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Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Sun Jul 13 15:26:53 EDT 1997
Version: 6.1.1
Current maintainer: Bruce Momjian (maillist@candle.pha.pa.us)
The most recent version of this document can be viewed at the
postgreSQL Web site, http://postgreSQL.org.
Linux-specific questions are answered in
http://postgreSQL.org/docs/FAQ-Linux.phtml.
Irix-specific questions are answered in
http://postgreSQL.org/docs/FAQ-Irix.phtml.
Changes in this version (* = modified, + = new):
* 3.42) What is Genetic Query Optimization?
* 3.43) I am running Solaris and my dates display wrong. Why?
_________________________________________________________________
Questions answered:
1) General questions
1.1) What is PostgreSQL?
1.2) What does PostgreSQL run on?
1.3) Where can I get PostgreSQL?
1.4) What's the copyright on PostgreSQL?
1.5) Support for PostgreSQL
1.6) Latest release of PostgreSQL
1.7) Is there a commercial version of PostgreSQL?
1.9) What version of SQL does PostgreSQL use?
1.10) Does PostgreSQL work with databases from earlier versions of
postgres?
1.11) How many people use PostgreSQL?
2) Installation questions
2.1) initdb doesn't run
2.2) when I start up the postmaster, I get "FindBackend: could not
find a backend to execute..." "postmaster: could not find backend to
execute..."
2.3) The system seems to be confused about commas, decimal points, and
date formats.
2.4) How do I install PostgreSQL somewhere other than
/usr/local/pgsql?
2.5) When I run postmaster, I get a Bad System Call core dumped
message.
2.6) When I try to start the postmaster, I get IpcMemoryCreate errors.
2.7) I have changed a source file, but a recompile does not see the
change?
3) Operational questions
3.1) How do I specify a KEY or other constraints on a column?
3.2) Does PostgreSQL support nested subqueries?
3.3) How do I define a unique indices?
3.4) I've having a lot of problems using rules.
3.5) I can't seem to write into the middle of large objects reliably.
3.6) Does PostgreSQL have a graphical user interface? A report
generator? A embedded query language interface?
3.7) How can I write client applications to PostgreSQL?
3.8) How do I prevent other hosts from accessing my PostgreSQL
3.9) How do I set up a pg_group?
3.10) What is the exact difference between binary cursors and normal
cursors?
3.11) Why doesn't the != operator work?
3.12) What is a R-tree index and what is it used for?
3.13) What is the maximum size for a tuple?
3.14) I defined indices but my queries don't seem to make use of them.
Why?
3.15) Are there ODBC drivers for PostgreSQL?
3.16) How do I use postgres for multi-dimensional indexing (> 2
dimensions)?
3.17) How do I do regular expression searches? case-insensitive regexp
searching?
3.18) I can't access the database as the 'root' user.
3.19) I experienced a server crash during a vacuum. How do I remove
the lock file?
3.20) What is the difference between the various character types?
3.21) In a query, how do I detect if a field is NULL?
3.22) How do I see how the query optimizer is evaluating my query?
3.23) How do I create a serial field?
3.24) How do I create a multi-column index?
3.25) What are the temp_XXX files in my database directory?
3.26) Why are my table files not getting any smaller after a delete?
3.27) Why can't I connect to my database from another machine?
3.28) I get the error 'default index class unsupported' when creating
an index. How do I do it?
3.29) Why does creating an index crash the backend server?
3.30) How do I find out what indexes or operations are defined in the
database?
3.31) Why do statements require an extra character at the end? Why
does 'createuser' return 'unexpected last match in input()'? Why does
pg_dump fail?
3.32) All my servers crash under concurrent table access. Why?
3.33) What tools are available for hooking postgres to Web pages?
3.34) What is the time-warp feature and how does it relate to vacuum?
3.35) How do I tune the database engine for better performance?
3.36) What debugging features are available in PostgreSQL?
3.37) What is an oid? What is a tid?
3.38) What is the meaning of some of the terms used in Postgres?
3.39) What is Genetic Query Optimization?
3.40) I am running Solaris and my dates display wrong. Why? 3.41) How
do I enable more than 32 concurrent backends?
4) Questions about extending PostgreSQL
4.1) I wrote a user-defined function and when I run it in psql, it
dumps core.
4.2) I get messages of the type NOTICE:PortalHeapMemoryFree:
0x402251d0
4.3) I've written some nifty new types and functions for PostgreSQL.
4.4) How do I write a C function to return a tuple?
5) Bugs
5.1) How do I make a bug report?
_________________________________________________________________
Section 1: 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.
1.2) What does PostgreSQL run on?
The authors have compiled and tested PostgreSQL on the following
platforms(some of these compiles require gcc 2.7.0):
* aix - IBM on AIX 3.2.5 or 4.x
* alpha - DEC Alpha AXP on OSF/1 2.0
* BSD44_derived - OSs derived from 4.4-lite BSD (NetBSD, FreeBSD)
* bsdi - BSD/OS 2.0, 2.01, 2.1, 3.0
* dgux - DG/UX 5.4R3.10
* hpux - HP PA-RISC on HP-UX 9.0
* i386_solaris - i386 Solaris
* irix5 - SGI MIPS on IRIX 5.3
* linux - Intel x86 on Linux 1.2 and Linux ELF (For non-ELF Linux,
see LINUX_ELF below).
* sparc_solaris - SUN SPARC on Solaris 2.4
* sunos4 - SUN SPARC on SunOS 4.1.3
* svr4 - Intel x86 on Intel SVR4
* ultrix4 - DEC MIPS on Ultrix 4.4
The following platforms have known problems/bugs:
* nextstep - Motorola MC68K or Intel x86 on NeXTSTEP 3.2
1.3) Where can I get PostgreSQL?
The primary anonymous ftp site for PostgreSQL is:
* ftp://ftp.postgreSQL.org/pub
A mirror site exists at:
* ftp://postgres95.vnet.net/pub/postgres95
* ftp://ftp.luga.or.at/pub/postgres95
* ftp://cal011111.student.utwente.nl/pub/postgres95
* ftp://ftp.uni-trier.de/pub/database/rdbms/postgres/postgres95
* ftp://rocker.sch.bme.hu
1.4) What's the copyright on PostgreSQL?
PostgreSQL is subject to the following COPYRIGHT.
PostgreSQL Data Base Management System
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.
1.5) Support for PostgreSQL
There is no official support for PostgreSQL from the original
maintainers or from University of California, Berkeley. It is
maintained through volunteer effort only.
The main mailing list is: questions@postgreSQL.org. It is available
for discussion o f matters pertaining to PostgreSQL, including but not
limited to bug reports and fixes. For info on how to subscribe, send a
mail with the lines in the body (not the subject line)
subscribe
end
to questions-request@postgreSQL.org.
There is also a digest list available. To subscribe to this list, send
email to: questions-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.
There is a bugs mailing list available. To subscribe to this list,
send email to bugs-request@postgreSQL.org with a BODY of:
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 information about PostgreSQL can be found via the
PostgreSQL WWW home page at:
http://postgreSQL.org
1.6) Latest release of PostgreSQL
The latest release of PostgreSQL is version 6.1.1, which will be
released in July, 1997. For information about what is new in 6.1.1,
see our TODO list on our WWW page.
We expect a 7.0 release in several months that will remove time-travel
and reduce by 50% the size of on-disk system columns maintained for
each row in a table. This release will also require a dump and
restore.
1.7) Is there a commercial version of PostgreSQL?
Illustra Information Technology (a wholly owned subsidiary of Informix
Software, Inc.) sells an object-relational DBMS called Illustra that
was originally based on postgres. Illustra has cosmetic similarities
to PostgreSQL but has more features, is more robust, performs better,
and offers real documentation and support. On the flip side, it costs
money. For more information, contact sales@illustra.com
1.8) What documentation is available for PostgreSQL?
A user manual, manual pages, and some small test examples are included
in the distribution. The sql and built-in manual pages are
particularly important.
The www page contains pointers to an implementation guide and five
papers written about postgres design concepts and features.
1.9) What version of SQL does PostgreSQL use?
PostgreSQL supports a subset of SQL-92. It has most of the important
constructs but lacks some of the functionality. The most visible
differences are:
* no support for nested subqueries
* no HAVING clause under a GROUP BY
On the other hand, you get to create user-defined types, functions,
inheritance etc. If you're willing to help with PostgreSQL coding,
eventually we can also add the missing features listed above.
1.10) Does PostgreSQL work with databases from earlier versions of postgres?
PostgreSQL v1.09 is compatible with databases created with v1.01.
Those upgrading from 1.0 should read the directions in the
MIGRATION_1.0_TO_1.02 directory.
Upgrading to 6.0 requires a dump and restore from previous releases.
Upgrading to 6.1 requires a dump and restore from previous releases.
Upgrading from 6.1 to 6.1.1 requires a compile of the new release,
recompile of all your custom applications to use the new libpq, and
then an install while the postmaster is temporarily stopped.
Those ugrading from versions earlier than 1.09 must upgrade to 1.09
first without a dump/reload, then dump the data from 1.09, and then
load it into 6.1.1.
1.11) How many people use PostgreSQL?
Since we don't have any licensing or registration scheme, it's
impossible to tell. We do know hundreds copies of PostgreSQL v1.* have
been downloaded, and that there many hundreds of subscribers to the
mailing lists.
_________________________________________________________________
Section 2: Installation Questions
2.1) initdb doesn't run
* check to see that you have the proper paths set
* check that the 'postgres' user owns all the right files
* ensure that there are files in $PGDATA/files, and that they are
non-empty. If they aren't, then "gmake install" failed for some
reason
2.2) when I start up the postmaster, I get "FindBackend: could not find a
backend to execute..." "postmaster: could not find backend to execute..."
You probably do not have the right path set up. The 'postgres'
executable needs to be in your path.
2.3) 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. Set those accordingly for
your operating environment.
2.4) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
You need to edit Makefile.global and change POSTGRESDIR accordingly,
or create a Makefile.custom and define POSTGRESDIR there.
2.5) When I run postmaster, I get a Bad System Call core dumped message.
It could be a variety of problems, but first check to see that you
have system V extensions installed on your kernel. PostgreSQL requires
kernel support for shared memory.
2.6) When I try to start the postmaster, I get IpcMemoryCreate errors.
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 you configure postmaster to run with. For most systems, with
default buffer sizes, you need a minimum of ~760K.
2.7) I have changed a source file, but a 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'.
_________________________________________________________________
Section 3: PostgreSQL Features
3.1) How do I specify a KEY or other constraints on a column?
Column constraints are not supported in PostgreSQL. As a consequence,
the system does not check for duplicates.
Under 6.0, create a unique index on the column. Attempts to create
duplicate of that column will report an error.
3.2) Does PostgreSQL support nested subqueries?
Subqueries are not implemented, but they can be simulated using sql
functions.
3.3) How do I define a unique indices?
PostgreSQL 6.0 supports unique indices.
3.4) I've having a lot of problems using rules.
Currently, the rule system in PostgreSQL is mostly broken. It works
enough to support the view mechanism, but that's about it. Use
PostgreSQL rules at your own peril.
3.5) I can't seem to write into the middle of large objects reliably.
The Inversion large object system in PostgreSQL is also mostly broken.
It works well enough for storing large wads of data and reading them
back out, but the implementation has some underlying problems. Use
PostgreSQL large objects at your own peril.
3.6) Does PostgreSQL have a graphical user interface? A report generator? A
embedded query language interface?
No. No. No. Not in the official distribution at least. Some users have
reported some success at using 'pgbrowse' and 'onyx' as frontends to
PostgreSQL. Several contributions are working on tk based frontend
tools. Ask on the mailing list.
3.7) How can I write client applications to PostgreSQL?
PostgreSQL supports a C-callable library interface called libpq as
well as a Tcl-based library interface called libtcl.
Others have contributed a perl interface and a WWW gateway to
PostgreSQL. See the PostgreSQL home pages for more details.
3.8) How do I prevent other hosts from accessing my PostgreSQL backend?
Use host-based authentication by modifying the file $PGDATA/pg_hba
accordingly.
3.9) 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 char16 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[].
3.10) What is the exact difference between binary cursors and normal cursors?
Normal cursors return data back in ASCII format. Since data is stored
natively in binary format, the system must do a conversion to produce
the ASCII format. In addition, ASCII formats are often large in size
than binary format. Once the attributes come back in ASCII, often the
client application then has to convert it to a binary format to
manipulate it anyway.
Binary cursors give you back the data in the native binary
representation. Thus, binary cursors will tend to be a little faster
since there's less overhead of conversion.
However, ASCII is architectural neutral whereas binary representation
can differ between different machine architecture. Thus, if your
client machine uses a different representation than you server
machine, getting back attributes in binary format is probably not what
you want. Also, if your main purpose is displaying the data in ASCII,
then getting it back in ASCII will save you some effort on the client
side.
3.11) Why doesn't the != operator work?
SQL specifies <> as the inequality operator, and that is what we have
defined for the built-in types.
In 6.0, != is equivalent to <>.
3.12) What is a R-tree index and what is it used for?
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 a 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"
3.13) What is the maximum size for a tuple?
Tuples are limited to 8K bytes. Taking into account system attributes
and other overhead, one should stay well shy of 8,000 bytes to be on
the safe side. To use attributes larger than 8K, try using the large
objects interface.
Tuples do not cross 8k boundaries so a 5k tuple will require 8k of
storage.
3.14) I defined indices but my queries don't seem to make use of them. 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 has a better shot at using indices. Note
that the optimizer is limited and does not use indices in some
circumstances (such as OR clauses).
If the system still does not see the index, it is probably because you
have created an index on a field with the improper *_ops type. For
example, you have created a CHAR(4) field, but have specified a
char_ops index type_class.
See the create_index manual page for information on what type classes
are available. It must match the field type.
Postgres does not warn the user when the improper index is created.
Indexes not used for ORDER BY operations.
3.15) Are there ODBC drivers for PostgreSQL?
There are two ODBC drivers available, PostODBC and OpenLink ODBC.
For all people being interested in PostODBC, there are now two mailing
lists devoted to the discussion of PostODBC. The mailing lists are:
* postodbc-users@listserv.direct. net
* postodbc-developers@listse rv.direct.net
these lists are ordinary majordomo mailing lists. You can subscribe by
sending a mail to:
* majordomo@listserv.direct.net
OpenLink ODBC is currently in beta under Linux. You can get it from
http://www.openlinksw.com/postgres.html. It works with our standard
ODBC client software so you'll have Postgres ODBC available on every
client platform we support (Win, Mac, Unix, VMS).
We 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.
3.16) How do I use postgres for multi-dimensional indexing (> 2 dimensions)?
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.
3.17) How do I do regular expression searches? case-insensitive regexp
searching?
PostgreSQL supports the SQL LIKE syntax as well as more general
regular expression searching with the ~ operator. The !~ is the
negated regexp operator. ~* and !~* are the case-insensitive regular
expression operators.
3.18) I can't 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.
3.19) I experienced a server crash during a vacuum. How do I remove the lock
file?
If the server crashes during a vacuum command, chances are it will
leave a lock file hanging around. Attempts to re-run the vacuum
command result in
WARN:can't create lock file -- another vacuum cleaner running?
If you are sure that no vacuum is actually running, you can remove the
file called "pg_vlock" in your database directory (which is
$PGDATA/base/<dbName>)
3.20) What is the difference between the various character types?
Type Internal Name Notes
--------------------------------------------------
CHAR char 1 character }
CHAR2 char2 2 characters }
CHAR4 char4 4 characters } optimized for a fixed length
CHAR8 char8 8 characters }
CHAR16 char16 16 characters }
CHAR(#) bpchar blank padded to the specified fixed length
VARCHAR(#) varchar size specifies maximum length, no padding
TEXT text length limited only by maximum tuple length
BYTEA bytea variable-length array of bytes
Remember, you need to use the internal name when creating indexes on
these fields or when doing other internal operations.
The last four types above are "varlena" types (i.e. the first four
bytes is the length, followed by the data). CHAR(#) and VARCHAR(#)
allocate the maximum number of bytes no matter how much data is stored
in the field. TEXT and BYTEA are the only character types that have
variable length on the disk.
3.21) In a query, how do I detect if a field is NULL?
PostgreSQL has two builtin keywords, "isnull" and "notnull" (note no
spaces). Version 1.05 and later and 6.* understand IS NULL and IS NOT
NULL.
3.22) How do I see how the query optimizer is evaluating my query?
Place the word 'EXPLAIN' at the beginning of the query, for example:
EXPLAIN SELECT * FROM table1 WHERE age = 23;
3.23) How do I create a serial field?
Postgres does not allow the user to specifiy a user column as type
SERIAL. Instead, you can use each row's oid field as a unique value.
However, if you need to dump and reload the database, you need to be
using postgres version 1.07 or later or 6.* with pgdump's -o option or
COPY's WITH OIDS option to preserver the oids.
Another valid way of doing this is to create a function:
create table my_oids (f1 int4);
insert into my_oids values (1);
create function new_oid () returns int4 as
'update my_oids set f1 = f1 + 1; select f1 from my_oids; '
language 'sql';
then:
create table my_stuff (my_key int4, value text);
insert into my_stuff values (new_oid(), 'hello');
However, keep in mind there is a race condition here where one server
could do the update, then another one do an update, and they both
could select the same new id. This statement should be performed
within a transaction.
Sequences were implemented in 6.1
3.24) How do I create a multi-column index?
In 6.0, you can not directly create a multi-column index using create
index. You need to define a function which acts on the multiple
columns, then use create index with that function.
In 6.1, this feature is available.
3.25) What are the temp_XXX files in my database directory?
They are temp_ files generated by the query executor. For example, if
a sort needs to be done to satisfy an ORDER BY, some temp files are
generated as a result of the sort.
If you have no transactions or sorts running at the time, it is safe
to delete the temp_ files.
3.26) Why are my table files not getting any smaller after a delete?
If you run vacuum in pre-6.0, unused rows will be marked for reuse,
but the file blocks are not released.
In 6.0, vacuum properly shrinks tables.
3.27) Why can't I connect to my database from another machine?
The default configuration allows only connections from tcp/ip host
localhost. You need to add a host entry to the file pgsql/data/pg_hba.
3.28) I get the error 'default index class unsupported' when creating an
index. How do I do it?
You probably used:
create index idx1 on person using btree (name);
PostgreSQL indexes are extensible, and therefore in pre-6.0, you must
specify a class_type when creating an index. Read the manual page for
create index (called create_index).
Version 6.0, if you do not specify a class_type, it defaults to the
proper type for the column.
3.29) Why does creating an index crash the backend server?
You have probably defined an incorrect *_ops type class for the field
you are indexing.
3.30) How do I find out what indexes or operations are defined in the
database?
Run the file pgsql/src/tutorial/syscat.source. It illustrates many of
the 'select's needed to get information out of the database system
tables.
3.31) Why do statements require an extra character at the end? Why does
'createuser' return 'unexpected last match in input()'? Why does pg_dump
fail?
You have compile postgres with flex version 2.5.3. There is bug in
this version of flex. Use flex version 2.5.2 or flex 2.5.4 instead.
There is a doc/README.flex file which will properly patch the flex
2.5.3 source code.
3.32) All my servers crash under concurrent table access. Why?
This problem can be caused by a kernel that is not configured to
support semaphores.
3.33) What tools are available for hooking postgres to Web pages?
For web integration, PHP/FI is an excellent interface. The URL for
that is http://www.vex.net/php/
PHP is great for simple stuff, but for more complex stuff, some still
use the perl interface and CGI.pm.
An example of using WWW with C to talk to Postgres is can be tried at:
* http://postgreSQL.org/~mlc
An WWW gatway based on WDB using perl can be downloaded from:
* http://www.eol.ists.ca/~dunlop/wdb -p95
3.34) What is the time-warp feature and how does it relate to vacuum?
PostgreSQL handles data changes differently than most database
systems. When a row is changed in a table, the original row is marked
with the time it was changed, and a new row is created with the
current data. By default, only current rows are used in a table. If
you specify a date/time after the table name in a FROM clause, you can
access the data that was current at that time, i.e.
SELECT *
FROM employees ['July 24, 1996 09:00:00']
displays employee rows in the table at the specified time. You can
specify intervals like [date,date], [date,], [,date], or [,]. This
last option accesses all rows that ever existed.
INSERTed rows get a timestamp too, so rows that were not in the table
at the desired time will not appear.
Vacuum removes rows that are no longer current. This time-warp feature
is used by the engine for rollback and crash recovery. Expiration
times can be set with purge.
In 6.0, once a table is vacuumed, the creation time of a row may be
incorrect, causing time-traval to fail.
The time-travel feature will be removed in 7.0.
3.35) How do I tune the database engine for better performance?
There are two things that can be done. You can use Openlink's option
to 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 shared among the backend processes. If you make
this parameter too high, the process will not start or crash
unexpectedly. Each buffer is 8K and the defualt is 64 buffers.
3.36) What debugging features are available in PostgreSQL?
PostgreSQL has several features that report status information that
can be valuable for debugging purposes.
First, by compiling with DEBUG defined, 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 can contain 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
1-3 that specifies the debug level. The query plans in a verbose debug
file can be formatted using the 'indent' program. (You may need to
remove the '====' lines in 1.* releases.) Be warned that a debug level
greater than one generates large log files in 1.* releases.
You can actuall 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
perhaps 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.
The postgres program has a -s, -A, -t options that can be very usefull
for debugging and performance measurements.
The EXPLAIN command (see this FAQ) allows you to see how PostgreSQL is
iterpreting your query.
3.37) What is an oid? What is a tid?
Oids are Postgres's answer to unique row ids or serial columns. Every
row that is created in Postgres gets a unique oid. All oids generated
by initdb are less than 16384 (from backend/access/transam.h). All
post-initdb (user-created) oids are equal or greater that this. All
these oids are unique not only within a table, or database, but unique
within the entire postgres installation.
Postgres uses oids in its internal system tables to link rows in
separate 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.
Tids are used to indentify 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. They can not be
accessed through sql.
3.38) What is the meaning of some of the terms used in Postgres?
Some of the source code and older documentation use terms that have
more common usage. Here are some:
* row, record, tuple
* attribute, field, column
* table, class
* retrieve, select
* replace, update
* append, insert
* oid, serial value
* portal, cursor
* range variable, table name, table alias
Please let me know if you think of any more.
3.39) 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 README.GEQO <utesch@aut.tu-freiberg.de>.
3.40) I am running Solaris and my dates display wrong. Why?
There was a bug in 6.0 that caused this problem under Solaris with -O2
optimization. Upgrade to 6.1.1.
3.41) How do I enable more than 32 concurrent backends?
Edit include/storage/sinvaladt.h, and change the value of
MaxBackendId. In the future, we plan to make this a configurable
prameter.
_________________________________________________________________
Section 4: Extending PostgreSQL
4.1) I wrote a user-defined function and when I run it in psql, it dumps
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
4.2) I get messages of the type NOTICE:PortalHeapMemoryFree: 0x402251d0 not
in alloc set!
You are pfree'ing something that was not palloc'ed. When writing
user-defined functions, do not include the file "libpq-fe.h". Doing so
will cause your palloc to be a malloc instead of a free. Then, when
the backend pfrees the storage, you get the notice message.
4.3) I've written some nifty new types and functions for PostgreSQL.
Please share them with other PostgreSQL users. Send your extensions to
mailing list, and they will eventually end up in the contrib/
subdirectory.
4.4) How do I write a C function to return a tuple?
This requires extreme wizardry, so extreme that the authors have not
ever tried it, though in principle it can be done. The short answer is
... you can't. This capability is forthcoming in the future.
_________________________________________________________________
Section 5: Bugs
5.1) How do I make a bug report?
Check the current FAQ at http://postgreSQL.org
Also check out our ftp site ftp://ftp.postgreSQL.org/pub to see if
there is a more recent PostgreSQL version.
You can also fill out the "bug-template" file and send it to:
* bugs@postgreSQL.org
This is the address of the developers mailing list.
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