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Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Mon Nov 12 02:38:47 EST 2001
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
The most recent version of this document can be viewed at
http://www.PostgreSQL.org/docs/faq-english.html.
Platform-specific questions are answered at
http://www.PostgreSQL.org/users-lounge/docs/faq.html.
_________________________________________________________________
General Questions
1.1) What is PostgreSQL?
1.2) What's the copyright on PostgreSQL?
1.3) What Unix platforms does PostgreSQL run on?
1.4) What non-unix ports are available?
1.5) Where can I get PostgreSQL?
1.6) Where can I get support?
1.7) What is the latest release?
1.8) What documentation is available?
1.9) How do I find out about known bugs or missing features?
1.10) How can I learn SQL?
1.11) Is PostgreSQL Y2K compliant?
1.12) How do I join the development team?
1.13) How do I submit a bug report?
1.14) How does PostgreSQL compare to other DBMS's?
User Client Questions
2.1) Are there ODBC drivers for PostgreSQL?
2.2) What tools are available for use PostgreSQL with Web pages?
2.3) Does PostgreSQL have a graphical user interface? A report
generator? An embedded query language interface?
2.4) What languages are available to communicate with PostgreSQL?
Administrative Questions
3.1) How do I install PostgreSQL somewhere other than
/usr/local/pgsql?
3.2) When I start the postmaster, I get a Bad System Call or core
dumped message. Why?
3.3) When I try to start the postmaster, I get IpcMemoryCreate errors.
Why?
3.4) When I try to start the postmaster, I get IpcSemaphoreCreate
errors. Why?
3.5) How do I prevent other hosts from accessing my PostgreSQL
database?
3.6) Why can't I connect to my database from another machine?
3.7) How do I tune the database engine for better performance?
3.8) What debugging features are available?
3.9) I get "Sorry, too many clients" when trying to connect. Why?
3.10) What are the pg_sorttempNNN.NN files in my database directory?
Operational Questions
4.1) Why is the system confused about commas, decimal points, and date
formats.
4.2) What is the exact difference between binary cursors and normal
cursors?
4.3) How do I SELECT only the first few rows of a query?
4.4) How do I get a list of tables or other things I can see in psql?
4.5) How do you remove a column from a table?
4.6) What is the maximum size for a row, table, database?
4.7) How much database disk space is required to store data from a
typical text file?
4.8) How do I find out what tables or indexes are defined in the
database?
4.9) My queries are slow or don't make use of the indexes. Why?
4.10) How do I see how the query optimizer is evaluating my query?
4.11) What is an R-tree index?
4.12) What is the Genetic Query Optimizer?
4.13) How do I perform regular expression searches and
case-insensitive regular expression searches? How do I use an index
for case-insensitive searches?
4.14) In a query, how do I detect if a field is NULL?
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) Don't currval() and nextval() lead to a race condition with
other users?
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 "ERROR: Memory exhausted in
AllocSetAlloc()?"
4.20) How do I tell what PostgreSQL version I am running?
4.21) My large-object operations get invalid large obj descriptor.
Why?
4.22) How do I create a column that will default to the current time?
4.23) Why are my subqueries using IN so slow?
4.24) How do I perform an outer join?
4.25) How do I perform queries using multiple databases?
Extending PostgreSQL
5.1) I wrote a user-defined function. When I run it in psql, why does
it dump core?
5.2) How can I contribute some nifty new types and functions to
PostgreSQL?
5.3) How do I write a C function to return a tuple?
5.3) I have changed a source file. Why does the recompile not see the
change?
_________________________________________________________________
General Questions
1.1) What is PostgreSQL?
PostgreSQL is an enhancement of the POSTGRES database management
system, a next-generation DBMS research prototype. While PostgreSQL
retains the powerful data model and rich data types of POSTGRES, it
replaces the PostQuel query language with an extended subset of SQL.
PostgreSQL is free and the complete source is available.
PostgreSQL development is performed by a team of Internet developers
who all subscribe to the PostgreSQL development mailing list. The
current coordinator is Marc G. Fournier (scrappy@PostgreSQL.org). (See
below on how to join). This team is now responsible for all
development of PostgreSQL.
The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen. Many
others have contributed to the porting, testing, debugging, and
enhancement of the code. The original Postgres code, from which
PostgreSQL is derived, was the effort of many graduate students,
undergraduate students, and staff programmers working under the
direction of Professor Michael Stonebraker at the University of
California, Berkeley.
The original name of the software at Berkeley was Postgres. When SQL
functionality was added in 1995, its name was changed to Postgres95.
The name was changed at the end of 1996 to PostgreSQL.
It is pronounced Post-Gres-Q-L.
1.2) What's the copyright on PostgreSQL?
PostgreSQL is subject to the following COPYRIGHT:
PostgreSQL Data Base Management System
Portions copyright (c) 1996-2001, PostgreSQL Global Development Group
Portions Copyright (c) 1994-6 Regents of the University of California
Permission to use, copy, modify, and distribute this software and its
documentation for any purpose, without fee, and without a written
agreement is hereby granted, provided that the above copyright notice
and this paragraph and the following two paragraphs appear in all
copies.
IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY PARTY
FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES,
INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND
ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF CALIFORNIA HAS BEEN
ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY WARRANTIES,
INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE SOFTWARE
PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE UNIVERSITY OF
CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT,
UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
1.3) What Unix platforms does PostgreSQL run on?
In general, a modern Unix-compatible platform should be able to run
PostgreSQL. The platforms that had received explicit testing at the
time of release are listed in the installation instructions.
1.4) What non-unix ports are available?
Client
It is possible to compile the libpq C library, psql, and other
interfaces and binaries to run on MS Windows platforms. In this case,
the client is running on MS Windows, and communicates via TCP/IP to a
server running on one of our supported Unix platforms. A file
win31.mak is included in the distribution for making a Win32 libpq
library and psql. PostgreSQL also communicates with ODBC clients.
Server
The database server can run on Windows NT and Win2k using Cygwin, the
Cygnus Unix/NT porting library. See pgsql/doc/FAQ_MSWIN in the
distribution or the MS Windows FAQ on our web site. We have no plan to
do a native port to any Microsoft platform.
1.5) Where can I get PostgreSQL?
The primary anonymous ftp site for PostgreSQL is
ftp://ftp.PostgreSQL.org/pub. For mirror sites, see our main Web site.
1.6) Where can I get support?
The main mailing list is: pgsql-general@PostgreSQL.org. It is
available for discussion of matters pertaining to PostgreSQL. To
subscribe, send mail with the following lines in the body (not the
subject line)
subscribe
end
to pgsql-general-request@PostgreSQL.org.
There is also a digest list available. To subscribe to this list, send
email to: pgsql-general-digest-request@PostgreSQL.org with a body of:
subscribe
end
Digests are sent out to members of this list whenever the main list
has received around 30k of messages.
The bugs mailing list is available. To subscribe to this list, send
email to pgsql-bugs-request@PostgreSQL.org with a body of:
subscribe
end
There is also a developers discussion mailing list available. To
subscribe to this list, send email to
pgsql-hackers-request@PostgreSQL.org with a body of:
subscribe
end
Additional mailing lists and information about PostgreSQL can be found
via the PostgreSQL WWW home page at:
http://www.PostgreSQL.org
There is also an IRC channel on EFNet, channel #PostgreSQL. I use the
unix command irc -c '#PostgreSQL' "$USER" irc.phoenix.net.
A list of commercial support companies is available at
http://www.postgresql.org/users-lounge/commercial-support.html.
1.7) What is the latest release?
The latest release of PostgreSQL is version 7.1.3.
We plan to have major releases every four months.
1.8) What documentation is available?
Several manuals, manual pages, and some small test examples are
included in the distribution. See the /doc directory. You can also
browse the manual online at
http://www.PostgreSQL.org/users-lounge/docs/.
There is a PostgreSQL book available at
http://www.PostgreSQL.org/docs/awbook.html.
psql has some nice \d commands to show information about types,
operators, functions, aggregates, etc.
Our Web site contains even more documentation.
1.9) How do I find out about known bugs or missing features?
PostgreSQL supports an extended subset of SQL-92. See our TODO list
for known bugs, missing features, and future plans.
1.10) How can I learn SQL?
The PostgreSQL book at http://www.PostgreSQL.org/docs/awbook.html
teaches SQL. There is 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, Judith S.,
et al., Addison-Wesley. Others like The Complete Reference SQL, Groff
et al., McGraw-Hill.
1.11) Is PostgreSQL Y2K compliant?
Yes, we easily handle dates past the year 2000AD, and before 2000BC.
1.12) How do I join the development team?
First, download the latest source and read the PostgreSQL Developers
documentation on our Web site, or in the distribution. Second,
subscribe to the pgsql-hackers and pgsql-patches mailing lists. Third,
submit high-quality patches to pgsql-patches.
There are about a dozen people who have commit privileges to the
PostgreSQL CVS archive. They each have submitted so many high-quality
patches that it was impossible for the existing committers to keep up,
and we had confidence that patches they committed were of high
quality.
1.13) How do I submit a bug report?
Fill out the "bug-template" file and send it to:
pgsql-bugs@PostgreSQL.org
Also check out our ftp site ftp://ftp.PostgreSQL.org/pub to see if
there is a more recent PostgreSQL version or patches.
1.14) How does PostgreSQL compare to other DBMS's?
There are several ways of measuring software: features, performance,
reliability, support, and price.
Features
PostgreSQL has most features present in large commercial
DBMS's, like transactions, subselects, triggers, views, foreign
key referential integrity, and sophisticated locking. We have
some features they don't have, like user-defined types,
inheritance, rules, and multi-version concurrency control to
reduce lock contention.
Performance
PostgreSQL runs in two modes. Normal fsync mode flushes every
completed transaction to disk, guaranteeing that if the OS
crashes or loses power in the next few seconds, all your data
is safely stored on disk. In this mode, we are slower than most
commercial databases, partly because few of them do such
conservative flushing to disk in their default modes. In
no-fsync mode, we are usually faster than commercial databases,
though in this mode, an OS crash could cause data corruption.
We are working to provide an intermediate mode that suffers
less performance overhead than full fsync mode, and will allow
data integrity within 30 seconds of an OS crash.
In comparison to MySQL or leaner database systems, we are
slower on inserts/updates because we have transaction overhead.
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. 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
different CPU's.
Reliability
We realize that a DBMS must be reliable, or it is worthless. We
strive to release well-tested, stable code that has a minimum
of bugs. Each release has at least one month of beta testing,
and our release history shows that we can provide stable, solid
releases that are ready for production use. We believe we
compare favorably to other database software in this area.
Support
Our mailing list provides a large group of developers and users
to help resolve any problems encountered. While we can not
guarantee a fix, commercial DBMS's don't always supply a fix
either. Direct access to developers, the user community,
manuals, and the source code often make PostgreSQL support
superior to other DBMS's. There is commercial per-incident
support available for those who need it. (See support FAQ
item.)
Price
We are free for all use, both commercial and non-commercial.
You can add our code to your product with no limitations,
except those outlined in our BSD-style license stated above.
_________________________________________________________________
User Client Questions
2.1) Are there ODBC drivers for PostgreSQL?
There are two ODBC drivers available, PsqlODBC and OpenLink ODBC.
PsqlODBC is included in the distribution. More information about it
can be gotten from ftp://ftp.PostgreSQL.org/pub/odbc/.
OpenLink ODBC can be gotten from http://www.openlinksw.com. It works
with their standard ODBC client software so you'll have PostgreSQL
ODBC available on every client platform they support (Win, Mac, Unix,
VMS).
They will probably be selling this product to people who need
commercial-quality support, but a freeware version will always be
available. Questions to postgres95@openlink.co.uk.
See also the ODBC chapter of the Programmer's Guide.
2.2) What tools are available for using PostgreSQL with Web pages?
A nice introduction to Database-backed Web pages can be seen at:
http://www.webtools.com
There is also one at http://www.phone.net/home/mwm/hotlist/.
For Web integration, PHP is an excellent interface. It is at
http://www.php.net
For complex cases, many use the Perl interface and CGI.pm.
2.3) Does PostgreSQL have a graphical user interface? A report generator?
An embedded query language interface?
We have a nice graphical user interface called pgaccess, which is
shipped as part of the distribution. Pgaccess also has a report
generator. The Web page is http://www.flex.ro/pgaccess
We also include ecpg, which is an embedded SQL query language
interface for C.
2.4) What languages are available to communicate with PostgreSQL?
We have:
* C (libpq)
* C++ (libpq++)
* Embedded C (ecpg)
* Java (jdbc)
* Perl (perl5)
* ODBC (odbc)
* Python (PyGreSQL)
* TCL (libpgtcl)
* C Easy API (libpgeasy)
* Embedded HTML (PHP from http://www.php.net)
_________________________________________________________________
Administrative Questions
3.1) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
Specify the --prefix option when running configure.
3.2) When I start the postmaster, I get a Bad System Call or core dumped
message. Why?
It could be a variety of problems, but first check to see that you
have System V extensions installed in your kernel. PostgreSQL requires
kernel support for shared memory and semaphores.
3.3) When I try to start the postmaster, I get IpcMemoryCreate errors. Why?
You either do not have shared memory configured properly in your
kernel or you need to enlarge the shared memory available in the
kernel. The exact amount you need depends on your architecture and how
many buffers and backend processes you configure for the postmaster.
For most systems, with default numbers of buffers and processes, you
need a minimum of ~1MB. See the PostgreSQL Administrator's Guide for
more detailed information about shared memory and semaphores.
3.4) When I try to start the postmaster, I get IpcSemaphoreCreate errors.
Why?
If the error message is IpcSemaphoreCreate: semget failed (No space
left on device) then your kernel is not configured with enough
semaphores. Postgres needs one semaphore per potential backend
process. A temporary solution is to start the postmaster with a
smaller limit on the number of backend processes. Use -N with a
parameter less than the default of 32. A more permanent solution is to
increase your kernel's SEMMNS and SEMMNI parameters.
If the error message is something else, you might not have semaphore
support configured in your kernel at all. See the PostgreSQL
Administrator's Guide for more detailed information about shared
memory and semaphores.
3.5) How do I 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
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.
Inoperative semaphores can also cause crashes during heavy database
access.
3.6) Why can't I connect to my database from another machine?
The default configuration allows only unix domain socket connections
from the local machine. To enable TCP/IP connections, make sure the
postmaster has been started with the -i option, and add an appropriate
host entry to the file pgsql/data/pg_hba.conf.
3.7) How do I tune the database engine for better performance?
Certainly, indices can speed up queries. The EXPLAIN command allows
you to see how PostgreSQL is interpreting your query, and which
indices are being used.
If you are doing a lot of 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 indices when making large data changes.
There are several tuning options. You can disable fsync() by starting
the postmaster with a -o -F option. This will prevent fsync()'s from
flushing to disk after every transaction.
You can also use the postmaster -B option to increase the number of
shared memory buffers used by the backend processes. If you make this
parameter too high, the postmaster may not start because you've
exceeded your kernel's limit on shared memory space. Each buffer is 8K
and the default is 64 buffers.
You can also use the backend -S option to increase the maximum amount
of memory used by the backend process for temporary sorts. The -S
value is measured in kilobytes, and the default is 512 (ie, 512K).
You can also use the CLUSTER command to group data in tables to match
an index. See the CLUSTER manual page for more details.
3.8) What debugging features are available?
PostgreSQL has several features that report status information that
can be valuable for debugging purposes.
First, by running configure with the --enable-cassert option, many
assert()'s monitor the progress of the backend and halt the program
when something unexpected occurs.
Both postmaster and postgres have several debug options available.
First, whenever you start the postmaster, make sure you send the
standard output and error to a log file, like:
cd /usr/local/pgsql
./bin/postmaster >server.log 2>&1 &
This will put a server.log file in the top-level PostgreSQL directory.
This file contains useful information about problems or errors
encountered by the server. Postmaster has a -d option that allows even
more detailed information to be reported. The -d option takes a number
that specifies the debug level. Be warned that high debug level values
generate large log files.
If 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.
You can also compile with profiling to see what functions are taking
execution time. The backend profile files will be deposited in the
pgsql/data/base/dbname directory. The client profile file will be put
in the client's current directory.
3.9) 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.
The default limit is 32 processes. You can increase it by restarting
the postmaster with a suitable -N value or modifying postgresql.conf.
Note that if you make -N larger than 32, you must also increase -B
beyond its default of 64; -B must be at least twice -N, and probably
should be more than that for best performance. For large numbers of
backend processes, you are also likely to find that you need to
increase various Unix kernel configuration parameters. Things to check
include the maximum size of shared memory blocks, SHMMAX; the maximum
number of semaphores, SEMMNS and SEMMNI; the maximum number of
processes, NPROC; the maximum number of processes per user, MAXUPRC;
and the maximum number of open files, NFILE and NINODE. The reason
that PostgreSQL has a limit on the number of allowed backend processes
is so your system won't run out of resources.
In PostgreSQL versions prior to 6.5, the maximum number of backends
was 64, and changing it required a rebuild after altering the
MaxBackendId constant in include/storage/sinvaladt.h.
3.10) What are the pg_sorttempNNN.NN files in my database directory?
They are temporary files generated by the query executor. For example,
if a sort needs to be done to satisfy an ORDER BY, and the sort
requires more space than the backend's -S parameter allows, then
temporary files are created to hold the extra data.
The temporary files should be deleted automatically, but might not if
a backend crashes during a sort. If you have no backends running at
the time, it is safe to delete the pg_tempNNN.NN files.
_________________________________________________________________
Operational Questions
4.1) Why is system confused about commas, decimal points, and date formats.
Check your locale configuration. PostgreSQL uses the locale setting of
the user that ran the postmaster process. There are postgres and psql
SET commands to control the date format. Set those accordingly for
your operating environment.
4.2) What is the exact difference between binary cursors and normal
cursors?
See the DECLARE manual page for a description.
4.3) How do I SELECT only the first few rows of a query?
See the FETCH manual page, or use SELECT ... LIMIT....
The entire query may have to be evaluated, even if you only want the
first few rows. Consider a query that has an ORDER BY. If there is an
index that matches the ORDER BY, PostgreSQL may be able to evaluate
only the first few records requested, or the entire query may have to
be evaluated until the desired rows have been generated.
4.4) How do I get a list of tables or other things I can see in psql?
You can read the source code for psql in file
pgsql/src/bin/psql/describe.c. It contains SQL commands that generate
the output for psql's backslash commands. You can also start psql with
the -E option so it will print out the queries it uses to execute the
commands you give.
4.5) How do you remove a column from a table?
We do not support ALTER TABLE DROP COLUMN, but do this:
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;
4.6) What is the maximum size for a row, table, database?
These are the limits:
Maximum size for a database? unlimited (60GB databases exist)
Maximum size for a table? 16 TB
Maximum size for a row? unlimited in 7.1 and later
Maximum size for a field? 1GB in 7.1 and later
Maximum number of rows in a table? unlimited
Maximum number of columns in a table? 250-1600 depending on column types
Maximum number of indexes on a table? unlimited
Of course, these are not actually unlimited, but limited to available
disk space and memory/swap space. Performance may suffer when these
values get unusually large.
The maximum table size of 16TB does not require large file support
from the operating system. Large tables are stored as multiple 1GB
files so file system size limits are not important.
The maximum table size and maximum number of columns can be increased
if the default block size is increased to 32k.
4.7) How much database disk space is required to store data from a typical
text file?
A PostgreSQL database may need six-and-a-half times the disk space
required to store the data in a flat file.
Consider a file of 300,000 lines with two integers on each line. The
flat file is 2.4MB. The size of the PostgreSQL database file
containing this data can be estimated at 14MB:
36 bytes: each row header (approximate)
+ 8 bytes: two int fields @ 4 bytes each
+ 4 bytes: pointer on page to tuple
----------------------------------------
48 bytes per row
The data page size in PostgreSQL is 8192 bytes (8 KB), so:
8192 bytes per page
------------------- = 171 rows per database page (rounded up)
48 bytes per row
300000 data rows
-------------------- = 1755 database pages
171 rows per page
1755 database pages * 8192 bytes per page = 14,376,960 bytes (14MB)
Indexes do not require as much overhead, but do contain the data that
is being indexed, so they can be large also.
4.8) How do I find out what tables or indexes are defined in the database?
psql has a variety of backslash commands to show such information. Use
\? to see them.
Also try the file pgsql/src/tutorial/syscat.source. It illustrates
many of the SELECTs needed to get information from the database system
tables.
4.9) My queries are slow or don't make use of the indexes. Why?
PostgreSQL does not automatically maintain statistics. VACUUM must be
run to update the statistics. After statistics are updated, the
optimizer knows how many rows in the table, and can better decide if
it should use indices. Note that the optimizer does not use indices in
cases when the table is small because a sequential scan would be
faster.
For column-specific optimization statistics, use VACUUM ANALYZE.
VACUUM ANALYZE is important for complex multijoin queries, so the
optimizer can estimate the number of rows returned from each table,
and choose the proper join order. The backend does not keep track of
column statistics on its own, so VACUUM ANALYZE must be run to collect
them periodically.
Indexes are usually not used for ORDER BY or joins. A sequential scan
followed by an explicit sort is faster than an indexscan of all tuples
of a large table. This is because random disk access is very slow.
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 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?
See the EXPLAIN manual page.
4.11) What is an R-tree index?
An R-tree index is used for indexing spatial data. A hash index can't
handle range searches. A B-tree index only handles range searches in a
single dimension. R-tree's can handle multi-dimensional data. For
example, if an R-tree index can be built on an attribute of type
point, the system can more efficiently answer queries such as "select
all points within a bounding rectangle."
The canonical paper that describes the original R-tree design is:
Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
Searching." Proc of the 1984 ACM SIGMOD Int'l Conf on Mgmt of Data,
45-57.
You can also find this paper in Stonebraker's "Readings in Database
Systems".
Built-in R-trees can handle polygons and boxes. In theory, R-trees can
be extended to handle higher number of dimensions. In practice,
extending R-trees requires a bit of work and we don't currently have
any documentation on how to do it.
4.12) What is the Genetic Query Optimizer?
The GEQO module speeds query optimization when joining many tables by
means of a Genetic Algorithm (GA). It allows the handling of large
join queries through nonexhaustive search.
4.13) How do I perform regular expression searches and case-insensitive
regular expression searches? How do I use an index for case-insensitive
searches?
The ~ operator does regular expression matching, and ~* does
case-insensitive regular expression matching. The case-insensitive
variant of LIKE is called ILIKE in PostgreSQL 7.1 and later.
Case-insensitive equality comparisons are normally expressed as:
SELECT *
FROM tab
WHERE lower(col) = 'abc'
This will not use an standard index. However, if you create a
functional index, it will be used:
CREATE INDEX tabindex on tab (lower(col));
4.14) In a query, how do I detect if a field is NULL?
You test the column with IS NULLIS NOT NULL.
4.15) What is the difference between the various character types?
Type Internal Name Notes
--------------------------------------------------
"char" char 1 character
CHAR(#) bpchar blank padded to the specified fixed length
VARCHAR(#) varchar size specifies maximum length, no padding
TEXT text no specific upper limit on length
BYTEA bytea variable-length byte array (null-safe)
You will see the internal name when examining system catalogs and in
some error messages.
The last four types above are "varlena" types (i.e., the first four
bytes on disk are the length, followed by the data). Thus the actual
space used is slightly greater than the declared size. However, these
data types are also subject to compression or being stored out-of-line
by TOAST, so the space on disk might also be less than expected.
CHAR() is best when storing strings that are usually the same length.
VARCHAR() is best when storing variable-length strings but it limits
how long a string can be. TEXT is for strings of unlimited length,
maximum 1 gigabyte. BYTEA is for storing binary data, particularly
values that include NULL bytes.
4.16.1) How do I create a serial/auto-incrementing field?
PostgreSQL supports a SERIAL data type. It auto-creates a sequence and
index on the column. For example, this:
CREATE TABLE person (
id SERIAL,
name TEXT
);
is automatically translated into this:
CREATE SEQUENCE person_id_seq;
CREATE TABLE person (
id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
name TEXT
);
CREATE UNIQUE INDEX person_id_key ON person ( id );
See the create_sequence manual page for more information about
sequences. You can also use each row's OID field as a unique value.
However, if you need to dump and reload the database, you need to use
pg_dump's -o option or COPY WITH OIDS option to preserve the OIDs.
Numbering Rows.
4.16.2) How do I get the value of a SERIAL insert?
One approach is to 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 4.16.1, that might look like
this in Perl:
new_id = output of "SELECT nextval('person_id_seq')"
INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal');
You would then also have the new value stored in $newSerialID for use
in other queries (e.g., as a foreign key to the person table). Note
that the name of the automatically created SEQUENCE object will be
named <table>_<serialcolumn>_seq, where table and serialcolumn are the
names of your table and your SERIAL column, respectively.
Alternatively, you could retrieve the assigned SERIAL value with the
currval() function after it was inserted by default, e.g.,
INSERT INTO person (name) VALUES ('Blaise Pascal');
new_id = output of "SELECT currval('person_id_seq')";
Finally, you could use the OID returned from the INSERT statement to
look up the default value, though this is probably the least portable
approach. In Perl, using DBI with Edmund Mergl's DBD::Pg module, the
oid value is made available via $sth->{pg_oid_status} after
$sth->execute().
4.16.3) Don't currval() and nextval() lead to a race condition with other
users?
No. Currval() returns the current value assigned by your backend, not
by all users.
4.17) What is an OID? What is a TID?
OIDs are PostgreSQL's answer to unique row ids. Every row that is
created in PostgreSQL gets a unique OID. All OIDs generated during
initdb are less than 16384 (from backend/access/transam.h). All
user-created OIDs are equal to or greater than this. By default, all
these OIDs are unique not only within a table or database, but unique
within the entire PostgreSQL installation.
PostgreSQL uses OIDs in its internal system tables to link rows
between tables. These OIDs can be used to identify specific user rows
and used in joins. It is recommended you use column type OID to store
OID values. You can create an index on the OID field for faster
access.
Oids are assigned to all new rows from a central area that is used by
all databases. If you want to change the OID to something else, or if
you want to make a copy of the table, with the original OID's, there
is no reason you can't do it:
CREATE TABLE new_table(old_oid oid, mycol int);
SELECT old_oid, mycol INTO new FROM old;
COPY new TO '/tmp/pgtable';
DELETE FROM new;
COPY new WITH OIDS FROM '/tmp/pgtable';
OIDs are stored as 4-byte integers, and will overflow at 4 billion. No
one has reported this ever happening, and we plan to have the limit
removed before anyone does.
TIDs are used to identify specific physical rows with block and offset
values. Tids change after rows are modified or reloaded. They are used
by index entries to point to physical rows.
4.18) What is the meaning of some of the terms used in PostgreSQL?
Some of the source code and older documentation use terms that have
more common usage. Here are some:
* table, relation, class
* row, record, tuple
* column, field, attribute
* retrieve, select
* replace, update
* append, insert
* OID, serial value
* portal, cursor
* range variable, table name, table alias
A list of general database terms can be found at:
http://www.comptechnews.com/~reaster/dbdesign.html
4.19) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()?"
If you are running a version older than 7.1, an upgrade may fix the
problem. Also it is possible you have run out of virtual memory on
your system, or your kernel has a low limit for certain resources. Try
this before starting the postmaster:
ulimit -d 262144
limit datasize 256m
Depending on your shell, only one of these may succeed, but it will
set your process data segment limit much higher and perhaps allow the
query to complete. This command applies to the current process, and
all subprocesses created after the command is run. If you are having a
problem with the SQL client because the backend is returning too much
data, try it before starting the client.
4.20) How do I tell what PostgreSQL version I am running?
From psql, type select version();
4.21) My large-object operations get invalid large obj descriptor. Why?
You need to put BEGIN WORK and COMMIT around any use of a large object
handle, that is, surrounding lo_open ... lo_close.
Currently PostgreSQL enforces the rule by closing large object handles
at transaction commit. So the first attempt to do anything with the
handle will draw invalid large obj descriptor. So code that used to
work (at least most of the time) will now generate that error message
if you fail to use a transaction.
If you are using a client interface like ODBC you may need to set
auto-commit off.
4.22) How do I create a column that will default to the current time?
Use CURRENT_TIMESTAMP:
CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
4.23) Why are my subqueries using IN so slow?
Currently, we join subqueries to outer queries by sequentially
scanning the result of the subquery for each row of the outer query. A
workaround is to replace IN with EXISTS:
SELECT *
FROM tab
WHERE col1 IN (SELECT col2 FROM TAB2)
to:
SELECT *
FROM tab
WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 = col2)
We hope to fix this limitation in a future release.
4.24) How do I perform an outer join?
PostgreSQL 7.1 and later supports outer joins using the SQL standard
syntax. Here are two examples:
SELECT *
FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);
or
SELECT *
FROM t1 LEFT OUTER JOIN t2 USING (col);
These identical queries join t1.col to t2.col, and also return any
unjoined rows in t1 (those with no match in t2). A RIGHT join would
add unjoined rows of t2. A FULL join would return the matched rows
plus all unjoined rows from t1 and t2. The word OUTER is optional and
is assumed in LEFT, RIGHT, and FULL joins. Ordinary joins are called
INNER joins.
In previous releases, outer joins can be simulated using UNION and NOT
IN. For example, when joining tab1 and tab2, the following query does
an outer join of the two tables:
SELECT tab1.col1, tab2.col2
FROM tab1, tab2
WHERE tab1.col1 = tab2.col1
UNION ALL
SELECT tab1.col1, NULL
FROM tab1
WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
ORDER BY col1
4.25) How do I perform queries using multiple databases?
There is no way to query any database except the current one. Because
PostgreSQL loads database-specific system catalogs, it is uncertain
how a cross-database query should even behave.
Of course, a client can make simultaneous connections to different
databases and merge the information that way.
_________________________________________________________________
Extending PostgreSQL
5.1) I wrote a user-defined function. When I run it in psql, why does it
dump core?
The problem could be a number of things. Try testing your user-defined
function in a stand-alone test program first.
5.2) How can I contribute some nifty new types and functions to PostgreSQL?
Send your extensions to the pgsql-hackers mailing list, and they will
eventually end up in the contrib/ subdirectory.
5.3) How do I write a C function to return a tuple?
This requires wizardry so extreme that the authors have never tried
it, though in principle it can be done.
5.4) I have changed a source file. Why does the recompile not see the
change?
The Makefiles do not have the proper dependencies for include files.
You have to do a make clean and then another make. If you are using
GCC you can use the --enable-depend option of configure to have the
compiler compute the dependencies automatically.
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