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Frequently Asked Questions (FAQ) for PostgreSQL Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Sat Apr 23 14:56:41 EDT 2005 Last updated: Sat Apr 23 14:59:01 EDT 2005
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us) Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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General Questions General Questions
1.1) What is PostgreSQL? How is it pronounced? 1.1) What is PostgreSQL? How is it pronounced?
PostgreSQL is pronounced Post-Gres-Q-L, also called just Postgres. PostgreSQL is pronounced Post-Gres-Q-L, also called just Postgres.
PostgreSQL is an object-relational database system that has the PostgreSQL is an object-relational database system that has the
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company. To get involved, see the developer's FAQ at company. To get involved, see the developer's FAQ at
http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html
1.2) What is the copyright of PostgreSQL? 1.2) What is the copyright of PostgreSQL?
PostgreSQL is distributed under the classic BSD license. It has no PostgreSQL is distributed under the classic BSD license. It has no
restrictions on how the source code can be used. We like it and have restrictions on how the source code can be used. We like it and have
no intention of changing it. no intention of changing it.
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CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT, CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT,
UPDATES, ENHANCEMENTS, OR MODIFICATIONS. UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
1.3) What platforms does PostgreSQL support? 1.3) What platforms does PostgreSQL support?
In general, any modern Unix-compatible platform should be able to run In general, any modern Unix-compatible platform should be able to run
PostgreSQL. The platforms that had received explicit testing at the PostgreSQL. The platforms that had received explicit testing at the
time of release are listed in the installation instructions. time of release are listed in the installation instructions.
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http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgre http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgre
SQL&stype=all&sort=type&dir=%2F. SQL&stype=all&sort=type&dir=%2F.
1.4) Where can I get PostgreSQL? 1.4) Where can I get PostgreSQL?
Via web browser, use http://www.postgresql.org/ftp/, and via ftp, use Via web browser, use http://www.postgresql.org/ftp/, and via ftp, use
ftp://ftp.PostgreSQL.org/pub/. ftp://ftp.PostgreSQL.org/pub/.
1.5) Where can I get support? 1.5) Where can I get support?
The PostgreSQL community provides assistance to many of its users via The PostgreSQL community provides assistance to many of its users via
email. The main web site to subscribe to the email lists is email. The main web site to subscribe to the email lists is
http://www.postgresql.org/community/lists/. The general or bugs lists http://www.postgresql.org/community/lists/. The general or bugs lists
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A list of commercial support companies is available at A list of commercial support companies is available at
http://techdocs.postgresql.org/companies.php. http://techdocs.postgresql.org/companies.php.
1.6) How do I submit a bug report? 1.6) How do I submit a bug report?
Visit the PostgreSQL bug form at Visit the PostgreSQL bug form at
http://www.postgresql.org/support/submitbug. http://www.postgresql.org/support/submitbug.
Also check out our ftp site ftp://ftp.PostgreSQL.org/pub/ to see if Also check out our ftp site ftp://ftp.PostgreSQL.org/pub/ to see if
there is a more recent PostgreSQL version. there is a more recent PostgreSQL version.
1.7) What is the latest release? 1.7) What is the latest release?
The latest release of PostgreSQL is version 8.0.2. The latest release of PostgreSQL is version 8.0.2.
We plan to have a major release every year, with minor releases every We plan to have a major release every year, with minor releases every
few months. few months.
1.8) What documentation is available? 1.8) What documentation is available?
PostgreSQL includes extensive documentation, including a large manual, PostgreSQL includes extensive documentation, including a large manual,
manual pages, and some test examples. See the /doc directory. You can manual pages, and some test examples. See the /doc directory. You can
also browse the manuals online at http://www.PostgreSQL.org/docs. also browse the manuals online at http://www.PostgreSQL.org/docs.
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Our web site contains even more documentation. Our web site contains even more documentation.
1.9) How do I find out about known bugs or missing features? 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 PostgreSQL supports an extended subset of SQL-92. See our TODO list
for known bugs, missing features, and future plans. for known bugs, missing features, and future plans.
1.10) How can I learn SQL? 1.10) How can I learn SQL?
First, consider the PostgreSQL-specific books mentioned above. Another First, consider the PostgreSQL-specific books mentioned above. Another
one is "Teach Yourself SQL in 21 Days, Second Edition" at one is "Teach Yourself SQL in 21 Days, Second Edition" at
http://members.tripod.com/er4ebus/sql/index.htm http://members.tripod.com/er4ebus/sql/index.htm
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http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM, http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,
and at http://sqlcourse.com. and at http://sqlcourse.com.
1.11) How do I join the development team? 1.11) How do I join the development team?
See the Developer's FAQ. See the Developer's FAQ.
1.12) How does PostgreSQL compare to other DBMSs? 1.12) How does PostgreSQL compare to other DBMSs?
There are several ways of measuring software: features, performance, There are several ways of measuring software: features, performance,
reliability, support, and price. reliability, support, and price.
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User Client Questions User Client Questions
2.1) What interfaces are available for PostgreSQL? 2.1) What interfaces are available for PostgreSQL?
The PostgreSQL install includes only the C and embedded C interfaces. The PostgreSQL install includes only the C and embedded C interfaces.
All other interfaces are independent projects that are downloaded All other interfaces are independent projects that are downloaded
separately; being separate allows them to have their own release separately; being separate allows them to have their own release
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others are available at http://gborg.postgresql.org in the others are available at http://gborg.postgresql.org in the
Drivers/Interfaces section and via Internet search. Drivers/Interfaces section and via Internet search.
2.2) What tools are available for using PostgreSQL with Web pages? 2.2) What tools are available for using PostgreSQL with Web pages?
A nice introduction to Database-backed Web pages can be seen at: A nice introduction to Database-backed Web pages can be seen at:
http://www.webreview.com http://www.webreview.com
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For complex cases, many use the Perl and DBD::Pg with CGI.pm or For complex cases, many use the Perl and DBD::Pg with CGI.pm or
mod_perl. mod_perl.
2.3) Does PostgreSQL have a graphical user interface? 2.3) Does PostgreSQL have a graphical user interface?
Yes, see http://techdocs.postgresql.org/guides/GUITools for a detailed Yes, see http://techdocs.postgresql.org/guides/GUITools for a detailed
list. list.
_________________________________________________________________ _________________________________________________________________
Administrative Questions Administrative Questions
3.1) How do I install PostgreSQL somewhere other than /usr/local/pgsql? 3.1) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
Specify the --prefix option when running configure. Specify the --prefix option when running configure.
3.2) How do I control connections from other hosts? 3.2) How do I control connections from other hosts?
By default, PostgreSQL only allows connections from the local machine By default, PostgreSQL only allows connections from the local machine
using Unix domain sockets or TCP/IP connections. Other machines will using Unix domain sockets or TCP/IP connections. Other machines will
not be able to connect unless you modify listen_addresses in the not be able to connect unless you modify listen_addresses in the
postgresql.conf file, enable host-based authentication by modifying postgresql.conf file, enable host-based authentication by modifying
the $PGDATA/pg_hba.conf file, and restart the server. the $PGDATA/pg_hba.conf file, and restart the server.
3.3) How do I tune the database engine for better performance? 3.3) How do I tune the database engine for better performance?
There are three major areas for potential performance improvement: There are three major areas for potential performance improvement:
Query Changes Query Changes
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http://candle.pha.pa.us/main/writings/pgsql/hw_performance/inde http://candle.pha.pa.us/main/writings/pgsql/hw_performance/inde
x.html and http://www.powerpostgresql.com/PerfList/. x.html and http://www.powerpostgresql.com/PerfList/.
3.4) What debugging features are available? 3.4) What debugging features are available?
There are many log_* server configuration variables that enable There are many log_* server configuration variables that enable
printing of query and process statistics which can be very useful for printing of query and process statistics which can be very useful for
debugging and performance measurements. debugging and performance measurements.
3.5) Why do I get "Sorry, too many clients" when trying to connect? 3.5) Why do I get "Sorry, too many clients" when trying to connect?
You have reached the default limit is 100 database sessions. You need You have reached the default limit is 100 database sessions. You need
to increase the postmaster's limit on how many concurrent backend to increase the postmaster's limit on how many concurrent backend
processes it can start by changing the max_connections value in processes it can start by changing the max_connections value in
postgresql.conf and restarting the postmaster. postgresql.conf and restarting the postmaster.
3.6) Why do I need to do a dump and restore to upgrade between major 3.6) Why do I need to do a dump and restore to upgrade between major
PostgreSQL releases? PostgreSQL releases?
The PostgreSQL team makes only small changes between minor releases, The PostgreSQL team makes only small changes between minor releases,
so upgrading from 7.4.0 to 7.4.1 does not require a dump and restore. so upgrading from 7.4.0 to 7.4.1 does not require a dump and restore.
However, major releases (e.g. from 7.3 to 7.4) often change the However, major releases (e.g. from 7.3 to 7.4) often change the
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files. A dump outputs data in a generic format that can then be loaded files. A dump outputs data in a generic format that can then be loaded
in using the new internal format. in using the new internal format.
3.7) What computer hardware should I use? 3.7) What computer hardware should I use?
Because PC hardware is mostly compatible, people tend to believe that Because PC hardware is mostly compatible, people tend to believe that
all PC hardware is of equal quality. It is not. ECC RAM, SCSI, and all PC hardware is of equal quality. It is not. ECC RAM, SCSI, and
quality motherboards are more reliable and have better performance quality motherboards are more reliable and have better performance
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Operational Questions Operational Questions
4.1) How do I SELECT only the first few rows of a query? A random row? 4.1) How do I SELECT only the first few rows of a query? A random row?
To retrieve only a few rows, if you know at the number of rows needed To retrieve only a few rows, if you know at the number of rows needed
at the time of the SELECT use LIMIT . If an index matches the ORDER BY at the time of the SELECT use LIMIT . If an index matches the ORDER BY
it is possible the entire query does not have to be executed. If you it is possible the entire query does not have to be executed. If you
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ORDER BY random() ORDER BY random()
LIMIT 1; LIMIT 1;
4.2) How do I find out what tables, indexes, databases, and users are 4.2) How do I find out what tables, indexes, databases, and users are
defined? How do I see the queries used by psql to display them? defined? How do I see the queries used by psql to display them?
Use the \dt command to see tables in psql. For a complete list of Use the \dt command to see tables in psql. For a complete list of
commands inside psql you can use \?. Alternatively you can read the commands inside psql you can use \?. Alternatively you can read the
source code for psql in file pgsql/src/bin/psql/describe.c, it source code for psql in file pgsql/src/bin/psql/describe.c, it
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many of the SELECTs needed to get information from the database system many of the SELECTs needed to get information from the database system
tables. tables.
4.3) How do you change a column's data type? 4.3) How do you change a column's data type?
Changing the data type of a column can be done easily in 8.0 and later Changing the data type of a column can be done easily in 8.0 and later
with ALTER TABLE ALTER COLUMN TYPE. with ALTER TABLE ALTER COLUMN TYPE.
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You might then want to do VACUUM FULL tab to reclaim the disk space You might then want to do VACUUM FULL tab to reclaim the disk space
used by the expired rows. used by the expired rows.
4.4) What is the maximum size for a row, a table, and a database? 4.4) What is the maximum size for a row, a table, and a database?
These are the limits: These are the limits:
Maximum size for a database? unlimited (32 TB databases exist) Maximum size for a database? unlimited (32 TB databases exist)
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The maximum table size and maximum number of columns can be quadrupled The maximum table size and maximum number of columns can be quadrupled
by increasing the default block size to 32k. by increasing the default block size to 32k.
4.5) How much database disk space is required to store data from a typical 4.5) How much database disk space is required to store data from a typical
text file? text file?
A PostgreSQL database may require up to five times the disk space to A PostgreSQL database may require up to five times the disk space to
store data from a text file. store data from a text file.
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NULLs are stored as bitmaps, so they use very little space. NULLs are stored as bitmaps, so they use very little space.
4.6) Why are my queries slow? Why don't they use my indexes? 4.6) Why are my queries slow? Why don't they use my indexes?
Indexes are not used by every query. Indexes are used only if the Indexes are not used by every query. Indexes are used only if the
table is larger than a minimum size, and the query selects only a 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 small percentage of the rows in the table. This is because the random
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types exactly match the index's column types. This was particularly types exactly match the index's column types. This was particularly
true of int2, int8, and numeric column indexes. true of int2, int8, and numeric column indexes.
4.7) How do I see how the query optimizer is evaluating my query? 4.7) How do I see how the query optimizer is evaluating my query?
See the EXPLAIN manual page. See the EXPLAIN manual page.
4.8) How do I perform regular expression searches and case-insensitive 4.8) How do I perform regular expression searches and case-insensitive
regular expression searches? How do I use an index for case-insensitive regular expression searches? How do I use an index for case-insensitive
searches? searches?
The ~ operator does regular expression matching, and ~* does The ~ operator does regular expression matching, and ~* does
case-insensitive regular expression matching. The case-insensitive case-insensitive regular expression matching. The case-insensitive
variant of LIKE is called ILIKE. variant of LIKE is called ILIKE.
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expresssion index, it will be used: expresssion index, it will be used:
CREATE INDEX tabindex ON tab (lower(col)); CREATE INDEX tabindex ON tab (lower(col));
4.9) In a query, how do I detect if a field is NULL? How can I sort on 4.9) In a query, how do I detect if a field is NULL? How can I sort on
whether a field is NULL or not? whether a field is NULL or not?
You test the column with IS NULL and IS NOT NULL. You test the column with IS NULL and IS NOT NULL.
SELECT * SELECT *
FROM tab FROM tab
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FROM tab FROM tab
ORDER BY (col IS NOT NULL) ORDER BY (col IS NOT NULL)
4.10) What is the difference between the various character types? 4.10) What is the difference between the various character types?
Type Internal Name Notes Type Internal Name Notes
VARCHAR(n) varchar size specifies maximum length, no padding VARCHAR(n) varchar size specifies maximum length, no padding
CHAR(n) bpchar blank padded to the specified fixed length CHAR(n) bpchar blank padded to the specified fixed length
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particularly values that include NULL bytes. All the types described particularly values that include NULL bytes. All the types described
here have similar performance characteristics. here have similar performance characteristics.
4.11.1) How do I create a serial/auto-incrementing field? 4.11.1) How do I create a serial/auto-incrementing field?
PostgreSQL supports a SERIAL data type. It auto-creates a sequence. PostgreSQL supports a SERIAL data type. It auto-creates a sequence.
For example, this: For example, this:
CREATE TABLE person ( CREATE TABLE person (
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See the create_sequence manual page for more information about See the create_sequence manual page for more information about
sequences. sequences.
4.11.2) How do I get the value of a SERIAL insert? 4.11.2) How do I get the value of a SERIAL insert?
One approach is to retrieve the next SERIAL value from the sequence One approach is to retrieve the next SERIAL value from the sequence
object with the nextval() function before inserting and then insert it object with the nextval() function before inserting and then insert it
explicitly. Using the example table in 4.11.1, an example in a explicitly. Using the example table in 4.11.1, an example in a
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execute("INSERT INTO person (name) VALUES ('Blaise Pascal')"); execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
new_id = execute("SELECT currval('person_id_seq')"); new_id = execute("SELECT currval('person_id_seq')");
4.11.3) Doesn't currval() lead to a race condition with other users? 4.11.3) Doesn't currval() lead to a race condition with other users?
No. currval() returns the current value assigned by your session, not No. currval() returns the current value assigned by your session, not
by all sessions. by all sessions.
4.11.4) Why aren't my sequence numbers reused on transaction abort? Why are 4.11.4) Why aren't my sequence numbers reused on transaction abort? Why are
there gaps in the numbering of my sequence/SERIAL column? there gaps in the numbering of my sequence/SERIAL column?
To improve concurrency, sequence values are given out to running To improve concurrency, sequence values are given out to running
transactions as needed and are not locked until the transaction transactions as needed and are not locked until the transaction
completes. This causes gaps in numbering from aborted transactions. completes. This causes gaps in numbering from aborted transactions.
4.12) What is an OID? What is a TID? 4.12) What is an OID? What is a TID?
Every row that is created in PostgreSQL gets a unique OID unless Every row that is created in PostgreSQL gets a unique OID unless
created WITHOUT OIDS. OIDs are autotomatically assigned unique 4-byte created WITHOUT OIDS. OIDs are autotomatically assigned unique 4-byte
integers that are unique across the entire installation. However, they integers that are unique across the entire installation. However, they
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values. TIDs change after rows are modified or reloaded. They are used values. TIDs change after rows are modified or reloaded. They are used
by index entries to point to physical rows. by index entries to point to physical rows.
4.13) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"? 4.13) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
You probably have run out of virtual memory on your system, or your 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 kernel has a low limit for certain resources. Try this before starting
postmaster: postmaster:
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problem with the SQL client because the backend is returning too much problem with the SQL client because the backend is returning too much
data, try it before starting the client. data, try it before starting the client.
4.14) How do I tell what PostgreSQL version I am running? 4.14) How do I tell what PostgreSQL version I am running?
From psql, type SELECT version(); From psql, type SELECT version();
4.15) Why does my large-object operations get "invalid large obj 4.15) Why does my large-object operations get "invalid large obj descriptor"?
descriptor"?
You need to put BEGIN WORK and COMMIT around any use of a large object You need to put BEGIN WORK and COMMIT around any use of a large object
handle, that is, surrounding lo_open ... lo_close. handle, that is, surrounding lo_open ... lo_close.
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If you are using a client interface like ODBC you may need to set If you are using a client interface like ODBC you may need to set
auto-commit off. auto-commit off.
4.16) How do I create a column that will default to the current time? 4.16) How do I create a column that will default to the current time?
Use CURRENT_TIMESTAMP: Use CURRENT_TIMESTAMP:
CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP ); CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
4.17) How do I perform an outer join? 4.17) How do I perform an outer join?
PostgreSQL supports outer joins using the SQL standard syntax. Here PostgreSQL supports outer joins using the SQL standard syntax. Here
are two examples: are two examples:
SELECT * SELECT *
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is assumed in LEFT, RIGHT, and FULL joins. Ordinary joins are called is assumed in LEFT, RIGHT, and FULL joins. Ordinary joins are called
INNER joins. INNER joins.
4.18) How do I perform queries using multiple databases? 4.18) How do I perform queries using multiple databases?
There is no way to query a database other than the current one. There is no way to query a database other than the current one.
Because PostgreSQL loads database-specific system catalogs, it is Because PostgreSQL loads database-specific system catalogs, it is
uncertain how a cross-database query should even behave. uncertain how a cross-database query should even behave.
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course, a client can also make simultaneous connections to different course, a client can also make simultaneous connections to different
databases and merge the results on the client side. databases and merge the results on the client side.
4.19) How do I return multiple rows or columns from a function? 4.19) How do I return multiple rows or columns from a function?
It is easy using set-returning functions, It is easy using set-returning functions,
http://techdocs.postgresql.org/guides/SetReturningFunctions http://techdocs.postgresql.org/guides/SetReturningFunctions
. .
4.20) Why do I get "relation with OID ##### does not exist" errors when 4.20) Why do I get "relation with OID ##### does not exist" errors when
accessing temporary tables in PL/PgSQL functions? accessing temporary tables in PL/PgSQL functions?
PL/PgSQL caches function scripts, and an unfortunate side effect is PL/PgSQL caches function scripts, and an unfortunate side effect is
that if a PL/PgSQL function accesses a temporary table, and that table that if a PL/PgSQL function accesses a temporary table, and that table
is later dropped and recreated, and the function called again, the is later dropped and recreated, and the function called again, the
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table access in PL/PgSQL. This will cause the query to be reparsed table access in PL/PgSQL. This will cause the query to be reparsed
every time. every time.
4.21) What encryption options are available? 4.21) What encryption options are available?
* contrib/pgcrypto contains many encryption functions for use in SQL * contrib/pgcrypto contains many encryption functions for use in SQL
queries. queries.
* To encrypt transmission from the client to the server, the server * To encrypt transmission from the client to the server, the server
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Extending PostgreSQL Extending PostgreSQL
5.1) I wrote a user-defined function. When I run it in psql, why does it 5.1) I wrote a user-defined function. When I run it in psql, why does it dump
dump core? core?
The problem could be a number of things. Try testing your user-defined The problem could be a number of things. Try testing your user-defined
function in a stand-alone test program first. function in a stand-alone test program first.
5.2) How can I contribute some nifty new types and functions to PostgreSQL? 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 Send your extensions to the pgsql-hackers mailing list, and they will
eventually end up in the contrib/ subdirectory. eventually end up in the contrib/ subdirectory.
5.3) How do I write a C function to return a tuple? 5.3) How do I write a C function to return a tuple?
In versions of PostgreSQL beginning with 7.3, table-returning In versions of PostgreSQL beginning with 7.3, table-returning
functions are fully supported in C, PL/PgSQL, and SQL. See the functions are fully supported in C, PL/PgSQL, and SQL. See the
Programmer's Guide for more information. An example of a Programmer's Guide for more information. An example of a
table-returning function defined in C can be found in table-returning function defined in C can be found in
contrib/tablefunc. contrib/tablefunc.
5.4) I have changed a source file. Why does the recompile not see the 5.4) I have changed a source file. Why does the recompile not see the change?
change?
The Makefiles do not have the proper dependencies for include files. 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 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 GCC you can use the --enable-depend option of configure to have the

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alink="#0000ff"> alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1> <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Sat Apr 23 14:56:41 EDT 2005</P> <P>Last updated: Sat Apr 23 14:59:01 EDT 2005</P>
<P>Current maintainer: Bruce Momjian (<A href= <P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>) "mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)
@ -133,7 +133,7 @@
<H2 align="center">General Questions</H2> <H2 align="center">General Questions</H2>
<H4><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H4> <H3><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H3>
<P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>, also called just <P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>, also called just
<I>Postgres</I>.</P> <I>Postgres</I>.</P>
@ -152,8 +152,8 @@
http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html</A> http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html</A>
</P> </P>
<H4><A name="1.2">1.2</A>) What is the copyright of <H3><A name="1.2">1.2</A>) What is the copyright of
PostgreSQL?</H4> PostgreSQL?</H3>
<P>PostgreSQL is distributed under the classic BSD license. It has <P>PostgreSQL is distributed under the classic BSD license. It has
no restrictions on how the source code can be used. We like it and no restrictions on how the source code can be used. We like it and
@ -185,7 +185,7 @@
UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P> SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
<H4><A name="1.3">1.3</A>) What platforms does PostgreSQL support?</H4> <H3><A name="1.3">1.3</A>) What platforms does PostgreSQL support?</H3>
<P>In general, any modern Unix-compatible platform should be able to <P>In general, any modern Unix-compatible platform should be able to
run PostgreSQL. The platforms that had received explicit testing at run PostgreSQL. The platforms that had received explicit testing at
@ -204,14 +204,14 @@
"http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&amp;button=Search&amp;key=postgreSQL&amp;stype=all&amp;sort=type&amp;dir=%2F"> "http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&amp;button=Search&amp;key=postgreSQL&amp;stype=all&amp;sort=type&amp;dir=%2F">
http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&amp;button=Search&amp;key=postgreSQL&amp;stype=all&amp;sort=type&amp;dir=%2F</a>.</p> http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&amp;button=Search&amp;key=postgreSQL&amp;stype=all&amp;sort=type&amp;dir=%2F</a>.</p>
<H4><A name="1.4">1.4</A>) Where can I get PostgreSQL?</H4> <H3><A name="1.4">1.4</A>) Where can I get PostgreSQL?</H3>
<P>Via web browser, use <a href="http://www.postgresql.org/ftp/"> <P>Via web browser, use <a href="http://www.postgresql.org/ftp/">
http://www.postgresql.org/ftp/</a>, and via ftp, use http://www.postgresql.org/ftp/</a>, and via ftp, use
<A href="ftp://ftp.PostgreSQL.org/pub/"> <A href="ftp://ftp.PostgreSQL.org/pub/">
ftp://ftp.PostgreSQL.org/pub/</A>.</P> ftp://ftp.PostgreSQL.org/pub/</A>.</P>
<H4><A name="1.5">1.5</A>) Where can I get support?</H4> <H3><A name="1.5">1.5</A>) Where can I get support?</H3>
<P>The PostgreSQL community provides assistance to many of its users <P>The PostgreSQL community provides assistance to many of its users
via email. The main web site to subscribe to the email lists is via email. The main web site to subscribe to the email lists is
@ -230,7 +230,7 @@
"http://techdocs.postgresql.org/companies.php"> "http://techdocs.postgresql.org/companies.php">
http://techdocs.postgresql.org/companies.php</A>.</P> http://techdocs.postgresql.org/companies.php</A>.</P>
<H4><A name="1.6">1.6</A>) How do I submit a bug report?</H4> <H3><A name="1.6">1.6</A>) How do I submit a bug report?</H3>
<P>Visit the PostgreSQL bug form at <A href= <P>Visit the PostgreSQL bug form at <A href=
"http://www.postgresql.org/support/submitbug"> "http://www.postgresql.org/support/submitbug">
@ -240,14 +240,14 @@
"ftp://ftp.PostgreSQL.org/pub/">ftp://ftp.PostgreSQL.org/pub/</A> to "ftp://ftp.PostgreSQL.org/pub/">ftp://ftp.PostgreSQL.org/pub/</A> to
see if there is a more recent PostgreSQL version.</P> see if there is a more recent PostgreSQL version.</P>
<H4><A name="1.7">1.7</A>) What is the latest release?</H4> <H3><A name="1.7">1.7</A>) What is the latest release?</H3>
<P>The latest release of PostgreSQL is version 8.0.2.</P> <P>The latest release of PostgreSQL is version 8.0.2.</P>
<P>We plan to have a major release every year, with minor releases <P>We plan to have a major release every year, with minor releases
every few months.</P> every few months.</P>
<H4><A name="1.8">1.8</A>) What documentation is available?</H4> <H3><A name="1.8">1.8</A>) What documentation is available?</H3>
<P>PostgreSQL includes extensive documentation, including a large <P>PostgreSQL includes extensive documentation, including a large
manual, manual pages, and some test examples. See the <I>/doc</I> manual, manual pages, and some test examples. See the <I>/doc</I>
@ -272,15 +272,15 @@
<P>Our web site contains even more documentation.</P> <P>Our web site contains even more documentation.</P>
<H4><A name="1.9">1.9</A>) How do I find out about known bugs or <H3><A name="1.9">1.9</A>) How do I find out about known bugs or
missing features?</H4> missing features?</H3>
<P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92. <P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
See our <A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A> See our <A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A>
list for known bugs, missing features, and future plans.</P> list for known bugs, missing features, and future plans.</P>
<H4><A name="1.10">1.10</A>) How can I learn <H3><A name="1.10">1.10</A>) How can I learn
<SMALL>SQL</SMALL>?</H4> <SMALL>SQL</SMALL>?</H3>
<P>First, consider the PostgreSQL-specific books mentioned above. <P>First, consider the PostgreSQL-specific books mentioned above.
Another one is "Teach Yourself SQL in 21 Days, Second Edition" Another one is "Teach Yourself SQL in 21 Days, Second Edition"
@ -298,14 +298,14 @@
and at <A href= and at <A href=
"http://sqlcourse.com/">http://sqlcourse.com.</A></P> "http://sqlcourse.com/">http://sqlcourse.com.</A></P>
<H4><A name="1.11">1.11</A>) How do I join the development <H3><A name="1.11">1.11</A>) How do I join the development
team?</H4> team?</H3>
<P>See the <a href="http://www.postgresql.org/docs/faqs.FAQ_DEV.html"> <P>See the <a href="http://www.postgresql.org/docs/faqs.FAQ_DEV.html">
Developer's FAQ</A>. Developer's FAQ</A>.
<H4><A name="1.12">1.12</A>) How does PostgreSQL compare to other <H3><A name="1.12">1.12</A>) How does PostgreSQL compare to other
<SMALL>DBMS</SMALL>s?</H4> <SMALL>DBMS</SMALL>s?</H3>
<P>There are several ways of measuring software: features, <P>There are several ways of measuring software: features,
performance, reliability, support, and price.</P> performance, reliability, support, and price.</P>
@ -368,8 +368,8 @@
<H2 align="center">User Client Questions</H2> <H2 align="center">User Client Questions</H2>
<H4><A name="2.1">2.1</A>) What interfaces are available for <H3><A name="2.1">2.1</A>) What interfaces are available for
PostgreSQL?</H4> PostgreSQL?</H3>
<P>The PostgreSQL install includes only the <SMALL>C</SMALL> and embedded <P>The PostgreSQL install includes only the <SMALL>C</SMALL> and embedded
<SMALL>C</SMALL> interfaces. All other interfaces are independent projects <SMALL>C</SMALL> interfaces. All other interfaces are independent projects
@ -383,8 +383,8 @@
in the <I>Drivers/Interfaces</I> section and via Internet search. in the <I>Drivers/Interfaces</I> section and via Internet search.
</P> </P>
<H4><A name="2.2">2.2</A>) What tools are available for using <H3><A name="2.2">2.2</A>) What tools are available for using
PostgreSQL with Web pages?</H4> PostgreSQL with Web pages?</H3>
<P>A nice introduction to Database-backed Web pages can be seen at: <P>A nice introduction to Database-backed Web pages can be seen at:
<A href="http://www.webreview.com">http://www.webreview.com</A></P> <A href="http://www.webreview.com">http://www.webreview.com</A></P>
@ -396,8 +396,8 @@
<P>For complex cases, many use the Perl and DBD::Pg with CGI.pm or <P>For complex cases, many use the Perl and DBD::Pg with CGI.pm or
mod_perl.</P> mod_perl.</P>
<H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user <H3><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
interface?</H4> interface?</H3>
<P>Yes, see <a href="http://techdocs.postgresql.org/guides/GUITools"> <P>Yes, see <a href="http://techdocs.postgresql.org/guides/GUITools">
http://techdocs.postgresql.org/guides/GUITools</a> for a http://techdocs.postgresql.org/guides/GUITools</a> for a
@ -408,14 +408,14 @@
<H2 align="center">Administrative Questions</H2> <H2 align="center">Administrative Questions</H2>
<H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere <H3><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
other than <I>/usr/local/pgsql</I>?</H4> other than <I>/usr/local/pgsql</I>?</H3>
<P>Specify the <I>--prefix</I> option when running <P>Specify the <I>--prefix</I> option when running
<I>configure</I>.</P> <I>configure</I>.</P>
<H4><A name="3.2">3.2</A>) How do I control connections from other <H3><A name="3.2">3.2</A>) How do I control connections from other
hosts?</H4> hosts?</H3>
<P>By default, PostgreSQL only allows connections from the local <P>By default, PostgreSQL only allows connections from the local
machine using Unix domain sockets or TCP/IP connections. Other machine using Unix domain sockets or TCP/IP connections. Other
@ -424,8 +424,8 @@
host-based authentication by modifying the host-based authentication by modifying the
<I>$PGDATA/pg_hba.conf</I> file, and restart the server.</P> <I>$PGDATA/pg_hba.conf</I> file, and restart the server.</P>
<H4><A name="3.3">3.3</A>) How do I tune the database engine for <H3><A name="3.3">3.3</A>) How do I tune the database engine for
better performance?</H4> better performance?</H3>
<P>There are three major areas for potential performance <P>There are three major areas for potential performance
improvement:</P> improvement:</P>
@ -481,15 +481,15 @@
</DD> </DD>
</DL> </DL>
<H4><A name="3.4">3.4</A>) What debugging features are <H3><A name="3.4">3.4</A>) What debugging features are
available?</H4> available?</H3>
<P>There are many <CODE>log_*</CODE> server configuration variables <P>There are many <CODE>log_*</CODE> server configuration variables
that enable printing of query and process statistics which can be that enable printing of query and process statistics which can be
very useful for debugging and performance measurements.</P> very useful for debugging and performance measurements.</P>
<H4><A name="3.5">3.5</A>) Why do I get <I>"Sorry, too many <H3><A name="3.5">3.5</A>) Why do I get <I>"Sorry, too many
clients"</I> when trying to connect?</H4> clients"</I> when trying to connect?</H3>
<P>You have reached the default limit is 100 database sessions. You <P>You have reached the default limit is 100 database sessions. You
need to increase the <I>postmaster</I>'s limit on how many need to increase the <I>postmaster</I>'s limit on how many
@ -497,8 +497,8 @@
<I>max_connections</I> value in <I>postgresql.conf</I> and <I>max_connections</I> value in <I>postgresql.conf</I> and
restarting the <I>postmaster</I>.</P> restarting the <I>postmaster</I>.</P>
<H4><A name="3.6">3.6</A>) Why do I need to do a dump and restore <H3><A name="3.6">3.6</A>) Why do I need to do a dump and restore
to upgrade between major PostgreSQL releases?</H4> to upgrade between major PostgreSQL releases?</H3>
<P>The PostgreSQL team makes only small changes between minor releases, <P>The PostgreSQL team makes only small changes between minor releases,
so upgrading from 7.4.0 to 7.4.1 does not require a dump and restore. so upgrading from 7.4.0 to 7.4.1 does not require a dump and restore.
@ -508,7 +508,7 @@
data in a generic format that can then be loaded in using the new internal data in a generic format that can then be loaded in using the new internal
format.</P> format.</P>
<H4><A name="3.7">3.7</A>) What computer hardware should I use?</H4> <H3><A name="3.7">3.7</A>) What computer hardware should I use?</H3>
<P>Because PC hardware is mostly compatible, people tend to believe that <P>Because PC hardware is mostly compatible, people tend to believe that
all PC hardware is of equal quality. It is not. ECC RAM, SCSI, and all PC hardware is of equal quality. It is not. ECC RAM, SCSI, and
@ -522,8 +522,8 @@
<H2 align="center">Operational Questions</H2> <H2 align="center">Operational Questions</H2>
<H4><A name="4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the <H3><A name="4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the
first few rows of a query? A random row?</H4> first few rows of a query? A random row?</H3>
<P>To retrieve only a few rows, if you know at the number of rows <P>To retrieve only a few rows, if you know at the number of rows
needed at the time of the <SMALL>SELECT</SMALL> use needed at the time of the <SMALL>SELECT</SMALL> use
@ -541,9 +541,9 @@
LIMIT 1; LIMIT 1;
</PRE> </PRE>
<H4><A name="4.2">4.2</A>) How do I find out what tables, indexes, <H3><A name="4.2">4.2</A>) How do I find out what tables, indexes,
databases, and users are defined? How do I see the queries used databases, and users are defined? How do I see the queries used
by <I>psql</I> to display them?</H4> by <I>psql</I> to display them?</H3>
<P>Use the \dt command to see tables in <I>psql</I>. For a complete list of <P>Use the \dt command to see tables in <I>psql</I>. For a complete list of
commands inside psql you can use \?. Alternatively you can read the source commands inside psql you can use \?. Alternatively you can read the source
@ -564,7 +564,7 @@
illustrates many of the <SMALL>SELECT</SMALL>s needed to get illustrates many of the <SMALL>SELECT</SMALL>s needed to get
information from the database system tables.</P> information from the database system tables.</P>
<H4><A name="4.3">4.3</A>) How do you change a column's data type?</H4> <H3><A name="4.3">4.3</A>) How do you change a column's data type?</H3>
<P>Changing the data type of a column can be done easily in 8.0 <P>Changing the data type of a column can be done easily in 8.0
and later with <SMALL>ALTER TABLE ALTER COLUMN TYPE</SMALL>.</P> and later with <SMALL>ALTER TABLE ALTER COLUMN TYPE</SMALL>.</P>
@ -580,8 +580,8 @@
<P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the <P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the
disk space used by the expired rows.</P> disk space used by the expired rows.</P>
<H4><A name="4.4">4.4</A>) What is the maximum size for a row, a <H3><A name="4.4">4.4</A>) What is the maximum size for a row, a
table, and a database?</H4> table, and a database?</H3>
<P>These are the limits:</P> <P>These are the limits:</P>
<BLOCKQUOTE> <BLOCKQUOTE>
@ -611,8 +611,8 @@ table?</TD><TD>unlimited</TD></TR>
<P>The maximum table size and maximum number of columns can be <P>The maximum table size and maximum number of columns can be
quadrupled by increasing the default block size to 32k.</P> quadrupled by increasing the default block size to 32k.</P>
<H4><A name="4.5">4.5</A>) How much database disk space is required <H3><A name="4.5">4.5</A>) How much database disk space is required
to store data from a typical text file?</H4> to store data from a typical text file?</H3>
<P>A PostgreSQL database may require up to five times the disk <P>A PostgreSQL database may require up to five times the disk
space to store data from a text file.</P> space to store data from a text file.</P>
@ -648,8 +648,8 @@ table?</TD><TD>unlimited</TD></TR>
<P><SMALL>NULL</SMALL>s are stored as bitmaps, so they <P><SMALL>NULL</SMALL>s are stored as bitmaps, so they
use very little space.</P> use very little space.</P>
<H4><A name="4.6">4.6</A>) Why are my queries slow? Why don't they <H3><A name="4.6">4.6</A>) Why are my queries slow? Why don't they
use my indexes?</H4> use my indexes?</H3>
<P>Indexes are not used by every query. Indexes are used only if the <P>Indexes are not used by every query. Indexes are used only if the
table is larger than a minimum size, and the query selects only a table is larger than a minimum size, and the query selects only a
@ -712,14 +712,14 @@ table?</TD><TD>unlimited</TD></TR>
types exactly match the index's column types. This was particularly types exactly match the index's column types. This was particularly
true of int2, int8, and numeric column indexes.</P> true of int2, int8, and numeric column indexes.</P>
<H4><A name="4.7">4.7</A>) How do I see how the query optimizer is <H3><A name="4.7">4.7</A>) How do I see how the query optimizer is
evaluating my query?</H4> evaluating my query?</H3>
<P>See the <SMALL>EXPLAIN</SMALL> manual page.</P> <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
<H4><A name="4.8">4.8</A>) How do I perform regular expression <H3><A name="4.8">4.8</A>) How do I perform regular expression
searches and case-insensitive regular expression searches? How do I searches and case-insensitive regular expression searches? How do I
use an index for case-insensitive searches?</H4> use an index for case-insensitive searches?</H3>
<P>The <I>~</I> operator does regular expression matching, and <P>The <I>~</I> operator does regular expression matching, and
<I>~*</I> does case-insensitive regular expression matching. The <I>~*</I> does case-insensitive regular expression matching. The
@ -740,9 +740,9 @@ table?</TD><TD>unlimited</TD></TR>
CREATE INDEX tabindex ON tab (lower(col)); CREATE INDEX tabindex ON tab (lower(col));
</PRE> </PRE>
<H4><A name="4.9">4.9</A>) In a query, how do I detect if a field <H3><A name="4.9">4.9</A>) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>? How can I sort on whether a field is <SMALL> is <SMALL>NULL</SMALL>? How can I sort on whether a field is <SMALL>
NULL</SMALL> or not?</H4> NULL</SMALL> or not?</H3>
<P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS <P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
NOT NULL</SMALL>.</P> NOT NULL</SMALL>.</P>
@ -764,8 +764,8 @@ table?</TD><TD>unlimited</TD></TR>
ORDER BY (col IS NOT NULL) ORDER BY (col IS NOT NULL)
</PRE> </PRE>
<H4><A name="4.10">4.10</A>) What is the difference between the <H3><A name="4.10">4.10</A>) What is the difference between the
various character types?</H4> various character types?</H3>
<BLOCKQUOTE> <BLOCKQUOTE>
<TABLE > <TABLE >
<TR><TH>Type</TH><TH>Internal Name</TH><TH>Notes</TH></TR> <TR><TH>Type</TH><TH>Internal Name</TH><TH>Notes</TH></TR>
@ -801,8 +801,8 @@ length</TD></TR>
particularly values that include <SMALL>NULL</SMALL> bytes. All the particularly values that include <SMALL>NULL</SMALL> bytes. All the
types described here have similar performance characteristics.</P> types described here have similar performance characteristics.</P>
<H4><A name="4.11.1">4.11.1</A>) How do I create a <H3><A name="4.11.1">4.11.1</A>) How do I create a
serial/auto-incrementing field?</H4> serial/auto-incrementing field?</H3>
<P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It <P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
auto-creates a sequence. For example, auto-creates a sequence. For example,
@ -826,8 +826,8 @@ length</TD></TR>
See the <I>create_sequence</I> manual page for more information See the <I>create_sequence</I> manual page for more information
about sequences. about sequences.
<H4><A name="4.11.2">4.11.2</A>) How do I get the value of a <H3><A name="4.11.2">4.11.2</A>) How do I get the value of a
<SMALL>SERIAL</SMALL> insert?</H4> <SMALL>SERIAL</SMALL> insert?</H3>
<P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value <P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
from the sequence object with the <I>nextval()</I> function from the sequence object with the <I>nextval()</I> function
@ -855,23 +855,23 @@ length</TD></TR>
new_id = execute("SELECT currval('person_id_seq')"); new_id = execute("SELECT currval('person_id_seq')");
</PRE> </PRE>
<H4><A name="4.11.3">4.11.3</A>) Doesn't <I>currval()</I> <H3><A name="4.11.3">4.11.3</A>) Doesn't <I>currval()</I>
lead to a race condition with other users?</H4> lead to a race condition with other users?</H3>
<P>No. <I>currval()</I> returns the current value assigned by your <P>No. <I>currval()</I> returns the current value assigned by your
session, not by all sessions.</P> session, not by all sessions.</P>
<H4><A name="4.11.4">4.11.4</A>) Why aren't my sequence numbers <H3><A name="4.11.4">4.11.4</A>) Why aren't my sequence numbers
reused on transaction abort? Why are there gaps in the numbering of reused on transaction abort? Why are there gaps in the numbering of
my sequence/SERIAL column?</H4> my sequence/SERIAL column?</H3>
<P>To improve concurrency, sequence values are given out to running <P>To improve concurrency, sequence values are given out to running
transactions as needed and are not locked until the transaction transactions as needed and are not locked until the transaction
completes. This causes gaps in numbering from aborted completes. This causes gaps in numbering from aborted
transactions.</P> transactions.</P>
<H4><A name="4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is <H3><A name="4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is
a <SMALL>TID</SMALL>?</H4> a <SMALL>TID</SMALL>?</H3>
<P>Every row that is created in PostgreSQL gets a unique <P>Every row that is created in PostgreSQL gets a unique
<SMALL>OID</SMALL> unless created <SMALL>WITHOUT OIDS</SMALL>. <SMALL>OID</SMALL> unless created <SMALL>WITHOUT OIDS</SMALL>.
@ -893,8 +893,8 @@ length</TD></TR>
are modified or reloaded. They are used by index entries to point are modified or reloaded. They are used by index entries to point
to physical rows.</P> to physical rows.</P>
<H4><A name="4.13">4.13</A>) Why do I get the error <I>"ERROR: <H3><A name="4.13">4.13</A>) Why do I get the error <I>"ERROR:
Memory exhausted in AllocSetAlloc()"</I>?</H4> Memory exhausted in AllocSetAlloc()"</I>?</H3>
<P>You probably have run out of virtual memory on your system, <P>You probably have run out of virtual memory on your system,
or your kernel has a low limit for certain resources. Try this or your kernel has a low limit for certain resources. Try this
@ -912,13 +912,13 @@ length</TD></TR>
backend is returning too much data, try it before starting the backend is returning too much data, try it before starting the
client. client.
<H4><A name="4.14">4.14</A>) How do I tell what PostgreSQL version <H3><A name="4.14">4.14</A>) How do I tell what PostgreSQL version
I am running?</H4> I am running?</H3>
<P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P> <P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
<H4><A name="4.15">4.15</A>) Why does my large-object operations <H3><A name="4.15">4.15</A>) Why does my large-object operations
get <I>"invalid large obj descriptor"</I>?</H4> get <I>"invalid large obj descriptor"</I>?</H3>
<P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE> <P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
around any use of a large object handle, that is, surrounding around any use of a large object handle, that is, surrounding
@ -933,15 +933,15 @@ length</TD></TR>
<P>If you are using a client interface like <SMALL>ODBC</SMALL> you <P>If you are using a client interface like <SMALL>ODBC</SMALL> you
may need to set <CODE>auto-commit off.</CODE></P> may need to set <CODE>auto-commit off.</CODE></P>
<H4><A name="4.16">4.16</A>) How do I create a column that will <H3><A name="4.16">4.16</A>) How do I create a column that will
default to the current time?</H4> default to the current time?</H3>
<P>Use <I>CURRENT_TIMESTAMP</I>:</P> <P>Use <I>CURRENT_TIMESTAMP</I>:</P>
<PRE> <PRE>
CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP ); CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
</PRE> </PRE>
<H4><A name="4.17">4.17</A>) How do I perform an outer join?</H4> <H3><A name="4.17">4.17</A>) How do I perform an outer join?</H3>
<P>PostgreSQL supports outer joins using the SQL standard syntax. <P>PostgreSQL supports outer joins using the SQL standard syntax.
Here are two examples:</P> Here are two examples:</P>
@ -964,8 +964,8 @@ length</TD></TR>
<SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins <SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
are called <SMALL>INNER</SMALL> joins.</P> are called <SMALL>INNER</SMALL> joins.</P>
<H4><A name="4.18">4.18</A>) How do I perform queries using <H3><A name="4.18">4.18</A>) How do I perform queries using
multiple databases?</H4> multiple databases?</H3>
<P>There is no way to query a database other than the current one. <P>There is no way to query a database other than the current one.
Because PostgreSQL loads database-specific system catalogs, it is Because PostgreSQL loads database-specific system catalogs, it is
@ -976,16 +976,16 @@ length</TD></TR>
connections to different databases and merge the results on the connections to different databases and merge the results on the
client side.</P> client side.</P>
<H4><A name="4.19">4.19</A>) How do I return multiple rows or <H3><A name="4.19">4.19</A>) How do I return multiple rows or
columns from a function?</H4> columns from a function?</H3>
<P>It is easy using set-returning functions, <P>It is easy using set-returning functions,
<a href="http://techdocs.postgresql.org/guides/SetReturningFunctions"> <a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
http://techdocs.postgresql.org/guides/SetReturningFunctions</a></P>. http://techdocs.postgresql.org/guides/SetReturningFunctions</a></P>.
<H4><A name="4.20">4.20</A>) Why do I get "relation with OID ##### <H3><A name="4.20">4.20</A>) Why do I get "relation with OID #####
does not exist" errors when accessing temporary tables in PL/PgSQL does not exist" errors when accessing temporary tables in PL/PgSQL
functions?</H4> functions?</H3>
<P>PL/PgSQL caches function scripts, and an unfortunate side effect <P>PL/PgSQL caches function scripts, and an unfortunate side effect
is that if a PL/PgSQL function accesses a temporary table, and that is that if a PL/PgSQL function accesses a temporary table, and that
@ -995,8 +995,8 @@ length</TD></TR>
<SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This <SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
will cause the query to be reparsed every time.</P> will cause the query to be reparsed every time.</P>
<H4><A name="4.21">4.21</A>) What encryption options are available? <H3><A name="4.21">4.21</A>) What encryption options are available?
</H4> </H3>
<UL> <UL>
<LI><I>contrib/pgcrypto</I> contains many encryption functions for <LI><I>contrib/pgcrypto</I> contains many encryption functions for
use in <SMALL>SQL</SMALL> queries.</LI> use in <SMALL>SQL</SMALL> queries.</LI>
@ -1016,21 +1016,21 @@ length</TD></TR>
<H2 align="center">Extending PostgreSQL</H2> <H2 align="center">Extending PostgreSQL</H2>
<H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I <H3><A name="5.1">5.1</A>) I wrote a user-defined function. When I
run it in <I>psql</I>, why does it dump core?</H4> run it in <I>psql</I>, why does it dump core?</H3>
<P>The problem could be a number of things. Try testing your <P>The problem could be a number of things. Try testing your
user-defined function in a stand-alone test program first.</P> user-defined function in a stand-alone test program first.</P>
<H4><A name="5.2">5.2</A>) How can I contribute some nifty new <H3><A name="5.2">5.2</A>) How can I contribute some nifty new
types and functions to PostgreSQL?</H4> types and functions to PostgreSQL?</H3>
<P>Send your extensions to the <I>pgsql-hackers</I> mailing list, <P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
and they will eventually end up in the <I>contrib/</I> and they will eventually end up in the <I>contrib/</I>
subdirectory.</P> subdirectory.</P>
<H4><A name="5.3">5.3</A>) How do I write a C function to return a <H3><A name="5.3">5.3</A>) How do I write a C function to return a
tuple?</H4> tuple?</H3>
<P>In versions of PostgreSQL beginning with 7.3, table-returning <P>In versions of PostgreSQL beginning with 7.3, table-returning
functions are fully supported in C, PL/PgSQL, and SQL. See the functions are fully supported in C, PL/PgSQL, and SQL. See the
@ -1038,8 +1038,8 @@ length</TD></TR>
table-returning function defined in C can be found in table-returning function defined in C can be found in
<I>contrib/tablefunc</I>.</P> <I>contrib/tablefunc</I>.</P>
<H4><A name="5.4">5.4</A>) I have changed a source file. Why does <H3><A name="5.4">5.4</A>) I have changed a source file. Why does
the recompile not see the change?</H4> the recompile not see the change?</H3>
<P>The <I>Makefiles</I> do not have the proper dependencies for <P>The <I>Makefiles</I> do not have the proper dependencies for
include files. You have to do a <I>make clean</I> and then another include files. You have to do a <I>make clean</I> and then another