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Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Tue Mar 21 16:09:11 EST 2000
Last updated: Fri Jun 2 11:32:13 EDT 2000
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
@ -11,11 +11,14 @@
Linux-specific questions are answered in
http://www.PostgreSQL.org/docs/faq-linux.html.
HPUX-specific questions are answered in
http://www.PostgreSQL.org/docs/faq-hpux.html.
Solaris-specific questions are answered in
http://www.postgresql.org/docs/faq-solaris.html.
Irix-specific questions are answered in
http://www.PostgreSQL.org/docs/faq-irix.html.
HPUX-specific questions are answered in
http://www.PostgreSQL.org/docs/faq-hpux.shtml.
_________________________________________________________________
General Questions
@ -63,7 +66,6 @@
3.11) What debugging features are available in PostgreSQL?
3.12) I get 'Sorry, too many clients' when trying to connect. Why?
3.13) What are the pg_psort.XXX files in my database directory?
3.14) How do I set up a pg_group?
Operational Questions
@ -89,8 +91,8 @@
4.15) What is the difference between the various character types?
4.16.1) How do I create a serial/auto-incrementing field?
4.16.2) How do I get the value of a serial insert?
4.16.3) Wouldn't use of currval() and nextval() lead to a race
condition with other concurrent backend processes?
4.16.3) Don't currval() and nextval() lead to a race condition with
other concurrent backend processes?
4.17) What is an oid? What is a tid?
4.18) What is the meaning of some of the terms used in PostgreSQL?
4.19) Why do I get the error "FATAL: palloc failure: memory
@ -104,7 +106,7 @@
Extending PostgreSQL
5.1) I wrote a user-defined function. When I run it in psql, why does
it dumps core?
it dump core?
5.2) What does the message: NOTICE:PortalHeapMemoryFree: 0x402251d0
not in alloc set! mean?
5.3) How can I contribute some nifty new types and functions for
@ -142,6 +144,8 @@
functionality was added in 1995, its name was changed to Postgres95.
The name was changed at the end of 1996 to PostgreSQL.
It is pronounced Post-Gres-Q-L.
1.2) What's the copyright on PostgreSQL?
PostgreSQL is subject to the following COPYRIGHT.
@ -261,14 +265,19 @@
1.7) What is the latest release of PostgreSQL?
The latest release of PostgreSQL is version 6.5.2.
The latest release of PostgreSQL is version 7.0.2.
We plan to have major releases every four months.
1.8) What documentation is available for PostgreSQL?
Several manuals, manual pages, and some small test examples are
included in the distribution. See the /doc directory.
included in the distribution. See the /doc directory. You can also
browse the manual on-line at http://www.postgresql.org/docs/postgres.
in the distribution.
There is a PostgreSQL book availiable at
http://www.postgresql.org/docs/awbook.html
psql has some nice \d commands to show information about types,
operators, functions, aggregates, etc.
@ -282,16 +291,17 @@
1.10) How can I learn SQL?
There is a nice tutorial at http://w3.one.net/~jhoffman/sqltut.htm and
at
http://ourworld.compuserve.com/homepages/Graeme_Birchall/HTM_COOK.HTM.
The PostgreSQL book at http://www.postgresql.org/docs/awbook.html
teaches SQL. There is a nice tutorial at
http://w3.one.net/~jhoffman/sqltut.htm and at
http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM.
Another one is "Teach Yourself SQL in 21 Days, Second Edition" at
http://members.tripod.com/er4ebus/sql/index.htm
Many of our users like The Practical SQL Handbook, Bowman et al.,
Addison Wesley. Others like Lan Times Guide to SQL, Groff et al.,
Osborne McGraw-Hill.
Addison Wesley. Others like The Complete Reference SQL, Groff et al.,
McGraw-Hill.
1.11) Is PostgreSQL Y2K compliant?
@ -349,7 +359,9 @@
Of course, MySQL doesn't have any of the features mentioned in
the Features section above. We are built for flexibility and
features, though we continue to improve performance through
profiling and source code analysis.
profiling and source code analysis. There is an interesting web
page comparing PostgreSQL to MySQL at
http://openacs.org/why-not-mysql.html
We handle each user connection by creating a Unix process.
Backend processes share data buffers and locking information.
With multiple CPU's, multiple backends can easily run on
@ -434,7 +446,7 @@
* ODBC(odbc)
* Python(PyGreSQL)
* TCL(libpgtcl)
* A crude C/4GL(contrib/pginterface)
* C Easy API(libpgeasy)
* Embedded HTML(PHP from http://www.php.net)
_________________________________________________________________
@ -443,8 +455,7 @@
3.1) Why does initdb fail?
* check that you don't have any of the previous version's binaries
in your path (If you see the message WARN:heap_modifytuple: repl
is \ 9, this is the problem.)
in your path
* check to see that you have the proper paths set
* check that the postgres user owns the proper files
@ -459,7 +470,7 @@
message. Why?
It could be a variety of problems, but first check to see that you
have system V extensions installed on your kernel. PostgreSQL requires
have system V extensions installed in your kernel. PostgreSQL requires
kernel support for shared memory and semaphores.
3.4) When I try to start the postmaster, I get IpcMemoryCreate errors. Why?
@ -488,7 +499,7 @@
3.6) How do I prevent other hosts from accessing my PostgreSQL database?
By default, PostgreSQL only allows connections from the local machine
using unix domain sockets. Other machines will not be able to connect
using Unix domain sockets. Other machines will not be able to connect
unless you add the -i flag to the postmaster, and enable host-based
authentication by modifying the file $PGDATA/pg_hba.conf accordingly.
This will allow TCP/IP connections.
@ -538,10 +549,8 @@
and the default is 64 buffers.
You can also use the backend -S option to increase the maximum amount
of memory used by each backend process for temporary sorts. The -S
value is measured in kilobytes, and the default is 512 (ie, 512K). It
is unwise to make this value too large, or you may run out of memory
when a query invokes several concurrent sorts.
of memory used by the backend process for temporary sorts. The -S
value is measured in kilobytes, and the default is 512 (ie, 512K).
You can also use the CLUSTER command to group data in base tables to
match an index. See the cluster(l) manual page for more details.
@ -568,15 +577,22 @@
that specifies the debug level. Be warned that high debug level values
generate large log files.
You can actually run the postgres backend from the command line, and
type your SQL statement directly. This is recommended only for
debugging purposes. Note that a newline terminates the query, not a
semicolon. If you have compiled with debugging symbols, you can use a
debugger to see what is happening. Because the backend was not started
from the postmaster, it is not running in an identical environment and
locking/backend interaction problems may not be duplicated. Some
operating system can attach to a running backend directly to diagnose
problems.
If the postmaster is not running, you can actually run the postgres
backend from the command line, and type your SQL statement directly.
This is recommended only for debugging purposes. Note that a newline
terminates the query, not a semicolon. If you have compiled with
debugging symbols, you can use a debugger to see what is happening.
Because the backend was not started from the postmaster, it is not
running in an identical environment and locking/backend interaction
problems may not be duplicated.
If the postmaster is running, start psql in one window, then find the
PID of the postgres process used by psql. Use a debugger to attach to
the postgres PID. You can set breakpoints in the debugger and issue
queries from psql. If you are debugging postgres startup, you can set
PGOPTIONS="-W n", then start psql. This will cause startup to delay
for n seconds so you can attach with the debugger and trace through
the startup sequence.
The postgres program has -s, -A, and -t options that can be very
useful for debugging and performance measurements.
@ -584,30 +600,31 @@
You can also compile with profiling to see what functions are taking
execution time. The backend profile files will be deposited in the
pgsql/data/base/dbname directory. The client profile file will be put
in the current directory.
in the client's current directory.
3.12) I get 'Sorry, too many clients' when trying to connect. Why?
You need to increase the postmaster's limit on how many concurrent
backend processes it can start.
In Postgres 6.5.*, the default limit is 32 processes. You can increase
it by restarting the postmaster with a suitable -N value. With the
default configuration you can set -N as large as 1024; if you need
more, increase MAXBACKENDS in include/config.h and rebuild. You can
set the default value of -N at configuration time, if you like, using
configure's --with-maxbackends switch.
In Postgres 6.5 and up, the default limit is 32 processes. You can
increase it by restarting the postmaster with a suitable -N value.
With the default configuration you can set -N as large as 1024; if you
need more, increase MAXBACKENDS in include/config.h and rebuild. You
can set the default value of -N at configuration time, if you like,
using configure's --with-maxbackends switch.
Note that if you make -N larger than 32, you should consider
increasing -B beyond its default of 64. 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
Postgres has a limit on the number of allowed backend processes is so
that you can ensure that your system won't run out of resources.
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 Postgres has a limit on the number of allowed backend processes
is so that you can ensure that your system won't run out of resources.
In Postgres versions prior to 6.5, the maximum number of backends was
64, and changing it required a rebuild after altering the MaxBackendId
@ -623,25 +640,6 @@
The temp files should go away automatically, but might not if a
backend crashes during a sort. If you have no transactions running at
the time, it is safe to delete the pg_tempNNN.NN files.
3.14) How do I set up a pg_group?
Currently, there is no easy interface to set up user groups. You have
to explicitly insert/update the pg_group table. For example:
jolly=> insert into pg_group (groname, grosysid, grolist)
jolly=> values ('posthackers', '1234', '{5443, 8261}');
INSERT 548224
jolly=> grant insert on foo to group posthackers;
CHANGE
jolly=>
The fields in pg_group are:
* groname: the group name. This a name and should be purely
alphanumeric. Do not include underscores or other punctuation.
* grosysid: the group id. This is an int4. This should be unique for
each group.
* grolist: the list of pg_user id's that belong in the group. This
is an int4[].
_________________________________________________________________
Operational Questions
@ -673,9 +671,8 @@
You can read the source code for psql, file pgsql/src/bin/psql/psql.c.
It contains SQL commands that generate the output for psql's backslash
commands. Beginning in Postgres 6.5, you can also start psql with the
-E option so that it will print out the queries it uses to execute the
commands you give.
commands. You can also start psql with the -E option so that it will
print out the queries it uses to execute the commands you give.
4.5) How do you remove a column from a table?
@ -688,15 +685,22 @@
4.6) What is the maximum size for a row, table, database?
Rows are limited to 8K bytes, but this can be changed by editing
include/config.h and changing BLCKSZ. To use attributes larger than
8K, you can also use the large object interface.
These are the limits:
Maximum size for a database? unlimited (60GB databases exist)
Maximum size for a table? unlimited on all operating systems
Maximum size for a row? 8k, configurable to 32k
Maximum number of rows in a table? unlimited
Maximum number of columns table? unlimited
Maximum number of indexes on a table? unlimited
Of course, these are not actually unlimited, but limited to available
disk space.
Rows do not cross 8k boundaries so a 5k row will require 8k of
storage.
To change the maximum row size, edit include/config.h and change
BLCKSZ. To use attributes larger than 8K, you can also use the large
object interface.
Table and database sizes are unlimited. There are many databases that
are tens of gigabytes, and probably some that are hundreds.
Row length limit will be removed in 7.1.
4.7)How much database disk space is required to store data from a typical
flat file?
@ -754,13 +758,14 @@
column statistics on its own, so VACUUM ANALYZE must be run to collect
them periodically.
Indexes are not used for ORDER BY operations.
Indexes are usually not used for ORDER BY operations: a sequential
scan followed by an explicit sort is faster than an indexscan of all
tuples of a large table, because it takes fewer disk accesses.
When using wild-card operators such as LIKE or ~, indices can only be
used if the beginning of the search is anchored to the start of the
string. So, to use indices, LIKE searches can should not begin with %,
and ~(regular expression searches) should start with ^. If you have
locale enabled, indexes can't be used for wild-card searches.
string. So, to use indices, LIKE searches should not begin with %, and
~(regular expression searches) should start with ^.
4.10) How do I see how the query optimizer is evaluating my query?
@ -801,8 +806,12 @@
4.13) How do I do regular expression searches and case-insensitive regexp
searching?
~ and ~* are probably what you want. See psql's \do command.
The ~ operator does regular-expression matching, and ~* does
case-insensitive regular-expression matching. There is no
case-insensitive variant of the LIKE operator, but you can get the
effect of case-insensitive LIKE with this:
WHERE lower(textfield) LIKE lower(pattern)
4.14) In a query, how do I detect if a field is NULL?
You test the column with IS NULL and IS NOT NULL.
@ -811,13 +820,14 @@
Type Internal Name Notes
--------------------------------------------------
CHAR char 1 character
"char" char 1 character
CHAR(#) bpchar blank padded to the specified fixed length
VARCHAR(#) varchar size specifies maximum length, no padding
TEXT text length limited only by maximum row length
BYTEA bytea variable-length array of bytes
You need to use the internal name when doing internal operations.
You will see the internal name when examining system catalogs and in
some error messages.
The last four types above are "varlena" types (i.e. the first four
bytes are the length, followed by the data). char(#) allocates the
@ -877,8 +887,8 @@ BYTEA bytea variable-length array of bytes
oid value is made available via $sth->{pg_oid_status} after
$sth->execute().
4.16.3) Wouldn't use of currval() and nextval() lead to a race condition
with other concurrent backend processes?
4.16.3) Don't currval() and nextval() lead to a race condition with other
concurrent backend processes?
No. That has been handled by the backends.
@ -949,10 +959,6 @@ BYTEA bytea variable-length array of bytes
You need to put BEGIN WORK and COMMIT around any use of a large object
handle, that is, surrounding lo_open ... lo_close.
The documentation has always stated that lo_open must be wrapped in a
transaction, but PostgreSQL versions prior to 6.5 didn't enforce that
rule. Instead, they'd just fail occasionally if you broke it.
Current PostgreSQL enforces the rule by closing large object handles
at transaction commit, which will be instantly upon completion of the
lo_open command if you are not inside a transaction. So the first
@ -965,17 +971,12 @@ BYTEA bytea variable-length array of bytes
4.22) How do I create a column that will default to the current time?
The tempation is to do:
create table test (x int, modtime timestamp default 'now');
but this makes the column default to the time of table creation, not
the time of row insertion. Instead do:
This way always works:
CREATE TABLE test (x int, modtime timestamp default now() );
The calling of the function now() prevents the default value from
being computed at table creation time, and delays it until insertion
time. We believe this will not be a problem in post-6.5.* releases.
In releases 7.0 and later, you may use:
create table test (x int, modtime timestamp default 'now');
4.23) Why are my subqueries using IN so slow?
Currently, we join subqueries to outer queries by sequential scanning
@ -999,9 +1000,7 @@ BYTEA bytea variable-length array of bytes
dump core?
The problem could be a number of things. Try testing your user-defined
function in a stand alone test program first. Also, make sure you are
not sending elog NOTICES when the front-end is expecting data, such as
during a type_in() or type_out() functions
function in a stand alone test program first.
5.2) What does the message: NOTICE:PortalHeapMemoryFree: 0x402251d0 not in
alloc set! mean?
@ -1024,4 +1023,5 @@ BYTEA bytea variable-length array of bytes
change?
The Makefiles do not have the proper dependencies for include files.
You have to do a make clean and then another make.
You have to do a make clean and then another make. You have to do a
make clean and then another make.

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doc/src/FAQ.html
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@ -719,16 +719,17 @@ 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.<P>
You can actually run the postgres backend from the command line, and
type your SQL statement directly. This is recommended <B>only</B> for
debugging purposes. Note that a newline terminates the query, not a
semicolon. If you have compiled with debugging symbols, you can use a
debugger to see what is happening. Because the backend was not started
from the postmaster, it is not running in an identical environment and
locking/backend interaction problems may not be duplicated.<P>
If the <i>postmaster</i> is not running, you can actually run the
postgres backend from the command line, and type your SQL statement
directly. This is recommended <B>only</B> for debugging purposes. Note
that a newline terminates the query, not a semicolon. If you have
compiled with debugging symbols, you can use a debugger to see what is
happening. Because the backend was not started from the postmaster, it
is not running in an identical environment and locking/backend
interaction problems may not be duplicated.<P>
Another method is to start <I>psql</I> in one window, then find the
<small>PID</small> of the <i>postgres</i> process used by
If the <i>postmaster</i> is running, start <I>psql</I> in one window,
then find the <small>PID</small> of the <i>postgres</i> process used by
<i>psql.</i> Use a debugger to attach to the <i>postgres</i>
<small>PID.</small> You can set breakpoints in the debugger and issue
queries from <i>psql.</i> If you are debugging <i>postgres</i> startup,