postgresql/doc/FAQ

Frequently Asked Questions (FAQ) for PostgreSQL

Last updated: Sun Feb 1 16:13:28 EST 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 postgres?
1.11) Are there ODBC drivers for PostgreSQL?
1.12) What tools are available for hooking postgres to Web pages?
1.13) Does PostgreSQL have a graphical user interface? A report generator? A
embedded query language interface?
1.14) What is a good book to learn SQL?

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
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 32 concurrent backends?

3) Operational questions

3.1) Does PostgreSQL support nested subqueries?
3.2) I've having a lot of problems using rules.
3.3) I can't seem to write into the middle of large objects reliably.
3.4) How can I write client applications to PostgreSQL?
3.5) How do I set up a pg_group?
3.6) What is the exact difference between binary cursors and normal cursors?
3.7) What is a R-tree index and what is it used for?
3.8) What is the maximum size for a tuple?
3.9) I defined indices but my queries don't seem to make use of them. Why?
3.10) How do I do regular expression searches? case-insensitive regexp
searching?
3.11) I experienced a server crash during a vacuum. How do I remove the lock
file?
3.12) What is the difference between the various character types?
3.13) In a query, how do I detect if a field is NULL?
3.14) How do I see how the query optimizer is evaluating my query?
3.15) How do I create a serial field?
3.16) What are the pg_psort.XXX files in my database directory?
3.17) Why can't I connect to my database from another machine?
3.18) How do I find out what indexes or operations are defined in the
database?
3.19) What is the time-warp feature and how does it relate to vacuum?
3.20) What is an oid? What is a tid?
3.21) What is the meaning of some of the terms used in Postgres?
3.22) What is Genetic Query Optimization?
3.23) How do you remove a column from a table?
3.24) How do SELECT only the first few rows of a query?
3.25) Why can't I create a column named "time"?
3.26) How much database disk space is required to store data from a typical
flat file?

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: 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 6.2.1, which was released on
October 17th, 1997. We are testing 6.3 beta. For information about what is
new in 6.3, see our TODO list on our WWW page.

We plan to have major releases every three 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 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.

Upgrading to 6.3 from earlier releases requires a dump and restore.

Upgrading to 6.2.1 from pre-6.2 requires a dump and restore.

Upgrading to 6.2.1 from 6.2 does not require a dump, but see the appropriate
/migration file in the distribution.

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
6.2.1 or 6.3 beta.

1.11) 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 very popular. 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.

1.12) What tools are available for hooking postgres 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://php.iquest.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?

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.

1.14) What is a good book to learn SQL?

Many of our users like The Practical SQL Handbook, Bowman et al, Addison
Wesley.

  ------------------------------------------------------------------------

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) 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 backend?

Use 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?

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.

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.

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 32 concurrent backends?

Edit include/storage/sinvaladt.h, and change the value of MaxBackendId. In
the future, we plan to make this a configurable prameter.

  ------------------------------------------------------------------------

Section 3: PostgreSQL Features

3.1) Does PostgreSQL support nested subqueries?

Subqueries are not implemented, but they can be simulated using sql
functions.

3.2) 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.

3.3) 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.

3.4) 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.

3.5) 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[].

3.6) What is the exact difference between binary cursors and normal cursors?

See the declare manual page for a description.

3.7) 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"

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.

3.8) 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.

3.9) 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.

Postgres does not warn the user when the improper index is created.

Indexes not used for ORDER BY operations.

3.10) How do I do regular expression searches? case-insensitive regexp
searching?

See the pgbuiltin manual page. Search for regular expression.

3.11) I experienced a server crash during a vacuum. How do I remove the lock
file?

See the vacuum manual page.

3.12) 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

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.

3.13) In a query, how do I detect if a field is NULL?

You test the column with IS NULL and IS NOT NULL.

3.14) How do I see how the query optimizer is evaluating my query?

See the explain manual page.

3.15) 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 use pgdump's -o option or
COPY's WITH OIDS option to preserver the oids.

We also have a SEQUENCE function that is 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.

3.16) 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.

3.17) 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. See
the hba_conf manual page.

3.18) 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.

3.19) 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 6.3.

3.20) 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. You can create an
index on the oid field for faster access.

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.

3.21) 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.

3.22) 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>.

3.23) 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;

3.24) 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 first few rows.
Consider a query that has and ORDER BY. There is no way to return any rows
until the entire query is evaluated and sorted.

3.25) Why can't I create a column named "time"?

6.2.1 has added some new restricted keywords as we make PostgreSQL more
ANSI-92 compilant. The next release will have this restriction removed.
There is a patch on ftp.postgresql.org that will allow this feature now.

3.26)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.

  ------------------------------------------------------------------------

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.