORDER BY operations.
3.15) Are there ODBC drivers for PostgreSQL?
There are two ODBC drivers available, PostODBC and OpenLink ODBC.
For all people being interested in PostODBC, there are now two mailing lists
devoted to the discussion of PostODBC. The mailing lists are:
* postodbc-users@listserv.direct. net
* postodbc-developers@listse rv.direct.net
these lists are ordinary majordomo mailing lists. You can subscribe by
sending a mail to:
* majordomo@listserv.direct.net
OpenLink ODBC is currently in beta under Linux. You can get it from
http://www.openlinksw.com/postgres.html. It works with our standard ODBC
client software so you'll have Postgres ODBC available on every client
platform we support (Win, Mac, Unix, VMS).
We will probably be selling this product to people who need
commercial-quality support, but a freeware version will always be available.
Questions to postgres95@openlink.co.uk.
3.16) How do I use postgres for multi-dimensional indexing (> 2
dimensions">)?
Builtin R-Trees can handle polygons and boxes. In theory, R-trees can be
extended to handle higher number of dimensions. In practice, extending
R-trees require a bit of work and we don't currently have any documentation
on how to do it.
3.17) How do I do regular expression searches? case-insensitive regexp
searching?
PostgreSQL supports the SQL LIKE syntax as well as more general regular
expression searching with the ~ operator. The !~ is the negated regexp
operator. ~* and !~* are the case-insensitive regular expression operators.
3.18) I can't access the database as the 'root' user.
You should not create database users with user id 0(root). They will be
unable to access the database. This is a security precaution because of the
ability of any user to dynamically link object modules into the database
engine.
3.19) I experienced a server crash during a vacuum. How do I remove the lock
file?
If the server crashes during a vacuum command, chances are it will leave a
lock file hanging around. Attempts to re-run the vacuum command result in
WARN:can't create lock file -- another vacuum cleaner running?
If you are sure that no vacuum is actually running, you can remove the file
called "pg_vlock" in your database directory (which is $PGDATA/base/)
3.20) What is the difference between the various character types?
Type Internal Name Notes
--------------------------------------------------
CHAR char 1 character }
CHAR2 char2 2 characters }
CHAR4 char4 4 characters } optimized for a fixed length
CHAR8 char8 8 characters }
CHAR16 char16 16 characters }
CHAR(#) bpchar blank padded to the specified fixed length
VARCHAR(#) varchar size specifies maximum length, no padding
TEXT text length limited only by maximum tuple length
BYTEA bytea variable-length array of bytes
Remember, you need to use the internal name when creating indexes on these
fields or when doing other internal operations.
The last four types above are "varlena" types (i.e. the first four bytes is
the length, followed by the data). CHAR(#) and VARCHAR(#) allocate the
maximum number of bytes no matter how much data is stored in the field. TEXT
and BYTEA are the only character types that have variable length on the
disk.
3.21) In a query, how do I detect if a field is NULL?
PostgreSQL has two builtin keywords, "isnull" and "notnull" (note no
spaces). Version 1.05 and later and 6.* understand IS NULL and IS NOT NULL.
3.22) How do I see how the query optimizer is evaluating my query?
Place the word 'EXPLAIN' at the beginning of the query, for example:
EXPLAIN SELECT * FROM table1 WHERE age = 23;
3.23) How do I create a serial field?
Postgres does not allow the user to specifiy a user column as type SERIAL.
Instead, you can use each row's oid field as a unique value. However, if you
need to dump and reload the database, you need to be using postgres version
1.07 or later or 6.* with pgdump's -o option or COPY's WITH OIDS option to
preserver the oids.
Another valid way of doing this is to create a function:
create table my_oids (f1 int4);
insert into my_oids values (1);
create function new_oid () returns int4 as
'update my_oids set f1 = f1 + 1; select f1 from my_oids; '
language 'sql';
then:
create table my_stuff (my_key int4, value text);
insert into my_stuff values (new_oid(), 'hello');
However, keep in mind there is a race condition here where one server could
do the update, then another one do an update, and they both could select the
same new id. This statement should be performed within a transaction.
3.24) How do I create a multi-column index?
You can not directly create a multi-column index using create index. You
need to define a function which acts on the multiple columns, then use
create index with that function.
3.25) What are the temp_XXX files in my database directory?
They are temp_ files generated by the query executor. For example, if a sort
needs to be done to satisfy an ORDER BY, some temp files are generated as a
result of the sort.
If you have no transactions or sorts running at the time, it is safe to
delete the temp_ files.
3.26) Why are my table files not getting any smaller after a delete?
If you run vacuum in pre-6.0, unused rows will be marked for reuse, but the
file blocks are not released.
In 6.0, vacuum properly shrinks tables.
3.27) Why can't I connect to my database from another machine?
The default configuration allows only connections from tcp/ip host
localhost. You need to add a host entry to the file pgsql/data/pg_hba.
3.28) I get the error 'default index class unsupported' when creating an
index. How do I do it?
You probably used:
create index idx1 on person using btree (name);
PostgreSQL indexes are extensible, and therefore in pre-6.0, you must
specify a class_type when creating an index. Read the manual page for create
index (called create_index).
Version 6.0, if you do not specify a class_type, it defaults to the proper
type for the column.
3.29) Why does creating an index crash the backend server?
You have probably defined an incorrect *_ops type class for the field you
are indexing.
3.30) How do I specify a decimal constant as a float8, or a string as a
text? Why am I getting poor precision?
Use the :: operator. It is needed only when the default promotion rules
fail. i.e.:
insert into tab1 values (4.23::float8, '2343'::text)
The default floating-point constant is a float4 in releases prior to 1.05.
Later releases default to float8.
3.31) How do I find out what indexes or operations are defined in the
database?
Run the file pgsql/src/tutorial/syscat.source. It illustrates many of the
'select's needed to get information out of the database system tables.
3.32) My database is corrupt. I can't do anything. What should I do?
The 1.02 release has a README file and utility that describes a possible
cause of the problem and a workaround.
This bug is fixed in 1.02.1.
3.33) Createdb, destroydb, createuser,destroyuser don't run. Why?
Release 1.02 does not have this problem.
The 1.01 release of PostgreSQL uses a variable called PAGER to filter the
output of SELECT statements. Unfortunately, this PAGER is used even when the
standard output is not a terminal.
3.34) Why does 'createuser' return 'unexpected last match in input(">)'?
You have compile postgres with flex version 2.5.3. There is bug in this
version of flex. Use flex version 2.5.2 or flex 2.5.4 instead. There is a
doc/README.flex file which will properly patch the flex 2.5.3 source code.
3.35) All my servers crash under concurrent table access. Why?
This problem can be caused by a kernel that is not configured to support
semaphores.
3.36) What tools are available for hooking postgres to Web pages?
For web integration, PHP/FI is an excellent interface. The URL for that is
http://www.vex.net/php/
PHP is great for simple stuff, but for more complex stuff, some still use
the perl interface and CGI.pm.
An example of using WWW with C to talk to Postgres is can be tried at:
* http://postgreSQL.org/~mlc
An WWW gatway based on WDB using perl can be downloaded from:
* http://www.eol.ists.ca/~dunlop/wdb -p95
3.37) What is the time-warp feature and how does it relate to vacuum?
PostgreSQL handles data changes differently than most database systems. When
a row is changed in a table, the original row is marked with the time it was
changed, and a new row is created with the current data. By default, only
current rows are used in a table. If you specify a date/time after the table
name in a FROM clause, you can access the data that was current at that
time, i.e.
SELECT *
FROM employees ['July 24, 1996 09:00:00']
displays employee rows in the table at the specified time. You can specify
intervals like [date,date], [date,], [,date], or [,]. This last option
accesses all rows that ever existed.
INSERTed rows get a timestamp too, so rows that were not in the table at the
desired time will not appear.
Vacuum removes rows that are no longer current. This time-warp feature is
used by the engine for rollback and crash recovery. Expiration times can be
set with purge.
In 6.0, once a table is vacuumed, the creation time of a row may be
incorrect, causing time-traval to fail.
The time-travel feature will be removed in 7.0.
3.38) How do I tune the database engine for better performance?
There are two things that can be done. You can use Openlink's option to
disable fsync() by starting the postmaster with a '-o -F' option. This will
prevent fsync()'s from flushing to disk after every transaction.
You can also use the postmaster -B option to increase the number of shared
memory buffers shared among the backend processes. If you make this
parameter too high, the process will not start or crash unexpectedly. Each
buffer is 8K and the defualt is 64 buffers.
3.39) What debugging features are available in PostgreSQL?
PostgreSQL has several features that report status information that can be
valuable for debugging purposes.
First, by compiling with DEBUG defined, many assert()'s monitor the progress
of the backend and halt the program when something unexpected occurs.
Both postmaster and postgres have several debug options available. First,
whenever you start the postmaster, make sure you send the standard output
and error to a log file, like:
cd /usr/local/pgsql
./bin/postmaster >server.log 2>&1 &
This will put a server.log file in the top-level PostgreSQL directory. This
file can contain useful information about problems or errors encountered by
the server. Postmaster has a -d option that allows even more detailed
information to be reported. The -d option takes a number 1-3 that specifies
the debug level. The query plans in a verbose debug file can be formatted
using the 'indent' program. (You may need to remove the '====' lines in 1.*
releases.) Be warned that a debug level greater than one generates large log
files in 1.* releases.
You can actuall run the postgres backend from the command line, and type
your SQL statement directly. This is recommended ONLY for debugging
purposes. Note that a newline terminates the query, not a semicolon. If you
have compiled with debugging symbols, you can perhaps use a debugger to see
what is happening. Because the backend was not started from the postmaster,
it is not running in an identical environment and locking/backend
interaction problems may not be duplicated. Some operating system can attach
to a running backend directly to diagnose problems.
The postgres program has a -s, -A, -t options that can be very usefull for
debugging and performance measurements.
The EXPLAIN command (see this FAQ) allows you to see how PostgreSQL is
iterpreting your query.
3.40) What is an oid? What is a tid?
Oids are Postgres's answer to unique row ids or serial columns. Every row
that is created in Postgres gets a unique oid. All oids generated by initdb
are less than 16384 (from backend/access/transam.h). All post-initdb
(user-created) oids are equal or greater that this. All these oids are
unique not only within a table, or database, but unique within the entire
postgres installation.
Postgres uses oids in its internal system tables to link rows in separate
tables. These oids can be used to identify specific user rows and used in
joins. It is recommended you use column type oid to store oid values. See
the sql(l) manual page to see the other internal columns.
Tids are used to indentify specific physical rows with block and offset
values. Tids change after rows are modified or reloaded. They are used by
index entries to point to physical rows. They can not be accessed through
sql.
3.41) What is the meaning of some of the terms used in Postgres?
Some of the source code and older documentation use terms that have more
common usage. Here are some:
* row, record, tuple
* attribute, field, column
* table, class
* retrieve, select
* replace, update
* append, insert
* oid, serial value
* portal, cursor
* range variable, table name, table alias
Please let me know if you think of any more.
----------------------------------------------------------------------------
Section 4: Extending PostgreSQL
4.1) I wrote a user-defined function and when I run it in psql, it dumps
core.
The problem could be a number of things. Try testing your user-defined
function in a stand alone test program first. Also, make sure you are not
sending elog NOTICES when the front-end is expecting data, such as during a
type_in() or type_out() functions
4.2) I get messages of the type NOTICE:PortalHeapMemoryFree: 0x402251d0 not
in alloc set!
You are pfree'ing something that was not palloc'ed. When writing
user-defined functions, do not include the file "libpq-fe.h". Doing so will
cause your palloc to be a malloc instead of a free. Then, when the backend
pfrees the storage, you get the notice message.
4.3) I've written some nifty new types and functions for PostgreSQL.
Please share them with other PostgreSQL users. Send your extensions to
mailing list, and they will eventually end up in the contrib/ subdirectory.
4.4) How do I write a C function to return a tuple?
This requires extreme wizardry, so extreme that the authors have not ever
tried it, though in principle it can be done. The short answer is ... you
can't. This capability is forthcoming in the future.
----------------------------------------------------------------------------
Section 5: Bugs
5.1) How do I make a bug report?
Check the current FAQ at http://postgreSQL.org
Also check out our ftp site ftp://ftp.postgreSQL.org/pub to see if there is
a more recent PostgreSQL version.
You can also fill out the "bug-template" file and send it to:
* bugs@postgreSQL.org
This is the address of the developers mailing list.
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