
                Frequently Asked Questions (FAQ) for PostgreSQL
                                       
   Last updated: Mon Sep 29 22:45:58 EDT 1997
   Version: 6.2
   
   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.
   
   Changes in this version (* = modified, + = new):
     _________________________________________________________________
   
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 version of SQL does PostgreSQL use?
   1.9) What documentation is available for PostgreSQL?
   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?
   
  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?
   
  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 6.1.1, which was released
   on July 22, 1997. For information about what is new in 6.1.1, see our
   TODO list on our WWW page.
   
   We expect a 6.2 release at the end of September.
   
  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 6.1 requires a dump and restore from previous releases.
   
   Upgrading from 6.1 to 6.1.1 requires running configure, compile of the
   new release, recompile of all your custom applications to use the new
   libpq, and then an install while the postmaster is temporarily
   stopped.
   
   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 6.1.1.
   
  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?
  
   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
       
  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.
     _________________________________________________________________
   
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 postmaster -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

   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.
   
  3.13) 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.
   
  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 7.0.
   
  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.
   
   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;

  3.23) 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.
     _________________________________________________________________
   
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. 
