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Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Fri Jun 2 11:32:13 EDT 2000
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
The most recent version of this document can be viewed at the
postgreSQL Web site, http://www.PostgreSQL.org.
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.
_________________________________________________________________
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 for PostgreSQL?
1.7) What is the latest release of PostgreSQL?
1.8) What documentation is available for PostgreSQL?
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 hooking PostgreSQL to 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) Why does initdb fail?
3.2) How do I install PostgreSQL somewhere other than
/usr/local/pgsql?
3.3) When I start the postmaster, I get a Bad System Call or core
dumped message. Why?
3.4) When I try to start the postmaster, I get IpcMemoryCreate
errors3. Why?
3.5) When I try to start the postmaster, I get IpcSemaphoreCreate
errors. Why?
3.6) How do I prevent other hosts from accessing my PostgreSQL
database?
3.7) Why can't I connect to my database from another machine?
3.8) Why can't I access the database as the root user?
3.9) All my servers crash under concurrent table access. Why?
3.10) How do I tune the database engine for better performance?
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?
Operational Questions
4.1) The system seems to be 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 flat file?
4.8) How do I find out what indices or operations 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 Genetic Query Optimization?
4.13) How do I do regular expression searches and case-insensitive
regexp searching?
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 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
exhausted?"
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 do an outer join?
Extending PostgreSQL
5.1) I wrote a user-defined function. When I run it in psql, why does
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
PostgreSQL?
5.4) How do I write a C function to return a tuple?
5.5) I have changed a source file. Why does the recompile does 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 being 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 current and future 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) 1996-2000, PostgreSQL, Inc 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?
The authors have compiled and tested PostgreSQL on the following
platforms (some of these compiles require gcc):
* aix - IBM on AIX 3.2.5 or 4.x
* alpha - DEC Alpha AXP on Digital Unix 2.0, 3.2, 4.0
* BSD44_derived - OSs derived from 4.4-lite BSD (NetBSD, FreeBSD)
* bsdi - BSD/OS 2.x, 3.x, 4.x
* dgux - DG/UX 5.4R4.11
* hpux - HP PA-RISC on HP-UX 9.*, 10.*
* i386_solaris - i386 Solaris
* irix5 - SGI MIPS on IRIX 5.3
* linux - Intel i86 Alpha SPARC PPC M68k
* sco - SCO 3.2v5 Unixware
* sparc_solaris - SUN SPARC on Solaris 2.4, 2.5, 2.5.1
* sunos4 - SUN SPARC on SunOS 4.1.3
* svr4 - Intel x86 on Intel SVR4 and MIPS
* ultrix4 - DEC MIPS on Ultrix 4.4
1.4) What non-unix ports are available?
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.
The database server is now working on Windows NT using the Cygnus
Unix/NT porting library. See pgsql/doc/README.NT in the distribution.
There is also a web page at
http://www.freebsd.org/~kevlo/postgres/portNT.html. There is another
port using U/Win at http://surya.wipro.com/uwin/ported.html.
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 for PostgreSQL?
There is no official support for PostgreSQL from the University of
California, Berkeley. It is maintained through volunteer effort.
The main mailing list is: pgsql-general@postgreSQL.org. It is
available for discussion of matters pertaining to PostgreSQL. To
subscribe, send a mail with the 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 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 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://postgreSQL.org
There is also an IRC channel on EFNet, channel #PostgreSQL. I use the
unix command irc -c '#PostgreSQL' "$USER" irc.phoenix.net
Commercial support for PostgreSQL is available at
http://www.pgsql.com/
1.7) What is the latest release of PostgreSQL?
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. 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.
The 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 for a
list of 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 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 sources 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. All of them have submitted so many
high-quality patches that it was a pain for the existing committers to
keep up, and we had confidence that patches they committed were likely
to be of high quality.
1.13) How do I submit a bug report?
Fill out the "bug-template" file and send it to: 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, 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. We don't have
foreign key referential integrity or outer joins, but are
working on them for our next release.
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. The mode is
select-able by the database administrator.
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, PostODBC and OpenLink ODBC.
PostODBC is included in the distribution. More information about it
can be gotten from: http://www.insightdist.com/psqlodbc
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.
2.2) What tools are available for hooking PostgreSQL to 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
PHP is great for simple stuff, but for more complex cases, many use
the perl interface and CGI.pm.
A WWW gateway based on WDB using perl can be downloaded from
http://www.eol.ists.ca/~dunlop/wdb-p95
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) Why does initdb fail?
* check that you don't have any of the previous version's binaries
in your path
* check to see that you have the proper paths set
* check that the postgres user owns the proper files
3.2) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
The simplest way is to specify the --prefix option when running
configure. If you forgot to do that, you can edit Makefile.global and
change POSTGRESDIR accordingly, or create a Makefile.custom and define
POSTGRESDIR there.
3.3) 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.
3.4) When I try to start the postmaster, I get IpcMemoryCreate errors. Why?
You either do not have shared memory configured properly in 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 postmaster to run with.
For most systems, with default numbers of buffers and processes, you
need a minimum of ~1MB.
3.5) 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.
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
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.
3.7) 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. See the pg_hba.conf
manual page.
3.8) Why can't I access the database as the root user?
You should not create database users with user id 0 (root). They will
be unable to access the database. This is a security precaution
because of the ability of any user to dynamically link object modules
into the database engine.
3.9) All my servers crash under concurrent table access. Why?
This problem can be caused by a kernel that is not configured to
support semaphores.
3.10) 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 single
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 things that can be done. 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 up 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 base tables to
match an index. See the cluster(l) manual page for more details.
3.11) What debugging features are available in PostgreSQL?
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.
3.12) 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 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 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
constant in include/storage/sinvaladt.h.
3.13) 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
requires more space than the backend's -S parameter allows, then temp
files are created to hold the extra data.
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.
_________________________________________________________________
Operational Questions
4.1) The system seems to be confused about commas, decimal points, and date
formats.
Check your locale configuration. PostgreSQL uses the locale settings
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 information I see in psql?
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. 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?
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? 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.
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.
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?
A Postgres database can require about 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 contain 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 indices or operations 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. One has to make
an explicit VACUUM call 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 multi-join 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 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 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 efficient answer queries like 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"
Builtin 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 require a bit of work and we don't currently have
any documentation on how to do it.
4.12) What is Genetic Query Optimization?
The GEQO module in PostgreSQL is intended to solve the query
optimization problem of joining many tables by means of a Genetic
Algorithm (GA). It allows the handling of large join queries through
non-exhaustive search.
For further information see the documentation.
4.13) How do I do regular expression searches and case-insensitive regexp
searching?
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.
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 length limited only by maximum row length
BYTEA bytea variable-length array of bytes
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
maximum number of bytes no matter how much data is stored in the
field. text, varchar(#), and bytea all have variable length on the
disk, and because of this, there is a small performance penalty for
using them. Specifically, the penalty is for access to all columns
after the first column of this type.
4.16.1) How do I create a serial/auto-incrementing field?
PostgreSQL supports 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.
For more details, see Bruce Momjian's chapter on Numbering Rows.
4.16.2) How do I get the back the generated SERIAL value after an insert?
Probably the simplest 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:
$newSerialID = nextval('person_id_seq');
INSERT INTO person (id, name) VALUES ($newSerialID, '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.
Similarly, you could retrieve the just-assigned SERIAL value with the
currval() function after it was inserted by default, e.g.,
INSERT INTO person (name) VALUES ('Blaise Pascal');
$newID = currval('person_id_seq');
Finally, you could use the oid returned from the INSERT statement to
lookup 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
concurrent backend processes?
No. That has been handled by the backends.
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 or greater that 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. See the sql(l) manual page to see the other internal
columns. 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 INTO new SELECT old_oid, mycol FROM old;
COPY new TO '/tmp/pgtable';
DELETE FROM new;
COPY new WITH OIDS FROM '/tmp/pgtable';
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
4.19) Why do I get the error "FATAL: palloc failure: memory exhausted?"
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 65536
limit datasize 64m
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.
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
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?
This way always works:
CREATE TABLE test (x int, modtime timestamp default now() );
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
the result of the subquery for each row of the outer query. A
workaround is to replace IN with EXISTS. For example, change:
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 do an outer join?
PostgreSQL does not support outer joins in the current release. They
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 tab1.col1
_________________________________________________________________
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) What does the message: NOTICE:PortalHeapMemoryFree: 0x402251d0 not in
alloc set! mean?
You are pfree'ing something that was not palloc'ed. Beware of mixing
malloc/free and palloc/pfree.
5.3) How can I contribute some nifty new types and functions for
PostgreSQL?
Send your extensions to the pgsql-hackers mailing list, and they will
eventually end up in the contrib/ subdirectory.
5.4) 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.
5.5) I have changed a source file. Why does the recompile does 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. You have to do a
make clean and then another make.
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