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Backpatch FAQ changes to 8.0.X.

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@@ -1,7 +1,7 @@
Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Sat Jan 29 23:44:48 EST 2005
Last updated: Mon Jan 31 21:40:28 EST 2005
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
@@ -15,18 +15,17 @@
General Questions
1.1) What is PostgreSQL? How is it pronounced?
1.2) What is the copyright on PostgreSQL?
1.2) What is the copyright of PostgreSQL?
1.3) What platforms does PostgreSQL support?
1.4) Where can I get PostgreSQL?
1.5) Where can I get support?
1.6) What is the latest release?
1.7) What documentation is available?
1.8) How do I find out about known bugs or missing features?
1.9) How can I learn SQL?
1.10) How do I join the development team?
1.11) How do I submit a bug report?
1.6) How do I submit a bug report?
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) How do I join the development team?
1.12) How does PostgreSQL compare to other DBMSs?
1.13) How can I financially assist PostgreSQL?
User Client Questions
@@ -52,7 +51,7 @@
4.1) How do I SELECT only the first few rows of a query? A random row?
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?
4.3) How do you remove a column from a table, or change its data type?
4.3) How do you change a column's data type?
4.4) What is the maximum size for a row, a table, and a database?
4.5) How much database disk space is required to store data from a
typical text file?
@@ -69,19 +68,18 @@
4.11.3) Why aren't my sequence numbers reused on transaction abort?
Why are there gaps in the numbering of my sequence/SERIAL column?
4.12) What is an OID? What is a TID?
4.13) What is the meaning of some of the terms used in PostgreSQL?
4.14) Why do I get the error "ERROR: Memory exhausted in
4.13) Why do I get the error "ERROR: Memory exhausted in
AllocSetAlloc()"?
4.15) How do I tell what PostgreSQL version I am running?
4.16) Why does my large-object operations get "invalid large obj
4.14) How do I tell what PostgreSQL version I am running?
4.15) Why does my large-object operations get "invalid large obj
descriptor"?
4.17) How do I create a column that will default to the current time?
4.18) How do I perform an outer join?
4.19) How do I perform queries using multiple databases?
4.20) How do I return multiple rows or columns from a function?
4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL
4.16) How do I create a column that will default to the current time?
4.17) How do I perform an outer join?
4.18) How do I perform queries using multiple databases?
4.19) How do I return multiple rows or columns from a function?
4.20) Why can't I reliably create/drop temporary tables in PL/PgSQL
functions?
4.22) What encryption options are available?
4.21) What encryption options are available?
Extending PostgreSQL
@@ -98,38 +96,26 @@
1.1) What is PostgreSQL? How is it pronounced?
PostgreSQL is pronounced Post-Gres-Q-L.
PostgreSQL is pronounced Post-Gres-Q-L, also called just Postgres.
PostgreSQL is an enhancement of the POSTGRES database management
system (and is still sometimes reffered to as simply "Postgres"), 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 is an object-relational database system that has the
features of traditional commercial database systems with enhancements
to be found in next-generation DBMS systems. PostgreSQL is free and
the complete source code is available.
PostgreSQL development is performed by a team of developers who all
subscribe to the PostgreSQL development mailing list. The current
coordinator is Marc G. Fournier (scrappy@PostgreSQL.org). (See section
1.6 on how to join). This team is now responsible for all development
of PostgreSQL. It is a community project and is not controlled by any
PostgreSQL development is performed by a team of mostly volunteer
developers spread throughout the world and communicating via the
Internet. It is a community project and is not controlled by any
company. To get involved, see the developer's FAQ at
http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html
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?
1.2) What is the copyright of PostgreSQL?
PostgreSQL is subject to the following COPYRIGHT:
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
no intention of changing it.
This is the BSD license we use:
PostgreSQL Data Base Management System
@@ -155,10 +141,6 @@
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 platforms does PostgreSQL support?
In general, any modern Unix-compatible platform should be able to run
@@ -183,60 +165,40 @@
1.5) 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.
The PostgreSQL community provides assistance to many of its users via
email. The main web site to subscribe to the email lists is
http://www.postgresql.org/community/lists/. The general or bugs lists
are a good place to start.
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
The major IRC channel is #postgresql on Freenode (irc.freenode.net).
To connect you can use the Unix command irc -c '#postgresql' "$USER"
To connect you can use the Unix program irc -c '#postgresql' "$USER"
irc.freenode.net or use any of the other popular IRC clients. A
Spanish one also exists on the same network, (#postgresql-es), and a
French one, (#postgresqlfr). There is also a PostgreSQL channel on
EFNet.
A list of commercial support companies is available at
http://techdocs.postgresql.org/companies.php.
http://techdocs.postg resql.org/companies.php.
1.6) What is the latest release?
1.6) How do I submit a bug report?
Visit the PostgreSQL bug form at
http://www.postgresql.org/support/submitbug.
Also check out our ftp site ftp://ftp.PostgreSQL.org/pub to see if
there is a more recent PostgreSQL version.
1.7) What is the latest release?
The latest release of PostgreSQL is version 8.0.0.
We plan to have major releases every six to eight months.
We plan to have major releases every ten to twelve months.
1.7) What documentation is available?
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 manuals online at http://www.PostgreSQL.org/docs.
PostgreSQL includes extensive documentation, including a large manual,
manual pages, and some test examples. See the /doc directory. You can
also browse the manuals online at http://www.PostgreSQL.org/docs.
There are two PostgreSQL books available online at
http://www.PostgreSQL.org/docs/awbook.html and
@@ -252,12 +214,12 @@
Our web site contains even more documentation.
1.8) 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
for known bugs, missing features, and future plans.
1.9) How can I learn SQL?
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
@@ -273,10 +235,10 @@
et al., Addison-Wesley. Others like The Complete Reference SQL, Groff
et al., McGraw-Hill.
1.10) How do I join the development team?
1.11) 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,
FAQ and 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.
@@ -286,14 +248,6 @@
and we had confidence that patches they committed were of high
quality.
1.11) How do I submit a bug report?
Visit the PostgreSQL bug form at
http://www.postgresql.org/support/submitbug.
Also check out our ftp site ftp://ftp.PostgreSQL.org/pub to see if
there is a more recent PostgreSQL version or patches.
1.12) How does PostgreSQL compare to other DBMSs?
There are several ways of measuring software: features, performance,
@@ -340,29 +294,6 @@
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.
1.13) How can I financially assist PostgreSQL?
PostgreSQL has had a first-class infrastructure since we started in
1996. 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://store.pgsql.com/shopping/ 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.
Also, if you have a success story about PostgreSQL, please email it to
our advocacy list at pgsql-advocacy@postgresql.org.
_________________________________________________________________
User Client Questions
@@ -384,16 +315,16 @@
A nice introduction to Database-backed Web pages can be seen at:
http://www.webreview.com
For Web integration, PHP is an excellent interface. It is at
http://www.php.net.
For Web integration, PHP (http://www.php.net) is an excellent
interface.
For complex cases, many use the Perl interface and CGI.pm or mod_perl.
For complex cases, many use the Perl and CGI.pm or mod_perl.
2.3) Does PostgreSQL have a graphical user interface?
Yes, there are several graphical interfaces to PostgreSQL available.
These include PgAccess http://www.pgaccess.org), pgAdmin III
(http://www.pgadmin.org, RHDB Admin (http://sources.redhat.com/rhdb/
These include pgAdmin III (http://www.pgadmin.org, PgAccess
http://www.pgaccess.org), RHDB Admin (http://sources.redhat.com/rhd b/
), TORA (http://www.globecom.net/tora/, partly commercial), and Rekall
( http://www.rekallrevealed.org/). There is also PhpPgAdmin (
http://phppgadmin.sourceforge.net/ ), a web-based interface to
@@ -414,61 +345,58 @@
By default, PostgreSQL only allows connections from the local machine
using Unix domain sockets or TCP/IP connections. Other machines will
not be able to connect unless you modify listen_addresses in the
postgresql.conf and enable host-based authentication by modifying the
file $PGDATA/pg_hba.conf accordingly.
postgresql.conf file, enable host-based authentication by modifying
the $PGDATA/pg_hba.conf file, and restart the server.
3.3) How do I tune the database engine for better performance?
Certainly, indexes can speed up queries. The EXPLAIN ANALYZE 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 in the Administration Guide/Server
Run-time Environment/Run-time Configuration. You can disable fsync()
by using fsync option. This will prevent fsync()s from flushing to
disk after every transaction.
You can use the shared_buffers 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 1000 buffers.
You can also use the sort_mem (from PostgreSQL 8.0: work_mem) options
to increase the maximum amount of memory used by the backend processes
for each temporary sort. The default is 1024 (i.e. 1MB).
You can also use the CLUSTER command to group data in tables to match
an index. See the CLUSTER manual page for more details.
There are three major areas for potential performance improvement:
Query Changes
This involves modifying queries to obtain better performance:
+ Creation of indexes, including expression and partial indexes
+ Use of COPY instead of multiple INSERTs
+ Grouping of multiple statements into a single transaction to
reduce commit overhead
+ Use of CLUSTER when retrieving many rows from an index
+ Use of LIMIT for returning a subset of a query's output
+ Use of Prepared queries
+ Use of ANALYZE to maintain accurate optimizer statistics
+ Regular use of VACUUM or pg_autovacuum
+ Dropping of indexes during large data changes
Server Configuration
A number of postgresql.conf settings affect performance. For
more details, see Administration Guide/Server Run-time
Environment/Run-time Configuration for a full listing, and for
commentary see
http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_co
nf_e.html and
http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html.
Hardware Selection
The effect of hardware on performance is detailed in
http://candle.pha.pa.us/main/writings/pgsql/hw_performance/inde
x.html.
3.4) What debugging features are available?
PostgreSQL has several features that report status information that
can be valuable for debugging purposes.
There are many log_* server configuration variables that enable
printing of query and process statistics which can be very useful for
debugging and performance measurements.
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.
The following detailed debug instructions are to be used to provide
more detailed information for server developers debugging a problem.
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
It is also possible to debug the server if it isn't operating
properly. 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.
The 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
@@ -489,10 +417,6 @@
process with the debugger, set any breakpoints, and continue through
the startup sequence.
There are several log_* server configuration variables that enable
printing of process statistics which 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
@@ -501,23 +425,10 @@
3.5) 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.
You have reached the default limit is 100 database sessions. You need
to increase the postmaster's limit on how many concurrent backend
processes it can start by changing the max_connections value in
postgresql.conf and restarting the postmaster.
3.6) What is in the pgsql_tmp directory?
@@ -534,17 +445,13 @@
PostgreSQL releases?
The PostgreSQL team makes only small changes between minor releases,
so upgrading from 7.2 to 7.2.1 does not require a dump and restore.
However, major releases (e.g. from 7.2 to 7.3) often change the
so upgrading from 7.4 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
internal format of system tables and data files. These changes are
often complex, so we don't maintain backward compatability for data
often complex, so we don't maintain backward compatibility for data
files. A dump outputs data in a generic format that can then be loaded
in using the new internal format.
In releases where the on-disk format does not change, the pg_upgrade
script can be used to upgrade without a dump/restore. The release
notes mention whether pg_upgrade is available for the release.
3.8) What computer hardware should I use?
Because PC hardware is mostly compatible, people tend to believe that
@@ -560,13 +467,10 @@
4.1) How do I SELECT only the first few rows of a query? A random row?
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 using 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.
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
it is possible the entire query does not have to be executed. If you
don't know the number of rows at SELECT time, use a cursor and FETCH.
To SELECT a random row, use:
SELECT col
@@ -587,25 +491,16 @@
query to get information about the database.
There are also system tables beginning with pg_ that describe these
too. Use psql -l will list all databases.
too.
Use 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.
4.3) How do you remove a column from a table, or change its data type?
4.3) How do you change a column's data type?
DROP COLUMN functionality was added in release 7.3 with ALTER TABLE
DROP COLUMN. In earlier versions, you can 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;
Changing the data type of a column can be done easily in 8.0 and later
with ALTER TABLE ALTER COLUMN TYPE.
@@ -622,14 +517,16 @@
4.4) What is the maximum size for a row, a table, and a database?
These are the limits:
Maximum size for a database? unlimited (32 TB databases exist)
Maximum size for a table? 32 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
Maximum size for a database? unlimited (32 TB databases exist)
Maximum size for a table? 32 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.
@@ -716,15 +613,15 @@
* The search string can not start with a character class, e.g.
[a-e].
* Case-insensitive searches such as ILIKE and ~* do not utilize
indexes. Instead, use functional indexes, which are described in
section 4.10.
indexes. Instead, use expression indexes, which are described in
section 4.8.
* The default C locale must be used during initdb because it is not
possible to know the next-greater character in a non-C locale. You
can create a special text_pattern_ops index for such cases that
work only for LIKE indexing.
possible to know the next-greatest character in a non-C locale.
You can create a special text_pattern_ops index for such cases
that work only for LIKE indexing.
In pre-8.0 releases, indexes often can not be used unless the data
types exactly match the index's column types. This is particularly
types exactly match the index's column types. This was particularly
true of int2, int8, and numeric column indexes.
4.7) How do I see how the query optimizer is evaluating my query?
@@ -745,7 +642,7 @@
WHERE lower(col) = 'abc';
This will not use an standard index. However, if you create a
functional index, it will be used:
expresssion index, it will be used:
CREATE INDEX tabindex ON tab (lower(col));
4.9) In a query, how do I detect if a field is NULL?
@@ -754,14 +651,13 @@
4.10) What is the difference between the various character types?
Type Internal Name Notes
--------------------------------------------------
VARCHAR(n) varchar size specifies maximum length, no padding
CHAR(n) bpchar blank padded to the specified fixed length
TEXT text no specific upper limit on length
BYTEA bytea variable-length byte array (null-byte safe)
"char" char one character
Type Internal Name Notes
VARCHAR(n) varchar size specifies maximum length, no padding
CHAR(n) bpchar blank padded to the specified fixed length
TEXT text no specific upper limit on length
BYTEA bytea variable-length byte array (null-byte safe)
"char" char one character
You will see the internal name when examining system catalogs and in
some error messages.
@@ -797,9 +693,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
);
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.
sequences.
4.11.2) How do I get the value of a SERIAL insert?
@@ -821,16 +715,10 @@ BYTEA bytea variable-length byte array (null-byte safe)
execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
new_id = execute("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, and the oid value will wrap around when it reaches 4
billion. In Perl, using DBI with the DBD::Pg module, the oid value is
made available via $sth->{pg_oid_status} after $sth->execute().
4.11.3) Doesn't currval() lead to a race condition with other users?
No. currval() returns the current value assigned by your backend, not
by all users.
No. currval() returns the current value assigned by your session, not
by all sessions.
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?
@@ -856,25 +744,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
values. TIDs change after rows are modified or reloaded. They are used
by index entries to point to physical rows.
4.13) 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://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary
/glossary.html
4.14) 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
kernel has a low limit for certain resources. Try this before starting
@@ -889,11 +759,11 @@ BYTEA bytea variable-length byte array (null-byte safe)
problem with the SQL client because the backend is returning too much
data, try it before starting the client.
4.15) 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();
4.16) Why does my large-object operations get "invalid large obj
4.15) 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
@@ -908,12 +778,12 @@ BYTEA bytea variable-length byte array (null-byte safe)
If you are using a client interface like ODBC you may need to set
auto-commit off.
4.17) 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:
CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
4.18) 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
are two examples:
@@ -931,37 +801,25 @@ BYTEA bytea variable-length byte array (null-byte safe)
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
4.19) 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.
Because PostgreSQL loads database-specific system catalogs, it is
uncertain how a cross-database query should even behave.
contrib/dblink allows cross-database queries using function calls. Of
course, a client can make simultaneous connections to different
course, a client can also make simultaneous connections to different
databases and merge the results on the client side.
4.20) How do I return multiple rows or columns from a function?
4.19) How do I return multiple rows or columns from a function?
In 7.3, you can easily return multiple rows or columns from a
function, http://techdocs.postgresql.org/guides/SetReturningFunctions.
It is easy using set-returning functions,
http://techdocs.postgresql.org/guides/SetReturningFunctions.
4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL
4.20) Why can't I reliably create/drop temporary tables in PL/PgSQL
functions?
PL/PgSQL caches function contents, 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
is later dropped and recreated, and the function called again, the
function will fail because the cached function contents still point to
@@ -969,7 +827,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
table access in PL/PgSQL. This will cause the query to be reparsed
every time.
4.22) What encryption options are available?
4.21) What encryption options are available?
* contrib/pgcrypto contains many encryption functions for use in SQL
queries.
@@ -980,8 +838,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
possible to use a third-party encrypted transport, such as stunnel
or ssh, rather than PostgreSQL's native SSL connections.)
* Database user passwords are automatically encrypted when stored in
version 7.3. In previous versions, you must enable the option
PASSWORD_ENCRYPTION in postgresql.conf.
the system tables.
* The server can run using an encrypted file system.
_________________________________________________________________

609
doc/src/FAQ/FAQ.html
View File

@@ -10,7 +10,7 @@
alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Sat Jan 29 23:44:48 EST 2005</P>
<P>Last updated: Mon Jan 31 21:40:28 EST 2005</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)
@@ -27,21 +27,19 @@
<H2 align="center">General Questions</H2>
<A href="#1.1">1.1</A>) What is PostgreSQL? How is it pronounced?<BR>
<A href="#1.2">1.2</A>) What is the copyright on PostgreSQL?<BR>
<A href="#1.2">1.2</A>) What is the copyright of PostgreSQL?<BR>
<A href="#1.3">1.3</A>) What platforms does PostgreSQL support?<BR>
<A href="#1.4">1.4</A>) Where can I get PostgreSQL?<BR>
<A href="#1.5">1.5</A>) Where can I get support?<BR>
<A href="#1.6">1.6</A>) What is the latest release?<BR>
<A href="#1.7">1.7</A>) What documentation is available?<BR>
<A href="#1.8">1.8</A>) How do I find out about known bugs or
<A href="#1.6">1.6</A>) How do I submit a bug report?<BR>
<A href="#1.7">1.7</A>) What is the latest release?<BR>
<A href="#1.8">1.8</A>) What documentation is available?<BR>
<A href="#1.9">1.9</A>) How do I find out about known bugs or
missing features?<BR>
<A href="#1.9">1.9</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
<A href="#1.10">1.10</A>) How do I join the development team?<BR>
<A href="#1.11">1.11</A>) How do I submit a bug report?<BR>
<A href="#1.10">1.10</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
<A href="#1.11">1.11</A>) How do I join the development team?<BR>
<A href="#1.12">1.12</A>) How does PostgreSQL compare to other
<SMALL>DBMS</SMALL>s?<BR>
<A href="#1.13">1.13</A>) How can I financially assist
PostgreSQL?<BR>
<H2 align="center">User Client Questions</H2>
@@ -76,8 +74,7 @@
<A href="#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
by <I>psql</I> to display them?<BR>
<A href="#4.3">4.3</A>) How do you remove a column from a
table, or change its data type?<BR>
<A href="#4.3">4.3</A>) How do you change a column's data type?<BR>
<A href="#4.4">4.4</A>) What is the maximum size for a row, a
table, and a database?<BR>
<A href="#4.5">4.5</A>) How much database disk space is required
@@ -104,24 +101,22 @@
my sequence/SERIAL column?<BR>
<A href="#4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is a
<SMALL>TID</SMALL>?<BR>
<A href="#4.13">4.13</A>) What is the meaning of some of the terms
used in PostgreSQL?<BR>
<A href="#4.14">4.14</A>) Why do I get the error <I>"ERROR: Memory
<A href="#4.12">4.13</A>) Why do I get the error <I>"ERROR: Memory
exhausted in AllocSetAlloc()"</I>?<BR>
<A href="#4.15">4.15</A>) How do I tell what PostgreSQL version I
<A href="#4.14">4.14</A>) How do I tell what PostgreSQL version I
am running?<BR>
<A href="#4.16">4.16</A>) Why does my large-object operations get
<A href="#4.15">4.15</A>) Why does my large-object operations get
<I>"invalid large obj descriptor"</I>?<BR>
<A href="#4.17">4.17</A>) How do I create a column that will
<A href="#4.16">4.16</A>) How do I create a column that will
default to the current time?<BR>
<A href="#4.18">4.18</A>) How do I perform an outer join?<BR>
<A href="#4.19">4.19</A>) How do I perform queries using multiple
<A href="#4.17">4.17</A>) How do I perform an outer join?<BR>
<A href="#4.18">4.18</A>) How do I perform queries using multiple
databases?<BR>
<A href="#4.20">4.20</A>) How do I return multiple rows or columns
<A href="#4.19">4.19</A>) How do I return multiple rows or columns
from a function?<BR>
<A href="#4.21">4.21</A>) Why can't I reliably create/drop
<A href="#4.20">4.20</A>) Why can't I reliably create/drop
temporary tables in PL/PgSQL functions?<BR>
<A href="#4.22">4.22</A>) What encryption options are available?<BR>
<A href="#4.21">4.21</A>) What encryption options are available?<BR>
<H2 align="center">Extending PostgreSQL</H2>
@@ -140,45 +135,31 @@
<H4><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H4>
<P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>.</P>
<P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>, also called just
<I>Postgres</I>.</P>
<P>PostgreSQL is an enhancement of the POSTGRES database management
system (and is still sometimes reffered to as simply "Postgres"),
a next-generation <SMALL>DBMS</SMALL> 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 <SMALL>SQL</SMALL>. PostgreSQL is free and the
complete source is available.</P>
<P>PostgreSQL is an object-relational database system that has the
features of traditional commercial database systems with
enhancements to be found in next-generation <SMALL>DBMS</SMALL>
systems. PostgreSQL is free and the complete source code is
available.</P>
<P>PostgreSQL development is performed by a team of
developers who all subscribe to the PostgreSQL development mailing
list. The current coordinator is Marc G. Fournier (<A href=
"mailto:scrappy@PostgreSQL.org">scrappy@PostgreSQL.org</A>). (See
section <a href="#1.6">1.6</a> on how to join). This team is now
responsible for all development of PostgreSQL. It is a community
project and is not controlled by any company. To get involved, see
the developer's FAQ at <A href=
<P>PostgreSQL development is performed by a team of mostly volunteer
developers spread throughout the world and communicating via the
Internet. It is a community project and is not controlled by any
company. To get involved, see the developer's FAQ at <A href=
"http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html">
http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html</A>
</P>
<P>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.</P>
<P>The original name of the software at Berkeley was Postgres. When
<SMALL>SQL</SMALL> functionality was added in 1995, its name was
changed to Postgres95. The name was changed at the end of 1996 to
PostgreSQL.</P>
<H4><A name="1.2">1.2</A>) What is the copyright on
<H4><A name="1.2">1.2</A>) What is the copyright of
PostgreSQL?</H4>
<P>PostgreSQL is subject to the following COPYRIGHT:</P>
<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
have no intention of changing it.</P>
<P>This is the BSD license we use:</P>
<P>PostgreSQL Data Base Management System</P>
@@ -204,10 +185,6 @@
UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
<P>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.</P>
<H4><A name="1.3">1.3</A>) What platforms does PostgreSQL support?</H4>
<P>In general, any modern Unix-compatible platform should be able to
@@ -236,78 +213,46 @@
<H4><A name="1.5">1.5</A>) Where can I get support?</H4>
<P>The main mailing list is: <A href=
"mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>.
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):</P>
<PRE>
subscribe
end
</PRE>
<P>to <A href=
"mailto:pgsql-general-request@PostgreSQL.org">pgsql-general-request@PostgreSQL.org</A>.</P>
<P>There is also a digest list available. To subscribe to this
list, send email to: <A href=
"mailto:pgsql-general-digest-request@PostgreSQL.org">pgsql-general-digest-request@PostgreSQL.org</A>
with a body of:</P>
<PRE>
subscribe
end
</PRE>
Digests are sent out to members of this list whenever the main list
has received around 30k of messages.
<P>The bugs mailing list is available. To subscribe to this list,
send email to <A href=
"mailto:pgsql-bugs-request@PostgreSQL.org">pgsql-bugs-request@PostgreSQL.org</A>
with a body of:</P>
<PRE>
subscribe
end
</PRE>
There is also a developers discussion mailing list available. To
subscribe to this list, send email to <A href=
"mailto:pgsql-hackers-request@PostgreSQL.org">pgsql-hackers-request@PostgreSQL.org</A>
with a body of:
<PRE>
subscribe
end
</PRE>
<P>Additional mailing lists and information about PostgreSQL can be
found via the PostgreSQL WWW home page at:</P>
<BLOCKQUOTE>
<A href="http://www.PostgreSQL.org">http://www.PostgreSQL.org</A>
</BLOCKQUOTE>
<P>The PostgreSQL community provides assistance to many of its users
via email. The main web site to subscribe to the email lists is
<a href="http://www.postgresql.org/community/lists/">
http://www.postgresql.org/community/lists/</a>. The <I>general</I>
or <I>bugs</I> lists are a good place to start.
<P>The major IRC channel is <I>#postgresql</I> on Freenode
(<I>irc.freenode.net</I>). To connect you can use the Unix
command <CODE>irc -c '#postgresql' "$USER" irc.freenode.net</CODE>
program <CODE>irc -c '#postgresql' "$USER" irc.freenode.net</CODE>
or use any of the other popular IRC clients. A Spanish one also exists
on the same network, (<I>#postgresql-es</I>), and a French one,
(<I>#postgresqlfr</I>). There is also a PostgreSQL channel on EFNet.
<P>A list of commercial support companies is available at <A href=
"http://techdocs.postgresql.org/companies.php">http://techdocs.postgresql.org/companies.php</A>.</P>
"http://techdocs.postgresql.org/companies.php">http://techdocs.postg
resql.org/companies.php</A>.</P>
<H4><A name="1.6">1.6</A>) What is the latest release?</H4>
<H4><A name="1.6">1.6</A>) How do I submit a bug report?</H4>
<P>Visit the PostgreSQL bug form at <A href=
"http://www.postgresql.org/support/submitbug">
http://www.postgresql.org/support/submitbug</A>.</P>
<P>Also check out our ftp site <A href=
"ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
see if there is a more recent PostgreSQL version.</P>
<H4><A name="1.7">1.7</A>) What is the latest release?</H4>
<P>The latest release of PostgreSQL is version 8.0.0.</P>
<P>We plan to have major releases every six to eight months.</P>
<P>We plan to have major releases every ten to twelve months.</P>
<H4><A name="1.7">1.7</A>) What documentation is available?</H4>
<H4><A name="1.8">1.8</A>) What documentation is available?</H4>
<P>Several manuals, manual pages, and some small test examples are
included in the distribution. See the <I>/doc</I> directory. You
can also browse the manuals online at <A href=
"http://www.PostgreSQL.org/docs">http://www.PostgreSQL.org/docs</A>.</P>
<P>PostgreSQL includes extensive documentation, including a large
manual, manual pages, and some test examples. See the <I>/doc</I>
directory. You can also browse the manuals online at <A href=
"http://www.PostgreSQL.org/docs">http://www.PostgreSQL.org/docs</A>.
</P>
<P>There are two PostgreSQL books available online at <A href=
"http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
@@ -326,14 +271,14 @@
<P>Our web site contains even more documentation.</P>
<H4><A name="1.8">1.8</A>) How do I find out about known bugs or
<H4><A name="1.9">1.9</A>) How do I find out about known bugs or
missing features?</H4>
<P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
See our <A href="http://developer.PostgreSQL.org/todo.php">TODO</A>
list for known bugs, missing features, and future plans.</P>
<H4><A name="1.9">1.9</A>) How can I learn
<H4><A name="1.10">1.10</A>) How can I learn
<SMALL>SQL</SMALL>?</H4>
<P>The PostgreSQL book at <A href=
@@ -357,12 +302,12 @@
Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
<H4><A name="1.10">1.10</A>) How do I join the development
<H4><A name="1.11">1.11</A>) How do I join the development
team?</H4>
<P>First, download the latest source and read the PostgreSQL
Developers documentation on our web site, or in the distribution.
Second, subscribe to the <I>pgsql-hackers</I> and
Developers FAQ and documentation on our web site, or in the
distribution. Second, subscribe to the <I>pgsql-hackers</I> and
<I>pgsql-patches</I> mailing lists. Third, submit high quality
patches to pgsql-patches.</P>
@@ -372,16 +317,6 @@
committers to keep up, and we had confidence that patches they
committed were of high quality.</P>
<H4><A name="1.11">1.11</A>) How do I submit a bug report?</H4>
<P>Visit the PostgreSQL bug form at <A href=
"http://www.postgresql.org/support/submitbug">
http://www.postgresql.org/support/submitbug</A>.</P>
<P>Also check out our ftp site <A href=
"ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
see if there is a more recent PostgreSQL version or patches.</P>
<H4><A name="1.12">1.12</A>) How does PostgreSQL compare to other
<SMALL>DBMS</SMALL>s?</H4>
@@ -448,31 +383,6 @@
</DD>
</DL>
<H4><A name="1.13">1.13</A>) How can I financially assist
PostgreSQL?</H4>
<P>PostgreSQL has had a first-class infrastructure since we started
in 1996. This is all thanks to Marc Fournier, who has created
and managed this infrastructure over the years.</P>
<P>Quality infrastructure is very important to an open-source
project. It prevents disruptions that can greatly delay forward
movement of the project.</P>
<P>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 <A href="http://store.pgsql.com/shopping/">http://store.pgsql.com/shopping/</A>
and make a donation.</P>
<P>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.</P>
<P>Also, if you have a success story about PostgreSQL, please email
it to our advocacy list at <a href="mailto:pgsql-advocacy@postgresql.org">
pgsql-advocacy@postgresql.org</a>.</P>
<HR>
@@ -499,27 +409,33 @@
<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>
<P>For Web integration, PHP is an excellent interface. It is at <A
href="http://www.php.net">http://www.php.net</A>.</P>
<P>For Web integration, PHP (<A
href="http://www.php.net">http://www.php.net</A>) is an excellent
interface.</P>
<P>For complex cases, many use the Perl interface and CGI.pm or mod_perl.</P>
<P>For complex cases, many use the Perl and CGI.pm or mod_perl.</P>
<H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
interface?</H4>
<P>Yes, there are several graphical interfaces to PostgreSQL available.
These include PgAccess <a href="http://www.pgaccess.org">
http://www.pgaccess.org</a>), pgAdmin III (<a
href="http://www.pgadmin.org">http://www.pgadmin.org</a>, RHDB Admin (<a
href="http://sources.redhat.com/rhdb/">http://sources.redhat.com/rhdb/
</a>), TORA (<a href="http://www.globecom.net/tora/">http://www.globecom.net/tora/</a>,
partly commercial), and Rekall (<a href="http://www.rekallrevealed.org/">
http://www.rekallrevealed.org/</a>). There is also PhpPgAdmin
(<a href="http://phppgadmin.sourceforge.net/">
http://phppgadmin.sourceforge.net/ </a>), a web-based interface to
<P>Yes, there are several graphical interfaces to PostgreSQL
available. These include pgAdmin III (<a
href="http://www.pgadmin.org">http://www.pgadmin.org</a>, PgAccess
<a href="http://www.pgaccess.org"> http://www.pgaccess.org</a>),
RHDB Admin (<a
href="http://sources.redhat.com/rhdb/">http://sources.redhat.com/rhd
b/ </a>), TORA (<a
href="http://www.globecom.net/tora/">http://www.globecom.net/tora/</a>,
partly commercial), and Rekall (<a
href="http://www.rekallrevealed.org/">
http://www.rekallrevealed.org/</a>). There is also PhpPgAdmin (<a
href="http://phppgadmin.sourceforge.net/">
http://phppgadmin.sourceforge.net/ </a>), a web-based interface to
PostgreSQL.</P>
<P>See <a href="http://techdocs.postgresql.org/guides/GUITools">http://techdocs.postgresql.org/guides/GUITools</a> for a more detailed list.</P>
<P>See <a href="http://techdocs.postgresql.org/guides/GUITools">
http://techdocs.postgresql.org/guides/GUITools</a> for a more
detailed list.</P>
<HR>
@@ -538,75 +454,85 @@
<P>By default, PostgreSQL only allows connections from the local
machine using Unix domain sockets or TCP/IP connections. Other
machines will not be able to connect unless you modify
listen_addresses in the postgresql.conf <B>and</B> enable
host-based authentication by modifying the file
<I>$PGDATA/pg_hba.conf</I> accordingly.</P>
<I>listen_addresses</I> in the <I>postgresql.conf</I> file, enable
host-based authentication by modifying the
<I>$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
better performance?</H4>
<P>Certainly, indexes can speed up queries. The
<SMALL>EXPLAIN ANALYZE</SMALL> command allows you to see how
PostgreSQL is interpreting your query, and which indexes are
being used.</P>
<P>There are three major areas for potential performance
improvement:</P>
<DL>
<DT><B>Query Changes</B></DT>
<P>If you are doing many <SMALL>INSERTs</SMALL>, consider doing
them in a large batch using the <SMALL>COPY</SMALL> command. This
is much faster than individual <SMALL>INSERTS</SMALL>. Second,
statements not in a <SMALL>BEGIN WORK/COMMIT</SMALL> 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.</P>
<DD>This involves modifying queries to obtain better
performance:
<ul>
<li>Creation of indexes, including expression and partial
indexes</li>
<li>Use of COPY instead of multiple <SMALL>INSERT</SMALL>s</li>
<li>Grouping of multiple statements into a single transaction to
reduce commit overhead</li>
<li>Use of <SMALL>CLUSTER</SMALL> when retrieving many rows from an
index</li>
<li>Use of <SMALL>LIMIT</SMALL> for returning a subset of a query's
output</li>
<li>Use of Prepared queries</li>
<li>Use of <SMALL>ANALYZE</SMALL> to maintain accurate optimizer
statistics</li>
<li>Regular use of <SMALL>VACUUM</SMALL> or <I>pg_autovacuum</I>
<li>Dropping of indexes during large data changes</li>
</ul><BR>
<BR>
</DD>
<P>There are several tuning options in the <a href=
<DT><B>Server Configuration</B></DT>
<DD>A number of <I>postgresql.conf</I> settings affect performance.
For more details, see <a href=
"http://www.postgresql.org/docs/current/static/runtime.html">
Administration Guide/Server Run-time Environment/Run-time Configuration</a>.
You can disable <I>fsync()</I> by using <i>fsync</I> option. This will
prevent <I>fsync()</I>s from flushing to disk after every
transaction.</P>
Administration Guide/Server Run-time Environment/Run-time
Configuration</a> for a full listing, and for commentary see <a
href="http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_conf_e.html">
http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_conf_e.html</a>
and <a href="http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html">
http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html</a>.
<BR>
<BR>
</DD>
<P>You can use the <I>shared_buffers</I> option to
increase the number of shared memory buffers used by the backend
processes. If you make this parameter too high, the
<I>postmaster</I> may not start because you have exceeded your
kernel's limit on shared memory space. Each buffer is 8K and the
default is 1000 buffers.</P>
<DT><B>Hardware Selection</B></DT>
<P>You can also use the <I>sort_mem</I> (from PostgreSQL 8.0: <I>work_mem</I>)
options to increase the maximum amount of memory used by the backend
processes for each temporary sort. The default is 1024 (i.e. 1MB).</P>
<P>You can also use the <SMALL>CLUSTER</SMALL> command to group
data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
manual page for more details.</P>
<DD>The effect of hardware on performance is detailed in <a
href="http://candle.pha.pa.us/main/writings/pgsql/hw_performance/index.html">
http://candle.pha.pa.us/main/writings/pgsql/hw_performance/index.html</a>.
<BR>
<BR>
</DD>
</DL>
<H4><A name="3.4">3.4</A>) What debugging features are
available?</H4>
<P>PostgreSQL has several features that report status information
that can be valuable for debugging purposes.</P>
<P>There are many <CODE>log_*</CODE> server configuration variables
that enable printing of query and process statistics which can be
very useful for debugging and performance measurements.</P>
<P>First, by running <I>configure</I> with the --enable-cassert
<P><B>The following detailed debug instructions are to be used to
provide more detailed information for server developers debugging a
problem.</B></P>
<P>It is also possible to debug the server if it isn't operating
properly. First, by running <I>configure</I> with the --enable-cassert
option, many <I>assert()</I>s monitor the progress of the backend
and halt the program when something unexpected occurs.</P>
<P>Both <I>postmaster</I> and <I>postgres</I> have several debug
options available. First, whenever you start <I>postmaster</I>,
make sure you send the standard output and error to a log file,
like:</P>
<PRE>
cd /usr/local/pgsql
./bin/postmaster &gt;server.log 2&gt;&amp;1 &amp;
</PRE>
<P>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. <I>Postmaster</I> has a <I>-d</I>
option that allows even more detailed information to be reported.
The <I>-d</I> option takes a number that specifies the debug level.
Be warned that high debug level values generate large log
files.</P>
<P>The <I>postmaster</I> has a <I>-d</I> option that allows even more
detailed information to be reported. The <I>-d</I> option takes a
number that specifies the debug level. Be warned that high debug
level values generate large log files.</P>
<P>If <I>postmaster</I> is not running, you can actually run the
<I>postgres</I> backend from the command line, and type your
@@ -630,10 +556,6 @@
the debugger, set any breakpoints, and continue through the startup
sequence.</P>
<P>There are several <CODE>log_*</CODE> server configuration variables
that enable printing of process statistics which can be very useful
for debugging and performance measurements.</P>
<P>You can also compile with profiling to see what functions are
taking execution time. The backend profile files will be deposited
in the <I>pgsql/data/base/dbname</I> directory. The client profile
@@ -643,27 +565,11 @@
<H4><A name="3.5">3.5</A>) Why do I get <I>"Sorry, too many
clients"</I> when trying to connect?</H4>
<P>You need to increase <I>postmaster</I>'s limit on how many
concurrent backend processes it can start.</P>
<P>The default limit is 32 processes. You can increase it by
restarting <I>postmaster</I> with a suitable <I>-N</I> value or
modifying <I>postgresql.conf</I>.</P>
<P>Note that if you make <I>-N</I> larger than 32, you must also
increase <I>-B</I> beyond its default of 64; <I>-B</I> must be at
least twice <I>-N</I>, 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, <SMALL>SHMMAX;</SMALL> the maximum number
of semaphores, <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI;</SMALL> the
maximum number of processes, <SMALL>NPROC;</SMALL> the maximum
number of processes per user, <SMALL>MAXUPRC;</SMALL> and the
maximum number of open files, <SMALL>NFILE</SMALL> and
<SMALL>NINODE</SMALL>. The reason that PostgreSQL has a limit on
the number of allowed backend processes is so your system won't run
out of resources.</P>
<P>You have reached the default limit is 100 database sessions. You
need to increase the <I>postmaster</I>'s limit on how many
concurrent backend processes it can start by changing the
<I>max_connections</I> value in <I>postgresql.conf</I> and
restarting the <I>postmaster</I>.</P>
<H4><A name="3.6">3.6</A>) What is in the <I>pgsql_tmp</I> directory?</H4>
@@ -681,18 +587,13 @@
to upgrade between major PostgreSQL releases?</H4>
<P>The PostgreSQL team makes only small changes between minor releases,
so upgrading from 7.2 to 7.2.1 does not require a dump and restore.
However, major releases (e.g. from 7.2 to 7.3) often change the internal
so upgrading from 7.4 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 internal
format of system tables and data files. These changes are often complex,
so we don't maintain backward compatability for data files. A dump outputs
so we don't maintain backward compatibility for data files. A dump outputs
data in a generic format that can then be loaded in using the new internal
format.</P>
<P>In releases where the on-disk format does not change, the
<I>pg_upgrade</I> script can be used to upgrade without a dump/restore.
The release notes mention whether <I>pg_upgrade</I> is available for the
release.</P>
<H4><A name="3.8">3.8</A>) What computer hardware should I use?</H4>
<P>Because PC hardware is mostly compatible, people tend to believe that
@@ -710,15 +611,13 @@
<H4><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>
<P>See the <SMALL>FETCH</SMALL> manual page, or use
<SMALL>SELECT</SMALL> ... <SMALL>LIMIT</SMALL>....</P>
<P>The entire query may have to be evaluated, even if you only want
the first few rows. Consider using a query that has an <SMALL>ORDER
BY</SMALL>. If there is an index that matches the <SMALL>ORDER
BY</SMALL>, 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.</P>
<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
<SMALL>LIMIT</SMALL> . If an index matches the <SMALL>ORDER
BY</SMALL> it is possible the entire query does not have to be
executed. If you don't know the number of rows at
<SMALL>SELECT</SMALL> time, use a cursor and
<SMALL>FETCH</SMALL>.</P>
<P>To <SMALL>SELECT</SMALL> a random row, use:
<PRE>
@@ -743,28 +642,15 @@
database.</P>
<P>There are also system tables beginning with <I>pg_</I> that describe
these too. Use <I>psql -l</I> will list all databases.</P>
these too.</P>
<P>Use <I>psql -l</I> will list all databases.</P>
<P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
illustrates many of the <SMALL>SELECT</SMALL>s needed to get
information from the database system tables.</P>
<H4><A name="4.3">4.3</A>) How do you remove a column from a
table, or change its data type?</H4>
<P><SMALL>DROP COLUMN</SMALL> functionality was added in release 7.3
with <SMALL>ALTER TABLE DROP COLUMN</SMALL>. In earlier versions,
you can do this:</P>
<PRE>
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;
</PRE>
<H4><A name="4.3">4.3</A>) How do you change a column's data type?</H4>
<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>.
@@ -784,19 +670,25 @@
table, and a database?</H4>
<P>These are the limits:</P>
<PRE>
Maximum size for a database? unlimited (32 TB databases exist)
Maximum size for a table? 32 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
</PRE>
<CENTER>
<TABLE BORDER=1>
<TR><TD>Maximum size for a database?</TD><TD>unlimited (32 TB databases
exist)</TD></TR>
<TR><TD>Maximum size for a table?</TD><TD>32 TB</TD></TR>
<TR><TD>Maximum size for a row?</TD><TD>1.6TB</TD></TR>
<TR><TD>Maximum size for a field?</TD><TD>1 GB</TD></TR>
<TR><TD>Maximum number of rows in a table?</TD><TD>unlimited</TD></TR>
<TR><TD>Maximum number of columns in a table?</TD><TD>250-1600 depending
on column types</TD></TR>
<TR><TD>Maximum number of indexes on a
table?</TD><TD>unlimited</TD></TR>
</TABLE>
</CENTER>
<BR>
Of course, these are not actually unlimited, but limited to
<P>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.
when these values get unusually large.</P>
<P>The maximum table size of 32 TB does not require large file
support from the operating system. Large tables are stored as
@@ -893,10 +785,10 @@
<LI>The search string can not start with a character class,
e.g. [a-e].</LI>
<LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
<I>~*</I> do not utilize indexes. Instead, use functional
indexes, which are described in section <a href="#4.10">4.10</a>.</LI>
<I>~*</I> do not utilize indexes. Instead, use expression
indexes, which are described in section <a href="#4.8">4.8</a>.</LI>
<LI>The default <I>C</I> locale must be used during
<i>initdb</i> because it is not possible to know the next-greater
<i>initdb</i> because it is not possible to know the next-greatest
character in a non-C locale. You can create a special
<CODE>text_pattern_ops</CODE> index for such cases that work only
for <SMALL>LIKE</SMALL> indexing.
@@ -904,7 +796,7 @@
</UL>
<P>In pre-8.0 releases, indexes often can not be used unless the data
types exactly match the index's column types. This is particularly
types exactly match the index's column types. This was particularly
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
@@ -930,7 +822,7 @@
</PRE>
This will not use an standard index. However, if you create a
functional index, it will be used:
expresssion index, it will be used:
<PRE>
CREATE INDEX tabindex ON tab (lower(col));
</PRE>
@@ -943,16 +835,20 @@
<H4><A name="4.10">4.10</A>) What is the difference between the
various character types?</H4>
<PRE>
Type Internal Name Notes
--------------------------------------------------
VARCHAR(n) varchar size specifies maximum length, no padding
CHAR(n) bpchar blank padded to the specified fixed length
TEXT text no specific upper limit on length
BYTEA bytea variable-length byte array (null-byte safe)
"char" char one character
</PRE>
<CENTER>
<TABLE BORDER=1>
<TR><TH>Type</TH><TH>Internal Name</TH><TH>Notes</TH></TR>
<TR><TD>VARCHAR(n)</TD><TD>varchar</TD><TD>size specifies maximum
length, no padding</TD></TR>
<TR><TD>CHAR(n)</TD><TD>bpchar</TD><TD>blank padded to the specified
fixed length</TD></TR>
<TR><TD>TEXT</TD><TD>text</TD><TD>no specific upper limit on
length</TD></TR>
<TR><TD>BYTEA</TD><TD>bytea</TD><TD>variable-length byte array
(null-byte safe)</TD></TR>
<TR><TD>"char"</TD><TD>char</TD><TD>one character</TD></TR>
</TABLE>
</CENTER>
<P>You will see the internal name when examining system catalogs
and in some error messages.</P>
@@ -996,10 +892,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
</PRE>
See the <I>create_sequence</I> manual page for more information
about sequences. You can also use each row's <I>OID</I> field as a
unique value. However, if you need to dump and reload the database,
you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
about sequences.
<H4><A name="4.11.2">4.11.2</A>) How do I get the value of a
<SMALL>SERIAL</SMALL> insert?</H4>
@@ -1030,19 +923,11 @@ BYTEA bytea variable-length byte array (null-byte safe)
new_id = execute("SELECT currval('person_id_seq')");
</PRE>
<P>Finally, you could use the <A href="#4.12"><SMALL>OID</SMALL></A>
returned from the <SMALL>INSERT</SMALL> statement to look up the
default value, though this is probably the least portable approach,
and the oid value will wrap around when it reaches 4 billion.
In Perl, using DBI with the DBD::Pg module, the oid value is made
available via <I>$sth-&gt;{pg_oid_status}</I> after
<I>$sth-&gt;execute()</I>.</P>
<H4><A name="4.11.3">4.11.3</A>) Doesn't <I>currval()</I>
lead to a race condition with other users?</H4>
<P>No. <I>currval()</I> returns the current value assigned by your
backend, not by all users.</P>
session, not by all sessions.</P>
<H4><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
@@ -1076,36 +961,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
are modified or reloaded. They are used by index entries to point
to physical rows.</P>
<H4><A name="4.13">4.13</A>) What is the meaning of some of the
terms used in PostgreSQL?</H4>
<P>Some of the source code and older documentation use terms that
have more common usage. Here are some:</P>
<UL>
<LI>table, relation, class</LI>
<LI>row, record, tuple</LI>
<LI>column, field, attribute</LI>
<LI>retrieve, select</LI>
<LI>replace, update</LI>
<LI>append, insert</LI>
<LI><SMALL>OID</SMALL>, serial value</LI>
<LI>portal, cursor</LI>
<LI>range variable, table name, table alias</LI>
</UL>
<P>A list of general database terms can be found at: <A href=
"http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html">http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html</A></P>
<H4><A name="4.14">4.14</A>) Why do I get the error <I>"ERROR:
<H4><A name="4.13">4.13</A>) Why do I get the error <I>"ERROR:
Memory exhausted in AllocSetAlloc()"</I>?</H4>
<P>You probably have run out of virtual memory on your system,
@@ -1124,12 +980,12 @@ BYTEA bytea variable-length byte array (null-byte safe)
backend is returning too much data, try it before starting the
client.
<H4><A name="4.15">4.15</A>) How do I tell what PostgreSQL version
<H4><A name="4.14">4.14</A>) How do I tell what PostgreSQL version
I am running?</H4>
<P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
<H4><A name="4.16">4.16</A>) Why does my large-object operations
<H4><A name="4.15">4.15</A>) Why does my large-object operations
get <I>"invalid large obj descriptor"</I>?</H4>
<P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
@@ -1145,15 +1001,15 @@ BYTEA bytea variable-length byte array (null-byte safe)
<P>If you are using a client interface like <SMALL>ODBC</SMALL> you
may need to set <CODE>auto-commit off.</CODE></P>
<H4><A name="4.17">4.17</A>) How do I create a column that will
<H4><A name="4.16">4.16</A>) How do I create a column that will
default to the current time?</H4>
<P>Use <I>CURRENT_TIMESTAMP</I>:</P>
<PRE>
CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
</PRE>
<H4><A name="4.18">4.18</A>) How do I perform an outer join?</H4>
<H4><A name="4.17">4.17</A>) How do I perform an outer join?</H4>
<P>PostgreSQL supports outer joins using the SQL standard syntax.
Here are two examples:</P>
@@ -1176,24 +1032,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
<SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
are called <SMALL>INNER</SMALL> joins.</P>
<P>In previous releases, outer joins can be simulated using
<SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when
joining <I>tab1</I> and <I>tab2</I>, the following query does an
<I>outer</I> join of the two tables:<BR>
<BR>
</P>
<PRE>
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
</PRE>
<H4><A name="4.19">4.19</A>) How do I perform queries using
<H4><A name="4.18">4.18</A>) How do I perform queries using
multiple databases?</H4>
<P>There is no way to query a database other than the current one.
@@ -1201,29 +1040,29 @@ BYTEA bytea variable-length byte array (null-byte safe)
uncertain how a cross-database query should even behave.</P>
<P><I>contrib/dblink</I> allows cross-database queries using
function calls. Of course, a client can make simultaneous
function calls. Of course, a client can also make simultaneous
connections to different databases and merge the results on the
client side.</P>
<H4><A name="4.20">4.20</A>) How do I return multiple rows or
<H4><A name="4.19">4.19</A>) How do I return multiple rows or
columns from a function?</H4>
<P>In 7.3, you can easily return multiple rows or columns from a
function,
<P>It is easy using set-returning functions,
<a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
http://techdocs.postgresql.org/guides/SetReturningFunctions</a>.
<H4><A name="4.21">4.21</A>) Why can't I reliably create/drop
<H4><A name="4.20">4.20</A>) Why can't I reliably create/drop
temporary tables in PL/PgSQL functions?</H4>
<P>PL/PgSQL caches function contents, 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
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
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
<SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
will cause the query to be reparsed every time.</P>
<H4><A name="4.22">4.22</A>) What encryption options are available?
<H4><A name="4.21">4.21</A>) What encryption options are available?
</H4>
<UL>
<LI><I>contrib/pgcrypto</I> contains many encryption functions for
@@ -1236,8 +1075,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
encrypted transport, such as stunnel or ssh, rather than PostgreSQL's
native SSL connections.)
<LI>Database user passwords are automatically encrypted when stored in
version 7.3. In previous versions, you must enable the option
<I>PASSWORD_ENCRYPTION</I> in <I>postgresql.conf</I>.</LI>
the system tables.</LI>
<LI>The server can run using an encrypted file system.</LI>
</UL>
@@ -1277,4 +1115,3 @@ BYTEA bytea variable-length byte array (null-byte safe)
compiler compute the dependencies automatically.</P>
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