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Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Sun Aug 30 00:01:22 EDT 1998
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.shtml.
Irix-specific questions are answered in
http://postgreSQL.org/docs/faq-irix.shtml.
_________________________________________________________________
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?
1.8) What documentation is available for PostgreSQL?
1.9) What version of SQL does PostgreSQL use?
1.10) Does PostgreSQL work with databases from earlier versions of
PostgreSQL?
1.11) Are there ODBC drivers for PostgreSQL?
1.12) What tools are available for hooking PostgreSQL to Web pages?
1.13) Does PostgreSQL have a graphical user interface? A report
generator? A embedded query language interface?
1.14) How can I learn SQL?
1.15) What languages are available to communicate with PostgreSQL?
2) Installation/Configuration 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) How do I prevent other hosts from accessing my PostgreSQL
database?
2.9) I can't access the database as the 'root' user.
2.10) All my servers crash under concurrent table access. Why?
2.11) How do I tune the database engine for better performance?
2.12) What debugging features are available in PostgreSQL?
2.13) How do I enable more than 64 concurrent backends? 2.14) What
non-unix ports are available?
3) Operational questions
3.1) Does PostgreSQL support nested subqueries?
3.2) How can I write client applications to PostgreSQL?
3.3) How do I set up a pg_group?
3.4) What is the exact difference between binary cursors and normal
cursors?
3.5) What is an R-tree index and what is it used for?
3.6) What is the maximum size for a tuple?
3.7) I defined indices but my queries don't seem to make use of them.
Why?
3.8) How do I do regular expression searches? case-insensitive regexp
searching?
3.9) I experienced a server crash during a vacuum. How do I remove the
lock file?
3.10) What is the difference between the various character types?
3.11) In a query, how do I detect if a field is NULL?
3.12) How do I see how the query optimizer is evaluating my query?
3.13) How do I create a serial/auto-incrementing field?
3.14) What are the pg_psort.XXX files in my database directory?
3.15) Why can't I connect to my database from another machine?
3.16) How do I find out what indexes or operations are defined in the
database?
3.17) What is an oid? What is a tid?
3.18) What is the meaning of some of the terms used in PostgreSQL?
3.19) What is Genetic Query Optimization?
3.20) How do you remove a column from a table?
3.21) How do SELECT only the first few rows of a query?
3.22) How much database disk space is required to store data from a
typical flat file?
3.23) How do I get a list of tables, or other things I can see in
psql?
3.24) Why do I get the error "FATAL: palloc failure: memory
exhausted?"
3.25) How do I tell what PostgreSQL version I am running?
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 or 4.x
* alpha - DEC Alpha AXP on Digital Unix 2.0, 3.2, 4.0
* BSD44_derived - OSs derived from 4.4-lite BSD (NetBSD, FreeBSD)
* bsdi - BSD/OS 2.0, 2.01, 2.1, 3.0
* dgux - DG/UX 5.4R4.11
* hpux - HP PA-RISC on HP-UX 9.0, 10
* i386_solaris - i386 Solaris
* irix5 - SGI MIPS on IRIX 5.3
* linux - Intel x86 on Linux 2.0 and Linux ELF SPARC on Linux ELF
PPC on Linux Elf (For non-ELF Linux, see LINUX_ELF below).
* sco - SCO 3.2v5
* sparc_solaris - SUN SPARC on Solaris 2.4, 2.5, 2.5.1
* sunos4 - SUN SPARC on SunOS 4.1.3
* svr4 - Intel x86 on Intel SVR4 and MIPS
* ultrix4 - DEC MIPS on Ultrix 4.4
The following platforms have known problems/bugs:
* nextstep - Motorola MC68K or Intel x86 on NeXTSTEP 3.2
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: pgsql-general@postgreSQL.org. It is
available for discussion of matters pertaining to PostgreSQL, For info
on how to subscribe, send a mail with the lines in the body (not the
subject line)
subscribe
end
to pgsql-general-request@postgreSQL.org.
There is also a digest list available. To subscribe to this list, send
email to: pgsql-general-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.
The 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 mailing lists and information about PostgreSQL can be found
via the PostgreSQL WWW home page at:
http://postgreSQL.org
There also an IRC channel on EFNet, channel #PostgreSQL. I use the
unix command irc -c '#PostgreSQL' "$USER" irc.ais.net
1.6) Latest release of PostgreSQL
The latest release of PostgreSQL is version 6.4 beta.
We plan to have major releases every four months.
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 support. On the flip side, it costs money. For more
information, contact sales@illustra.com
1.8) What documentation is available for PostgreSQL?
Several manuals, manual pages, and some small test examples are
included in the distribution. See the /doc directory. The pgintro,
sql, and pgbuiltin manual pages are particularly important. pgintro
contains a list of all available manual pages.
psql has some nice \d commands to show information about types,
operators, functions, aggregates, etc.
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 an extended subset of SQL-92.
1.10) Does PostgreSQL work with databases from earlier versions of
PostgreSQL?
Upgrading to 6.4 from earlier releases requires a dump and restore.
Those ugrading from versions earlier than 1.09 must upgrade to 1.09
first without a dump/reload, then dump the data from 1.09, and then
load it into 6.4.
1.11) Are there ODBC drivers for PostgreSQL?
There are two ODBC drivers available, PostODBC and OpenLink ODBC.
PostODBC is included in the distribution. More information about it
can be gotten from: http://www.insightdist.com/psqlodbc
OpenLink ODBC can be gotten from http://www.openlinksw.com. It works
with their standard ODBC client software so you'll have PostgreSQL
ODBC available on every client platform they support (Win, Mac, Unix,
VMS).
They 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.
1.12) What tools are available for hooking PostgreSQL to Web pages?
A nice introduction to Database-backed Web pages can be seen at:
http://www.webtools.com
For web integration, PHP is an excellent interface. The URL for that
is http://www.php.net
PHP is great for simple stuff, but for more complex stuff, some still
use the perl interface and CGI.pm.
An WWW gatway based on WDB using perl can be downloaded from:
* http://www.eol.ists.ca/~dunlop/wdb -p95
1.13) Does PostgreSQL have a graphical user interface? A report generator? A
embedded query language interface?
We have a nice graphical user interface called pgaccess, which is
shipped as part of the distribtion. Pgaccess also has a report
generator.
We also have ecpg, which is an embedded SQL query language interface
for C. This is also included.
1.14) How can I learn SQL?
There is a nice tutorial at http://w3.one.net/~jhoffman/sqltut.htm and
at
http://ourworld.compuserve.com/homepages/Graeme_Birchall/DB2_COOK.HTM.
Many of our users like The Practical SQL Handbook, Bowman et al,
Addison Wesley.
1.15) What languages are available to communicate with PostgreSQL?
We have:
* C(interfaces/libpq)
* C++(interfaces/libpq++)
* Embedded C(interfaces/ecpg)
* Java(interfaces/jdbc)
* Perl(interfaces/perl5)
* ODBC(interfaces/odbc)
* Python(interfaces/python)
* TCL(interfaces/libpgtcl)
* A crude C/4GL(contrib/pginterface)
_________________________________________________________________
Section 2: Installation Questions
2.1) initdb doesn't run
* check that you don't have any of the previous version's binaries
in your path
* 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) 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.
2.7) 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'.
2.8) How do I prevent other hosts from accessing my PostgreSQL database?
By default, PostgreSQL only allows connections from the local machine
using unix domain. You must add the -i flag to the postmaster, and
enable host-based authentication by modifying the file $PGDATA/pg_hba
accordingly.
2.9) 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.
2.10) All my servers crash under concurrent table access. Why?
This problem can be caused by a kernel that is not configured to
support semaphores.
2.11) How do I tune the database engine for better performance?
If you are doing a lot of INSERTs, consider doing them in a large
batch using the COPY command. This is much faster than single
individual INSERTs. Second, statements not in a BEGIN WORK/COMMIT
transaction block are considered to be their in their own transaction.
Consider performing several statements in a single transaction block.
This reduces the transaction overhead. Also consider dropping and
recreating indexes when making large data changes.
There are several tuning things that can be done. You can 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.
You can also use the postgres -S option to increase the maximum amount
of memory used by each backend process for temporary sorts. Each
buffer is 1K and the defualt is 512 buffers.
You can also use the CLUSTER command to group data in base tables to
match an index. See the cluster(l) manual page for more details.
2.12) What debugging features are available in PostgreSQL?
PostgreSQL has several features that report status information that
can be valuable for debugging purposes.
First, by running configure with the -enable-cassert option, 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. Be warned that a debug level of 3
generates large log files.
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 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.
You can also compile with profiling to see what functions are taking
execution time. The backend profile files will be deposited in the
pgsql/data/base/dbname directory. The client profile file will be put
in the current directory.
The EXPLAIN command (see this FAQ) allows you to see how PostgreSQL is
interpreting your query.
2.13) How do I enable more than 64 concurrent backends?
Edit include/storage/sinvaladt.h, and change the value of
MaxBackendId. In the future, we plan to make this a configurable
prameter.
2.14) What non-unix ports are available?
It is possible to compile the libpq C library, psql, and other
interfaces and binaries to run on MS Windows platforms. In this case,
the client is running on MS Windows, and communicates via TCP/IP to a
server running on one of our supported Unix platforms.
A file win32.mak is included in the distributiion for making a Win32
libpq library and psql.
People have attempted to port our PostgreSQL database server to
Windows NT using the Cygnus Unix/NT porting library, but no one has
succeeded yet.
_________________________________________________________________
Section 3: PostgreSQL Features
3.1) Does PostgreSQL support nested subqueries?
Yes.
3.2) How can I write client applications to PostgreSQL?
PostgreSQL supports a C-callable library interface called libpq as
well as many others. See the /src/interfaces directory.
Others have contributed a perl interface and a WWW gateway to
PostgreSQL. See the PostgreSQL home pages for more details.
3.3) 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 name 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[].
3.4) What is the exact difference between binary cursors and normal cursors?
See the declare manual page for a description.
3.5) What is an 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 an 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"
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.
3.6) 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.
3.7) 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). For column-specific optimization
statistics, use 'vacuum analyze'.
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.
PostgreSQL does not warn the user when the improper index is created.
Indexes not used for ORDER BY operations.
3.8) How do I do regular expression searches? case-insensitive regexp
searching?
See psql's \do command.
3.9) I experienced a server crash during a vacuum. How do I remove the lock
file?
See the vacuum manual page.
3.10) What is the difference between the various character types?
Type Internal Name Notes
--------------------------------------------------
CHAR char 1 character
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
You need to use the internal name when doing internal operations.
The last four types above are "varlena" types (i.e. the first four
bytes are the length, followed by the data). CHAR(#) allocates the
maximum number of bytes no matter how much data is stored in the
field. TEXT, VARCHAR(#), and BYTEA all have variable length on the
disk, and because of this, there is a small performance penalty for
using them. Specifically, the penalty is for access to any columns
after the first column of this type.
3.11) In a query, how do I detect if a field is NULL?
You test the column with IS NULL and IS NOT NULL.
3.12) How do I see how the query optimizer is evaluating my query?
See the explain manual page.
3.13) How do I create a serial/auto-incrementing field?
PostgreSQL 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 use
pgdump's -o option or COPY's WITH OIDS option to preserver the oids.
We also have a SEQUENCE function that is very similar to SERIAL. See
the create_sequence manual page.
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.
Yet another way is to use general trigger function autoinc() from
contrib/spi/autoinc.c.
3.14) What are the pg_psort.XXX files in my database directory?
They are temporary sort 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 pg_psort.XXX files.
3.15) Why can't I connect to my database from another machine?
The default configuration allows only unix domain socket connections
from the local machine. To enable TCP/IP connections, use the
postmaster -i option You need to add a host entry to the file
pgsql/data/pg_hba. See the pg_hba.conf manual page.
3.16) How do I find out what indexes or operations are defined in the
database?
psql has a variety of backslash commands to show such information. Use
\? to see them.
Also try the file pgsql/src/tutorial/syscat.source. It illustrates
many of the 'select's needed to get information out of the database
system tables.
3.17) What is an oid? What is a tid?
Oids are PostgreSQL's answer to unique row ids or serial columns.
Every row that is created in PostgreSQL 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. By default, all these oids are unique not
only within a table, or database, but unique within the entire
PostgreSQL installation.
PostgreSQL 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. You can create an index on the oid field for faster access.
Oids are assigned to all new rows from a central area that is used by
all databases. If you want to change the oid to something else, or if
you want to make a copy of the table, with the original oid's, there
is no reason you can't do it:
CREATE TABLE new_table (mycol int);
INSERT INTO new_table SELECT oid, mycol FROM old_table;
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.
3.18) What is the meaning of some of the terms used in PostgreSQL?
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.
3.19) What is Genetic Query Optimization?
The GEQO module in PostgreSQL is intended to solve the query
optimization problem of joining many tables by means of a Genetic
Algorithm (GA). It allows the handling of large join queries through
non-exhaustive search.
For further information see README.GEQO <utesch@aut.tu-freiberg.de>.
3.20) How do you remove a column from a table?
We do not support ALTER TABLE DROP COLUMN, but do this:
SELECT ... -- select all columns but the one you want to remove
INTO TABLE new_table
FROM old_table;
DROP TABLE old_table;
ALTER TABLE new_table RENAME TO old_table;
3.21) How do SELECT only the first few rows of a query?
See the fetch manual page.
This only prevents all row results from being transfered to the
client. The entire query must be evaluated, even if you only want just
the first few rows. Consider a query that has and an ORDER BY. There
is no way to return any rows until the entire query is evaluated and
sorted.
3.22)How much database disk space is required to store data from a typical
flat file?
Consider a file with 300,000 lines with two integers on each line. The
flat file is 2.4MB. The size of the PostgreSQL database file
containing this data can be estimated:
40 bytes + each row header (approximate)
8 bytes + two int fields @ 4 bytes each
4 bytes + pointer on page to tuple
-------- =
52 bytes per row
The data page size in PostgreSQL is 8192(8k) bytes, so:
8192 bytes per page
------------------- = 157 rows per database page (rounded up)
52 bytes per row
300000 data rows
----------------- = 1911 database pages
157 rows per page
1911 database pages * 8192 bytes per page = 15,654,912 or 15.5MB
Indexes do not contain as much overhead, but do contain the data that is
being indexed, so they can be large also.
3.23) How do I get a list of tables, or other things I can see in psql?
See the file pgsql/src/bin/psql/psql.c. It contains SQL commands that
generate the output for psql's backslash commands.
3.24) Why do I get the error "FATAL: palloc failure: memory exhausted?"
It is possible you have run out of virtual memory on your system, or
your kernel has a low limit for certain resources. Try this before
starting the postmaster:
ulimit -d 65536
limit datasize 64m
Depending on your shell, only one of these may succeed, but it will
set your process data segment limit much higher and perhaps allow the
query to complete. This command applies the current process, and all
subprocesses created after the command is run. If are having a problem
with the SQL client because the backend is returning too much data,
try it before starting the client.
3.25) How do I tell what PostgreSQL version I am running?
From psql, type select version();
_________________________________________________________________
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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