FAQ

                Frequently Asked Questions (FAQ) for PostgreSQL
                                       
   Last updated: Mon Jul 29 19:56:45 EDT 2002
   
   Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
   
   The most recent version of this document can be viewed at
   http://www.PostgreSQL.org/docs/faq-english.html.
   
   Platform-specific questions are answered at
   http://www.PostgreSQL.org/users-lounge/docs/faq.html.
     _________________________________________________________________
   
                             General Questions
                                      
   1.1) What is PostgreSQL? How is it pronounced?
   1.2) What is the copyright on PostgreSQL?
   1.3) What Unix platforms does PostgreSQL run on?
   1.4) What non-Unix ports are available?
   1.5) Where can I get PostgreSQL?
   1.6) Where can I get support?
   1.7) What is the latest release?
   1.8) What documentation is available?
   1.9) How do I find out about known bugs or missing features?
   1.10) How can I learn SQL?
   1.11) Is PostgreSQL Y2K compliant?
   1.12) How do I join the development team?
   1.13) How do I submit a bug report?
   1.14) How does PostgreSQL compare to other DBMSs?
   1.15) How can I financially assist PostgreSQL?
   
                           User Client Questions
                                      
   2.1) Are there ODBC drivers for PostgreSQL?
   2.2) What tools are available for using PostgreSQL with Web pages?
   2.3) Does PostgreSQL have a graphical user interface? A report
   generator? An embedded query language interface?
   2.4) What languages are available to communicate with PostgreSQL?
   
                          Administrative Questions
                                      
   3.1) How do I install PostgreSQL somewhere other than
   /usr/local/pgsql?
   3.2) When I start postmaster, I get a Bad System Call or core dumped
   message. Why?
   3.3) When I try to start postmaster, I get IpcMemoryCreate errors.
   Why?
   3.4) When I try to start postmaster, I get IpcSemaphoreCreate errors.
   Why?
   3.5) How do I control connections from other hosts?
   3.6) How do I tune the database engine for better performance?
   3.7) What debugging features are available?
   3.8) Why do I get "Sorry, too many clients" when trying to connect?
   3.9) What are the pg_sorttempNNN.NN files in my database directory?
   3.10) Why do I need to do a dump and restore to upgrade PostgreSQL?
   
                           Operational Questions
                                      
   4.1) What is the difference between binary cursors and normal cursors?
   4.2) How do I SELECT only the first few rows of a query?
   4.3) How do I get a list of tables or other things I can see in psql?
   4.4) How do you remove a column from a table?
   4.5) What is the maximum size for a row, a table, and a database?
   4.6) How much database disk space is required to store data from a
   typical text file?
   4.7) How do I find out what tables, indexes, databases, and users are
   defined?
   4.8) My queries are slow or don't make use of the indexes. Why?
   4.9) How do I see how the query optimizer is evaluating my query?
   4.10) What is an R-tree index?
   4.11) What is the Genetic Query Optimizer?
   4.12) How do I perform regular expression searches and
   case-insensitive regular expression searches? How do I use an index
   for case-insensitive searches?
   4.13) In a query, how do I detect if a field is NULL?
   4.14) What is the difference between the various character types?
   4.15.1) How do I create a serial/auto-incrementing field?
   4.15.2) How do I get the value of a SERIAL insert?
   4.15.3) Don't currval() and nextval() lead to a race condition with
   other users?
   4.15.4) Why aren't my sequence numbers reused on transaction abort?
   Why are there gaps in the numbering of my sequence/SERIAL column?
   4.16) What is an OID? What is a TID?
   4.17) What is the meaning of some of the terms used in PostgreSQL?
   4.18) Why do I get the error "ERROR: Memory exhausted in
   AllocSetAlloc()"?
   4.19) How do I tell what PostgreSQL version I am running?
   4.20) Why does my large-object operations get "invalid large obj
   descriptor"?
   4.21) How do I create a column that will default to the current time?
   4.22) Why are my subqueries using IN so slow?
   4.23) How do I perform an outer join?
   4.24) How do I perform queries using multiple databases?
   4.25) How do I return multiple rows or columns from a function?
   4.26) Why can't I reliably create/drop temporary tables in PL/PgSQL
   functions?
   
                            Extending PostgreSQL
                                      
   5.1) I wrote a user-defined function. When I run it in psql, why does
   it dump core?
   5.2) How can I contribute some nifty new types and functions to
   PostgreSQL?
   5.3) How do I write a C function to return a tuple?
   5.4) I have changed a source file. Why does the recompile not see the
   change?
     _________________________________________________________________
   
                             General Questions
                                      
    1.1) What is PostgreSQL?
    
   PostgreSQL is pronounced Post-Gres-Q-L.
   
   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 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
   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 is the copyright on PostgreSQL?
    
   PostgreSQL is subject to the following COPYRIGHT:
   
   PostgreSQL Data Base Management System
   
   Portions copyright (c) 1996-2002, PostgreSQL Global Development Group
   Portions 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.
   
   The above is the BSD license, the classic open-source license. It has
   no restrictions on how the source code may be used. We like it and
   have no intention of changing it.
   
    1.3) What Unix platforms does PostgreSQL run on?
    
   In general, a modern Unix-compatible platform should be able to run
   PostgreSQL. The platforms that had received explicit testing at the
   time of release are listed in the installation instructions.
   
    1.4) What non-Unix ports are available?
    
   Client
   
   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
   win31.mak is included in the distribution for making a Win32 libpq
   library and psql. PostgreSQL also communicates with ODBC clients.
   
   Server
   
   The database server can run on Windows NT and Win2k using Cygwin, the
   Cygnus Unix/NT porting library. See pgsql/doc/FAQ_MSWIN in the
   distribution or the MS Windows FAQ on our web site. We have no plan to
   do a native port to any Microsoft platform.
   
    1.5) Where can I get PostgreSQL?
    
   The primary anonymous ftp site for PostgreSQL is
   ftp://ftp.PostgreSQL.org/pub. For mirror sites, see our main web site.
   
    1.6) Where can I get support?
    
   The main mailing list is: pgsql-general@PostgreSQL.org. It is
   available for discussion of matters pertaining to PostgreSQL. To
   subscribe, send mail with the following 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 is available. To subscribe to this list, send
   email to pgsql-bugs-request@PostgreSQL.org with a body of:
    subscribe
    end

   There is also a developers discussion mailing list available. To
   subscribe to this list, send email to
   pgsql-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://www.PostgreSQL.org
     
   There is also an IRC channel on EFNet, channel #PostgreSQL. I use the
   Unix command irc -c '#PostgreSQL' "$USER" irc.phoenix.net.
   
   A list of commercial support companies is available at
   http://www.postgresql.org/users-lounge/commercial-support.html.
   
    1.7) What is the latest release?
    
   The latest release of PostgreSQL is version 7.2.1.
   
   We plan to have major releases every four months.
   
    1.8) What documentation is available?
    
   Several manuals, manual pages, and some small test examples are
   included in the distribution. See the /doc directory. You can also
   browse the manual online at
   http://www.PostgreSQL.org/users-lounge/docs/.
   
   There are two PostgreSQL books available online at
   http://www.PostgreSQL.org/docs/awbook.html and
   http://www.commandprompt.com/ppbook/. There is a list of PostgreSQL
   books available for purchase at http://www.postgresql.org/books/.
   There is also a collection of PostgreSQL technical articles at
   http://techdocs.postgresql.org/.
   
   psql has some nice \d commands to show information about types,
   operators, functions, aggregates, etc.
   
   Our web site contains even more documentation.
   
    1.9) How do I find out about known bugs or missing features?
    
   PostgreSQL supports an extended subset of SQL-92. See our TODO list
   for known bugs, missing features, and future plans.
   
    1.10) How can I learn SQL?
    
   The PostgreSQL book at http://www.PostgreSQL.org/docs/awbook.html
   teaches SQL. There is another PostgreSQL book at
   http://www.commandprompt.com/ppbook. There is a nice tutorial at
   http://www.intermedia.net/support/sql/sqltut.shtm, at
   http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,
   and at http://sqlcourse.com.
   
   Another one is "Teach Yourself SQL in 21 Days, Second Edition" at
   http://members.tripod.com/er4ebus/sql/index.htm
   
   Many of our users like The Practical SQL Handbook, Bowman, Judith S.,
   et al., Addison-Wesley. Others like The Complete Reference SQL, Groff
   et al., McGraw-Hill.
   
    1.11) Is PostgreSQL Y2K compliant?
    
   Yes, we easily handle dates past the year 2000 AD, and before 2000 BC.
   
    1.12) How do I join the development team?
    
   First, download the latest source and read the PostgreSQL Developers
   documentation on our web site, or in the distribution. Second,
   subscribe to the pgsql-hackers and pgsql-patches mailing lists. Third,
   submit high quality patches to pgsql-patches.
   
   There are about a dozen people who have commit privileges to the
   PostgreSQL CVS archive. They each have submitted so many high-quality
   patches that it was impossible for the existing committers to keep up,
   and we had confidence that patches they committed were of high
   quality.
   
    1.13) How do I submit a bug report?
    
   Please visit the PostgreSQL BugTool page, which gives guidelines and
   directions on how to submit a bug.
   
   Also check out our ftp site ftp://ftp.PostgreSQL.org/pub to see if
   there is a more recent PostgreSQL version or patches.
   
    1.14) How does PostgreSQL compare to other DBMSs?
    
   There are several ways of measuring software: features, performance,
   reliability, support, and price.
   
   Features
          PostgreSQL has most features present in large commercial DBMSs,
          like transactions, subselects, triggers, views, foreign key
          referential integrity, and sophisticated locking. We have some
          features they do not have, like user-defined types,
          inheritance, rules, and multi-version concurrency control to
          reduce lock contention.
          
   Performance
          PostgreSQL has performance similar to other commercial and open
          source databases. it is faster for some things, slower for
          others. In comparison to MySQL or leaner database systems, we
          are slower on inserts/updates because of transaction overhead.
          Of course, MySQL does not have any of the features mentioned in
          the Features section above. We are built for reliability and
          features, though we continue to improve performance in every
          release. There is an interesting Web page comparing PostgreSQL
          to MySQL at http://openacs.org/why-not-mysql.html
          
   Reliability
          We realize that a DBMS must be reliable, or it is worthless. We
          strive to release well-tested, stable code that has a minimum
          of bugs. Each release has at least one month of beta testing,
          and our release history shows that we can provide stable, solid
          releases that are ready for production use. We believe we
          compare favorably to other database software in this area.
          
   Support
          Our mailing list provides a large group of developers and users
          to help resolve any problems encountered. While we can not
          guarantee a fix, commercial DBMSs do not always supply a fix
          either. Direct access to developers, the user community,
          manuals, and the source code often make PostgreSQL support
          superior to other DBMSs. There is commercial per-incident
          support available for those who need it. (See support FAQ
          item.)
          
   Price
          We are free for all use, both commercial and non-commercial.
          You can add our code to your product with no limitations,
          except those outlined in our BSD-style license stated above.
          
    1.15) How can I financially assist PostgreSQL?
    
   PostgreSQL has had a first-class infrastructure since we started six
   years ago. This is all thanks to Marc Fournier, who has created and
   managed this infrastructure over the years.
   
   Quality infrastructure is very important to an open-source project. It
   prevents disruptions that can greatly delay forward movement of the
   project.
   
   Of course, this infrastructure is not cheap. There are a variety of
   monthly and one-time expenses that are required to keep it going. If
   you or your company has money it can donate to help fund this effort,
   please go to http://www.pgsql.com/pg_goodies and make a donation.
   
   Although the web page mentions PostgreSQL, Inc, the "contributions"
   item is solely to support the PostgreSQL project and does not fund any
   specific company. If you prefer, you can also send a check to the
   contact address.
     _________________________________________________________________
   
                           User Client Questions
                                      
    2.1) Are there ODBC drivers for PostgreSQL?
    
   There are two ODBC drivers available, PsqlODBC and OpenLink ODBC.
   
   PsqlODBC is included in the distribution. More information about it
   can be gotten from ftp://ftp.PostgreSQL.org/pub/odbc/.
   
   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. Please send questions to postgres95@openlink.co.uk.
   
   See also the ODBC chapter of the Programmer's Guide.
   
    2.2) What tools are available for using PostgreSQL with Web pages?
    
   A nice introduction to Database-backed Web pages can be seen at:
   http://www.webreview.com
   
   There is also one at http://www.phone.net/home/mwm/hotlist/.
   
   For Web integration, PHP is an excellent interface. It is at
   http://www.php.net.
   
   For complex cases, many use the Perl interface and CGI.pm.
   
    2.3) Does PostgreSQL have a graphical user interface? A report generator?
    An embedded query language interface?
    
   We have a nice graphical user interface called pgaccess, which is
   shipped as part of the distribution. pgaccess also has a report
   generator. The Web page is http://www.flex.ro/pgaccess
   
   We also include ecpg, which is an embedded SQL query language
   interface for C.
   
    2.4) What languages are available to communicate with PostgreSQL?
    
   We have:
     * C (libpq)
     * C++ (libpq++)
     * Embedded C (ecpg)
     * Java (jdbc)
     * Perl (perl5)
     * ODBC (odbc)
     * Python (PyGreSQL)
     * TCL (libpgtcl)
     * C Easy API (libpgeasy)
     * Embedded HTML (PHP from http://www.php.net)
     _________________________________________________________________
   
                          Administrative Questions
                                      
    3.1) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
    
   Specify the --prefix option when running configure.
   
    3.2) When I start postmaster, I get a Bad System Call or core dumped
    message. Why?
    
   It could be a variety of problems, but first check to see that you
   have System V extensions installed in your kernel. PostgreSQL requires
   kernel support for shared memory and semaphores.
   
    3.3) When I try to start postmaster, I get IpcMemoryCreate errors. Why?
    
   You either do not have shared memory configured properly in your
   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 and backend processes you configure for postmaster. For
   most systems, with default numbers of buffers and processes, you need
   a minimum of ~1 MB. See the PostgreSQL Administrator's Guide for more
   detailed information about shared memory and semaphores.
   
    3.4) When I try to start postmaster, I get IpcSemaphoreCreate errors. Why?
    
   If the error message is IpcSemaphoreCreate: semget failed (No space
   left on device) then your kernel is not configured with enough
   semaphores. Postgres needs one semaphore per potential backend
   process. A temporary solution is to start postmaster with a smaller
   limit on the number of backend processes. Use -N with a parameter less
   than the default of 32. A more permanent solution is to increase your
   kernel's SEMMNS and SEMMNI parameters.
   
   Inoperative semaphores can also cause crashes during heavy database
   access.
   
   If the error message is something else, you might not have semaphore
   support configured in your kernel at all. See the PostgreSQL
   Administrator's Guide for more detailed information about shared
   memory and semaphores.
   
    3.5) How do I control connections from other hosts?
    
   By default, PostgreSQL only allows connections from the local machine
   using Unix domain sockets. Other machines will not be able to connect
   unless you add the -i flag to postmaster, and enable host-based
   authentication by modifying the file $PGDATA/pg_hba.conf accordingly.
   This will allow TCP/IP connections.
   
    3.6) How do I tune the database engine for better performance?
    
   Certainly, indexes can speed up queries. The EXPLAIN command allows
   you to see how PostgreSQL is interpreting your query, and which
   indexes are being used.
   
   If you are doing many INSERTs, consider doing them in a large batch
   using the COPY command. This is much faster than individual INSERTS.
   Second, statements not in a BEGIN WORK/COMMIT transaction block are
   considered to be 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 options. You can disable fsync() by starting
   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 used by the backend processes. If you make this
   parameter too high, the postmaster may not start because you have
   exceeded your kernel's limit on shared memory space. Each buffer is 8K
   and the default is 64 buffers.
   
   You can also use the backend -S option to increase the maximum amount
   of memory used by the backend process for temporary sorts. The -S
   value is measured in kilobytes, and the default is 512 (i.e. 512K).
   
   You can also use the CLUSTER command to group data in tables to match
   an index. See the CLUSTER manual page for more details.
   
    3.7) What debugging features are available?
    
   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 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 contains 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
   that specifies the debug level. Be warned that high debug level values
   generate large log files.
   
   If postmaster is not running, you can actually 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 postmaster, it is not running
   in an identical environment and locking/backend interaction problems
   may not be duplicated.
   
   If postmaster is running, start psql in one window, then find the PID
   of the postgres process used by psql. Use a debugger to attach to the
   postgres PID. You can set breakpoints in the debugger and issue
   queries from psql. If you are debugging postgres startup, you can set
   PGOPTIONS="-W n", then start psql. This will cause startup to delay
   for n seconds so you can attach to the process with the debugger, set
   any breakpoints, and continue through the startup sequence.
   
   The postgres program has -s, -A, and -t options that can be very
   useful 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 client's current directory. Linux requires a compile with
   -DLINUX_PROFILE for proper profiling.
   
    3.8) Why do I get "Sorry, too many clients" when trying to connect?
    
   You need to increase postmaster's limit on how many concurrent backend
   processes it can start.
   
   The default limit is 32 processes. You can increase it by restarting
   postmaster with a suitable -N value or modifying postgresql.conf.
   
   Note that if you make -N larger than 32, you must also increase -B
   beyond its default of 64; -B must be at least twice -N, and probably
   should be more than that for best performance. For large numbers of
   backend processes, you are also likely to find that you need to
   increase various Unix kernel configuration parameters. Things to check
   include the maximum size of shared memory blocks, SHMMAX; the maximum
   number of semaphores, SEMMNS and SEMMNI; the maximum number of
   processes, NPROC; the maximum number of processes per user, MAXUPRC;
   and the maximum number of open files, NFILE and NINODE. The reason
   that PostgreSQL has a limit on the number of allowed backend processes
   is so your system won't run out of resources.
   
   In PostgreSQL versions prior to 6.5, the maximum number of backends
   was 64, and changing it required a rebuild after altering the
   MaxBackendId constant in include/storage/sinvaladt.h.
   
    3.9) What are the pg_sorttempNNN.NN files in my database directory?
    
   They are temporary files generated by the query executor. For example,
   if a sort needs to be done to satisfy an ORDER BY, and the sort
   requires more space than the backend's -S parameter allows, then
   temporary files are created to hold the extra data.
   
   The temporary files should be deleted automatically, but might not if
   a backend crashes during a sort. If you have no backends running at
   the time, it is safe to delete the pg_tempNNN.NN files.
   
    3.10) Why do I need to do a dump and restore to upgrade PostgreSQL?
    
   The PostgreSQL team tries very heard to maintain compatability across
   minor releases. So upgrading from 7.2 to 7.2.1 does not require a dump
   a restore. However, new features are continuously being adding and
   sometimes this requires new fields to be added to system tables.
   
   These changes may be across many tables and so maintaining backward
   compatability would be quite difficult. Thus, restoring from a dump is
   required to make everything work.
   
   Note that the actual on-disk file format does not change very often, a
   feature the pg_upgrade script uses quite successfully. There the dump
   is used create the necessary information in the system tables. The
   data files are then just copied across. This method is not as
   guarenteed as the dump/restore method but when it works it can make
   upgrades very efficient.
     _________________________________________________________________
   
                           Operational Questions
                                      
    4.1) What is the difference between binary cursors and normal cursors?
    
   See the DECLARE manual page for a description.
   
    4.2) How do I SELECT only the first few rows of a query?
    
   See the FETCH manual page, or use SELECT ... LIMIT....
   
   The entire query may have to be evaluated, even if you only want the
   first few rows. Consider a query that has an ORDER BY. If there is an
   index that matches the ORDER BY, PostgreSQL may be able to evaluate
   only the first few records requested, or the entire query may have to
   be evaluated until the desired rows have been generated.
   
    4.3) How do I get a list of tables or other things I can see in psql?
    
   You can read the source code for psql in file
   pgsql/src/bin/psql/describe.c. It contains SQL commands that generate
   the output for psql's backslash commands. You can also start psql with
   the -E option so it will print out the queries it uses to execute the
   commands you give.
   
    4.4) How do you remove a column from a table?
    
   We do not support ALTER TABLE DROP COLUMN, but do this:
    BEGIN;
    LOCK TABLE old_table;
    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;
    COMMIT;

    4.5) What is the maximum size for a row, a table, and a database?
    
   These are the limits:
    Maximum size for a database?             unlimited (500 GB databases exist)
    Maximum size for a table?                16 TB
    Maximum size for a row?                  1.6TB
    Maximum size for a field?                1 GB
    Maximum number of rows in a table?       unlimited
    Maximum number of columns in a table?    250-1600 depending on column types
    Maximum number of indexes on a table?    unlimited

   Of course, these are not actually unlimited, but limited to available
   disk space and memory/swap space. Performance may suffer when these
   values get unusually large.
   
   The maximum table size of 16 TB does not require large file support
   from the operating system. Large tables are stored as multiple 1 GB
   files so file system size limits are not important.
   
   The maximum table size and maximum number of columns can be increased
   if the default block size is increased to 32k.
   
    4.6) How much database disk space is required to store data from a typical
    text file?
    
   A PostgreSQL database may require up to five times the disk space to
   store data from a text file.
   
   As an example, consider a file of 100,000 lines with an integer and
   text description on each line. Suppose the text string avergages
   twenty bytes in length. The flat file would be 2.8 MB. The size of the
   PostgreSQL database file containing this data can be estimated as 6.4
   MB:
    36 bytes: each row header (approximate)
    24 bytes: one int field and one text filed
   + 4 bytes: pointer on page to tuple
   ----------------------------------------
    64 bytes per row

   The data page size in PostgreSQL is 8192 bytes (8 KB), so:

   8192 bytes per page
   -------------------   =  128 rows per database page (rounded down)
     64 bytes per row

   100000 data rows
   --------------------  =  782 database pages (rounded up)
      128 rows per page

782 database pages * 8192 bytes per page  =  6,406,144 bytes (6.4 MB)

   Indexes do not require as much overhead, but do contain the data that
   is being indexed, so they can be large also.
   
   NULLs are stored in bitmaps, so they use very little space.
   
    4.7) How do I find out what tables, indexes, databases, and users are
    defined?
    
   psql has a variety of backslash commands to show such information. Use
   \? to see them. There are also system tables beginning with pg_ that
   describe these too. Also, psql -l will list all databases.
   
   Also try the file pgsql/src/tutorial/syscat.source. It illustrates
   many of the SELECTs needed to get information from the database system
   tables.
   
    4.8) My queries are slow or don't make use of the indexes. Why?
    
   Indexes are not automatically used by every query. Indexes are only
   used if the table is larger than a minimum size, and the query selects
   only a small percentage of the rows in the table. This is because the
   random disk access caused by an index scan can be slower than a
   straight read through the table, or sequential scan.
   
   To determine if an index should be used, PostgreSQL must have
   statistics about the table. These statistics are collected using
   VACUUM ANALYZE, or simply ANALYZE. Using statistics, the optimizer
   knows how many rows are in the table, and can better determine if
   indexes should be used. Statistics are also valuable in determining
   optimal join order and join methods. Statistics collection should be
   performed periodically as the contents of the table change.
   
   Indexes are normally not used for ORDER BY or to perform joins. A
   sequential scan followed by an explicit sort is usually faster than an
   index scan of a large table.
   However, LIMIT combined with ORDER BY often will use an index because
   only a small portion of the table is returned. In fact, though MAX()
   and MIN() don't use indexes, it is possible to retrieve such values
   using an index with ORDER BY and LIMIT:
    SELECT col
    FROM tab
    ORDER BY col [ DESC ]
    LIMIT 1

   When using wild-card operators such as LIKE or ~, indexes can only be
   used in certain circumstances:
     * The beginning of the search string must be anchored to the start
       of the string, i.e.:
       
     * LIKE patterns must not start with %.
     * ~ (regular expression) patterns must start with ^.
       
     The search string can not start with a character class, e.g. [a-e].
   
     Case-insensitive searches like ILIKE and ~* can not be used.
   Instead, use functional indexes, which are described later in this
   FAQ.
   
     The default C local must be used during initdb.
   
    4.9) How do I see how the query optimizer is evaluating my query?
    
   See the EXPLAIN manual page.
   
    4.10) What is an R-tree index?
    
   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-trees 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 efficiently answer queries such as "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." Proceedings 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".
   
   Built-in 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 requires a bit of work and we don't currently have
   any documentation on how to do it.
   
    4.11) What is the Genetic Query Optimizer?
    
   The GEQO module speeds query optimization when joining many tables by
   means of a Genetic Algorithm (GA). It allows the handling of large
   join queries through nonexhaustive search.
   
    4.12) How do I perform regular expression searches and case-insensitive
    regular expression searches? How do I use an index for case-insensitive
    searches?
    
   The ~ operator does regular expression matching, and ~* does
   case-insensitive regular expression matching. The case-insensitive
   variant of LIKE is called ILIKE.
   
   Case-insensitive equality comparisons are normally expressed as:
    SELECT *
    FROM tab
    WHERE lower(col) = 'abc'

   This will not use an standard index. However, if you create a
   functional index, it will be used:
    CREATE INDEX tabindex on tab (lower(col));

    4.13) In a query, how do I detect if a field is NULL?
    
   You test the column with IS NULL and IS NOT NULL.
   
    4.14) 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            no specific upper limit on length
BYTEA           bytea           variable-length byte array (null-byte safe)

   You will see the internal name when examining system catalogs and in
   some error messages.
   
   The last four types above are "varlena" types (i.e., the first four
   bytes on disk are the length, followed by the data). Thus the actual
   space used is slightly greater than the declared size. However, these
   data types are also subject to compression or being stored out-of-line
   by TOAST, so the space on disk might also be less than expected.
   
   CHAR() is best when storing strings that are usually the same length.
   VARCHAR() is best when storing variable-length strings but it limits
   how long a string can be. TEXT is for strings of unlimited length,
   maximum 1 gigabyte. BYTEA is for storing binary data, particularly
   values that include NULL bytes.
   
    4.15.1) How do I create a serial/auto-incrementing field?
    
   PostgreSQL supports a SERIAL data type. It auto-creates a sequence and
   index on the column. For example, this:
    CREATE TABLE person (
        id   SERIAL,
        name TEXT
    );

   is automatically translated into this:
    CREATE SEQUENCE person_id_seq;
    CREATE TABLE person (
        id   INT4 NOT NULL DEFAULT nextval('person_id_seq'),
        name TEXT
    );
    CREATE UNIQUE INDEX person_id_key ON person ( id );

   See the create_sequence manual page for more information about
   sequences. You can also use each row's OID field as a unique value.
   However, if you need to dump and reload the database, you need to use
   pg_dump's -o option or COPY WITH OIDS option to preserve the OIDs.
   
    4.15.2) How do I get the value of a SERIAL insert?
    
   One approach is to retrieve the next SERIAL value from the sequence
   object with the nextval() function before inserting and then insert it
   explicitly. Using the example table in 4.15.1, that might look like
   this in Perl:
    new_id = output of "SELECT nextval('person_id_seq')"
    INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal');

   You would then also have the new value stored in new_id for use in
   other queries (e.g., as a foreign key to the person table). Note that
   the name of the automatically created SEQUENCE object will be named
   <table>_<serialcolumn>_seq, where table and serialcolumn are the names
   of your table and your SERIAL column, respectively.
   
   Alternatively, you could retrieve the assigned SERIAL value with the
   currval() function after it was inserted by default, e.g.,
    INSERT INTO person (name) VALUES ('Blaise Pascal');
    new_id = output of "SELECT currval('person_id_seq')";

   Finally, you could use the OID returned from the INSERT statement to
   look up the default value, though this is probably the least portable
   approach. In Perl, using DBI with Edmund Mergl's DBD::Pg module, the
   oid value is made available via $sth->{pg_oid_status} after
   $sth->execute().
   
    4.15.3) Don't currval() and nextval() lead to a race condition with other
    users?
    
   No. Currval() returns the current value assigned by your backend, not
   by all users.
   
    4.15.4) Why aren't my sequence numbers reused on transaction abort? Why are
    there gaps in the numbering of my sequence/SERIAL column?
    
   To improve concurrency, sequence values are given out to running
   transactions as needed and are not locked until the transaction
   completes. This causes gaps in numbering from aborted transactions.
   
    4.16) What is an OID? What is a TID?
    
   OIDs are PostgreSQL's answer to unique row ids. Every row that is
   created in PostgreSQL gets a unique OID. All OIDs generated during
   initdb are less than 16384 (from backend/access/transam.h). All
   user-created OIDs are equal to or greater than 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
   between 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. 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 OIDs, there is
   no reason you can't do it:
        CREATE TABLE new_table(old_oid oid, mycol int);
        SELECT old_oid, mycol INTO new FROM old;
        COPY new TO '/tmp/pgtable';
        DELETE FROM new;
        COPY new WITH OIDS FROM '/tmp/pgtable';

   OIDs are stored as 4-byte integers, and will overflow at 4 billion. No
   one has reported this ever happening, and we plan to have the limit
   removed before anyone does.
   
   TIDs are used to identify 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.
   
    4.17) 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:
     * table, relation, class
     * row, record, tuple
     * column, field, attribute
     * retrieve, select
     * replace, update
     * append, insert
     * OID, serial value
     * portal, cursor
     * range variable, table name, table alias
       
   A list of general database terms can be found at:
   http://www.comptechnews.com/~reaster/dbdesign.html
   
    4.18) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
    
   You probably have run out of virtual memory on your system, or your
   kernel has a low limit for certain resources. Try this before starting
   postmaster:
    ulimit -d 262144
    limit datasize 256m

   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 to the current process, and
   all subprocesses created after the command is run. If you are having a
   problem with the SQL client because the backend is returning too much
   data, try it before starting the client.
   
    4.19) How do I tell what PostgreSQL version I am running?
    
   From psql, type select version();
   
    4.20) Why does my large-object operations get "invalid large obj
    descriptor"?
    
   You need to put BEGIN WORK and COMMIT around any use of a large object
   handle, that is, surrounding lo_open ... lo_close.
   
   Currently PostgreSQL enforces the rule by closing large object handles
   at transaction commit. So the first attempt to do anything with the
   handle will draw invalid large obj descriptor. So code that used to
   work (at least most of the time) will now generate that error message
   if you fail to use a transaction.
   
   If you are using a client interface like ODBC you may need to set
   auto-commit off.
   
    4.21) How do I create a column that will default to the current time?
    
   Use CURRENT_TIMESTAMP:
CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );

    4.22) Why are my subqueries using IN so slow?
    
   Currently, we join subqueries to outer queries by sequentially
   scanning the result of the subquery for each row of the outer query. A
   workaround is to replace IN with EXISTS:
SELECT *
    FROM tab
    WHERE col1 IN (SELECT col2 FROM TAB2)

   to:
SELECT *
    FROM tab
    WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 = col2)

   We hope to fix this limitation in a future release.
   
    4.23) How do I perform an outer join?
    
   PostgreSQL supports outer joins using the SQL standard syntax. Here
   are two examples:
    SELECT *
    FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);

   or
    SELECT *
    FROM t1 LEFT OUTER JOIN t2 USING (col);

   These identical queries join t1.col to t2.col, and also return any
   unjoined rows in t1 (those with no match in t2). A RIGHT join would
   add unjoined rows of t2. A FULL join would return the matched rows
   plus all unjoined rows from t1 and t2. The word OUTER is optional and
   is assumed in LEFT, RIGHT, and FULL joins. Ordinary joins are called
   INNER joins.
   
   In previous releases, outer joins can be simulated using UNION and NOT
   IN. For example, when joining tab1 and tab2, the following query does
   an outer join of the two tables:
    SELECT tab1.col1, tab2.col2
    FROM tab1, tab2
    WHERE tab1.col1 = tab2.col1
    UNION ALL
    SELECT tab1.col1, NULL
    FROM tab1
    WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
    ORDER BY col1

    4.24) How do I perform queries using multiple databases?
    
   There is no way to query any database except the current one. Because
   PostgreSQL loads database-specific system catalogs, it is uncertain
   how a cross-database query should even behave.
   
   Of course, a client can make simultaneous connections to different
   databases and merge the information that way.
   
    4.25) How do I return multiple rows or columns from a function?
    
   You can return result sets from PL/pgSQL functions using refcursors.
   See
   http://developer.postgresql.org/docs/postgres/plpgsql-cursors.html,
   section 23.7.3.3.
   
    4.26) Why can't I reliably create/drop temporary tables in PL/PgSQL
    functions?
    
   PL/PgSQL caches function contents, and an unfortunate side effect is
   that if a PL/PgSQL function accesses a temporary table, and that table
   is later dropped and recreated, and the function called again, the
   function will fail because the cached function contents still point to
   the old temporary table. The solution is to use EXECUTE for temporary
   table access in PL/PgSQL. This will cause the query to be reparsed
   every time.
     _________________________________________________________________
   
                            Extending PostgreSQL
                                      
    5.1) I wrote a user-defined function. When I run it in psql, why does it
    dump core?
    
   The problem could be a number of things. Try testing your user-defined
   function in a stand-alone test program first.
   
    5.2) How can I contribute some nifty new types and functions to PostgreSQL?
    
   Send your extensions to the pgsql-hackers mailing list, and they will
   eventually end up in the contrib/ subdirectory.
   
    5.3) How do I write a C function to return a tuple?
    
   This requires wizardry so extreme that the authors have never tried
   it, though in principle it can be done.
   
    5.4) I have changed a source file. Why does the recompile 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. If you are using
   GCC you can use the --enable-depend option of configure to have the
   compiler compute the dependencies automatically.