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FAQ updates from Ian Barwick.

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Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Fri Oct 11 13:54:56 EDT 2002
Last updated: Sun Oct 13 22:49:56 EDT 2002
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
@ -112,7 +112,7 @@
General Questions
1.1) What is PostgreSQL?
1.1) What is PostgreSQL? How is it pronounced?
PostgreSQL is pronounced Post-Gres-Q-L.
@ -122,11 +122,11 @@
replaces the PostQuel query language with an extended subset of SQL.
PostgreSQL is free and the complete source is available.
PostgreSQL development is performed by a team of Internet developers
who all subscribe to the PostgreSQL development mailing list. The
current coordinator is Marc G. Fournier (scrappy@PostgreSQL.org). (See
below on how to join). This team is now responsible for all
development of PostgreSQL.
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.
The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen. Many
others have contributed to the porting, testing, debugging, and
@ -183,18 +183,22 @@
Client
It is possible to compile the libpq C library, psql, and other
interfaces and binaries to run on MS Windows platforms. In this case,
the client is running on MS Windows, and communicates via TCP/IP to a
server running on one of our supported Unix platforms. A file
win31.mak is included in the distribution for making a Win32 libpq
library and psql. PostgreSQL also communicates with ODBC clients.
interfaces and client applications to run on MS Windows platforms. In
this case, the client is running on MS Windows, and communicates via
TCP/IP to a server running on one of our supported Unix platforms. A
file win32.mak is included in the distribution for making a Win32
libpq library and psql. PostgreSQL also communicates with ODBC
clients.
Server
The database server can run on Windows NT and Win2k using Cygwin, the
Cygnus Unix/NT porting library. See pgsql/doc/FAQ_MSWIN in the
distribution or the MS Windows FAQ on our web site. We have no plan to
do a native port to any Microsoft platform.
distribution or the MS Windows FAQ at
http://www.PostgreSQL.org/docs/faq-mswin.html.
A native port to some Microsoft platforms is currently being worked
upon.
1.5) Where can I get PostgreSQL?
@ -240,7 +244,7 @@
Unix command irc -c '#PostgreSQL' "$USER" irc.phoenix.net.
A list of commercial support companies is available at
http://www.postgresql.org/users-lounge/commercial-support.html.
http://www.PostgreSQL.org/users-lounge/commercial-support.html.
1.7) What is the latest release?
@ -258,9 +262,9 @@
There are two PostgreSQL books available online at
http://www.PostgreSQL.org/docs/awbook.html and
http://www.commandprompt.com/ppbook/. There is a list of PostgreSQL
books available for purchase at http://www.postgresql.org/books/.
books available for purchase at http://www.ca.PostgreSQL.org/books/.
There is also a collection of PostgreSQL technical articles at
http://techdocs.postgresql.org/.
http://techdocs.PostgreSQL.org/.
psql has some nice \d commands to show information about types,
operators, functions, aggregates, etc.
@ -307,8 +311,9 @@
1.13) How do I submit a bug report?
Please visit the PostgreSQL BugTool page, which gives guidelines and
directions on how to submit a bug.
Please visit the PostgreSQL BugTool page at
http://www.PostgreSQL.org/bugs/bugs.php, which gives guidelines and
directions on how to submit a bug report.
Also check out our ftp site ftp://ftp.PostgreSQL.org/pub to see if
there is a more recent PostgreSQL version or patches.
@ -346,14 +351,14 @@
compare favorably to other database software in this area.
Support
Our mailing list provides a large group of developers and users
to help resolve any problems encountered. While we can not
guarantee a fix, commercial DBMSs do not always supply a fix
either. Direct access to developers, the user community,
manuals, and the source code often make PostgreSQL support
superior to other DBMSs. There is commercial per-incident
support available for those who need it. (See support FAQ
item.)
Our mailing lists provide contact with a large group of
developers and users to help resolve any problems encountered.
While we cannot guarantee a fix, commercial DBMSs do not always
supply a fix either. Direct access to developers, the user
community, manuals, and the source code often make PostgreSQL
support superior to other DBMSs. There is commercial
per-incident support available for those who need it. (See FAQ
section 1.6.)
Price
We are free for all use, both commercial and non-commercial.
@ -362,9 +367,9 @@
1.15) How can I financially assist PostgreSQL?
PostgreSQL has had a first-class infrastructure since we started six
years ago. This is all thanks to Marc Fournier, who has created and
managed this infrastructure over the years.
PostgreSQL has had a first-class infrastructure since we started in
1994. 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
@ -373,8 +378,7 @@
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 https://store.pgsql.com/shopping/index.php?id=1 and make
a donation.
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
@ -407,39 +411,37 @@
A nice introduction to Database-backed Web pages can be seen at:
http://www.webreview.com
There is also one at http://www.phone.net/home/mwm/hotlist/.
For Web integration, PHP is an excellent interface. It is at
http://www.php.net.
For complex cases, many use the Perl interface and CGI.pm.
For complex cases, many use the Perl interface and CGI.pm or mod_perl.
2.3) Does PostgreSQL have a graphical user interface? A report generator?
An embedded query language interface?
We have a nice graphical user interface called pgaccess, which is
shipped as part of the distribution. pgaccess also has a report
generator. The Web page is http://www.flex.ro/pgaccess
We have a nice graphical user interface called PgAccess, which is
shipped as part of the distribution. PgAccess also has a report
generator. The Web page is http://www.pgaccess.org/.
We also include ecpg, which is an embedded SQL query language
interface for C.
2.4) What languages are available to communicate with PostgreSQL?
2.4) What languages are able to communicate with PostgreSQL?
We have:
* C (libpq)
* C++ (libpq++)
* Embedded C (ecpg)
* Java (jdbc)
* Perl (perl5)
* Perl (DBD::Pg)
* ODBC (odbc)
* Python (PyGreSQL)
* TCL (libpgtcl)
* C Easy API (libpgeasy)
* Embedded HTML (PHP from http://www.php.net)
* PHP ('pg_' functions, Pear::DB)
Additional interfaces are available at
http://www.postgresql.org/interfaces.html.
http://www.PostgreSQL.org/interfaces.html.
_________________________________________________________________
Administrative Questions
@ -594,7 +596,7 @@
was 64, and changing it required a rebuild after altering the
MaxBackendId constant in include/storage/sinvaladt.h.
3.9) What are the pg_sorttempNNN.NN files in my database directory?
3.9) What are the pg_tempNNN.NN files in my database directory?
They are temporary files generated by the query executor. For example,
if a sort needs to be done to satisfy an ORDER BY, and the sort
@ -610,11 +612,11 @@
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 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 data in
a generic format that can then be loaded in using the new internal
format.
However, major releases (e.g. from 7.2 to 7.3) 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 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
@ -632,10 +634,10 @@
See the FETCH manual page, or use SELECT ... LIMIT....
The entire query may have to be evaluated, even if you only want the
first few rows. Consider a query that has an ORDER BY. If there is an
index that matches the ORDER BY, PostgreSQL may be able to evaluate
only the first few records requested, or the entire query may have to
be evaluated until the desired rows have been generated.
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.
4.3) How do I get a list of tables or other things I can see in psql?
@ -647,7 +649,8 @@
4.4) How do you remove a column from a table?
We do not support ALTER TABLE DROP COLUMN, but do this:
Prior to version 7.3, ALTER TABLE DROP COLUMN is not supported. You
can do this instead:
BEGIN;
LOCK TABLE old_table;
SELECT ... -- select all columns but the one you want to remove
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PostgreSQL database file containing this data can be estimated as 6.4
MB:
36 bytes: each row header (approximate)
24 bytes: one int field and one text filed
24 bytes: one int field and one text field
+ 4 bytes: pointer on page to tuple
----------------------------------------
64 bytes per row
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SELECT col
FROM tab
ORDER BY col [ DESC ]
LIMIT 1
LIMIT 1;
When using wild-card operators such as LIKE or ~, indexes can only be
used in certain circumstances:
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The search string can not start with a character class, e.g. [a-e].
Case-insensitive searches like ILIKE and ~* can not be used.
Instead, use functional indexes, which are described later in this
FAQ.
Case-insensitive searches such as ILIKE and ~* do not utilise
indexes. Instead, use functional indexes, which are described in
section 4.12.
The default C local must be used during initdb.
The default C locale must be used during initdb.
4.9) How do I see how the query optimizer is evaluating my query?
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Case-insensitive equality comparisons are normally expressed as:
SELECT *
FROM tab
WHERE lower(col) = 'abc'
WHERE LOWER(col) = 'abc';
This will not use an standard index. However, if you create a
functional index, it will be used:
CREATE INDEX tabindex on tab (lower(col));
CREATE INDEX tabindex ON tab (LOWER(col));
4.13) In a query, how do I detect if a field is NULL?
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Type Internal Name Notes
--------------------------------------------------
"char" char 1 character
CHAR(#) bpchar blank padded to the specified fixed length
VARCHAR(#) varchar size specifies maximum length, no padding
CHAR(n) bpchar blank padded to the specified fixed length
VARCHAR(n) varchar size specifies maximum length, no padding
TEXT text no specific upper limit on length
BYTEA bytea variable-length byte array (null-byte safe)
@ -841,8 +844,8 @@ BYTEA bytea variable-length byte array (null-byte safe)
data types are also subject to compression or being stored out-of-line
by TOAST, so the space on disk might also be less than expected.
CHAR() is best when storing strings that are usually the same length.
VARCHAR() is best when storing variable-length strings but it limits
CHAR(n) is best when storing strings that are usually the same length.
VARCHAR(n) is best when storing variable-length strings but it limits
how long a string can be. TEXT is for strings of unlimited length,
maximum 1 gigabyte. BYTEA is for storing binary data, particularly
values that include NULL bytes.
@ -873,10 +876,10 @@ BYTEA bytea variable-length byte array (null-byte safe)
One approach is to retrieve the next SERIAL value from the sequence
object with the nextval() function before inserting and then insert it
explicitly. Using the example table in 4.15.1, that might look like
this in Perl:
new_id = output of "SELECT nextval('person_id_seq')"
INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal');
explicitly. Using the example table in 4.15.1, an example in a
pseudo-language would look like this:
new_id = execute("SELECT nextval('person_id_seq')");
execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
You would then also have the new value stored in new_id for use in
other queries (e.g., as a foreign key to the person table). Note that
@ -886,8 +889,8 @@ BYTEA bytea variable-length byte array (null-byte safe)
Alternatively, you could retrieve the assigned SERIAL value with the
currval() function after it was inserted by default, e.g.,
INSERT INTO person (name) VALUES ('Blaise Pascal');
new_id = output of "SELECT currval('person_id_seq')";
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
@ -898,7 +901,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
4.15.3) Don't currval() and nextval() lead to a race condition with other
users?
No. Currval() returns the current value assigned by your backend, not
No. currval() returns the current value assigned by your backend, not
by all users.
4.15.4) Why aren't my sequence numbers reused on transaction abort? Why are
@ -912,7 +915,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
OIDs are PostgreSQL's answer to unique row ids. Every row that is
created in PostgreSQL gets a unique OID. All OIDs generated during
initdb are less than 16384 (from backend/access/transam.h). All
initdb are less than 16384 (from include/access/transam.h). All
user-created OIDs are equal to or greater than this. By default, all
these OIDs are unique not only within a table or database, but unique
within the entire PostgreSQL installation.
@ -956,7 +959,8 @@ BYTEA bytea variable-length byte array (null-byte safe)
* range variable, table name, table alias
A list of general database terms can be found at:
http://www.comptechnews.com/~reaster/dbdesign.html
http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary
/glossary.html
4.18) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
@ -975,7 +979,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
4.19) How do I tell what PostgreSQL version I am running?
From psql, type select version();
From psql, type SELECT version();
4.20) Why does my large-object operations get "invalid large obj
descriptor"?
@ -1004,14 +1008,14 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
If the subquery returns only a few rows and the outer query returns
many rows, IN is fastest. To speed up other queries, replace IN with
EXISTS:
SELECT *
SELECT *
FROM tab
WHERE col IN (SELECT subcol FROM subtab)
WHERE col IN (SELECT subcol FROM subtab);
to:
SELECT *
SELECT *
FROM tab
WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col)
WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
For this to be fast, subcol should be an indexed column. We hope to
fix this limitation in a future release.
@ -1059,8 +1063,7 @@ SELECT *
4.25) How do I return multiple rows or columns from a function?
You can return result sets from PL/pgSQL functions using refcursors.
See
http://developer.postgresql.org/docs/postgres/plpgsql-cursors.html,
See http://www.PostgreSQL.org/idocs/index.php?plpgsql-cursors.html,
section 23.7.3.3.
4.26) Why can't I reliably create/drop temporary tables in PL/PgSQL
@ -1079,19 +1082,19 @@ SELECT *
There are several master/slave replication options available. These
allow only the master to make database changes and the slave can only
do database reads. The bottom of
http://gborg.postgresql.org/genpage?replication_research lists them. A
http://gborg.PostgreSQL.org/genpage?replication_research lists them. A
multi-master replication solution is being worked on at
http://gborg.postgresql.org/project/pgreplication/projdisplay.php.
http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php.
4.27) What encryption options are available?
4.28) What encryption options are available?
* /contrib/pgcrypto contains many encryption functions for use in
SQL queries.
* The only way to encrypt transmission from the client to the server
is by using hostssl in pg_hba.conf.
* Database user passwords are automatically encrypted when stored in
version 7.3. In previous versions, you must enable
password_encryption in postgresql.conf.
version 7.3. In previous versions, you must enable the option
PASSWORD_ENCRYPTION in postgresql.conf.
* The server can run using an encrypted file system.
_________________________________________________________________