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Frequently Asked Questions (FAQ) for PostgreSQL Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Tue Jul 29 10:09:15 EDT 1997 Last updated: Thu Sep 4 01:32:22 EDT 1997
Version: 6.1.1 Version: 6.1.1
Current maintainer: Bruce Momjian (maillist@candle.pha.pa.us) Current maintainer: Bruce Momjian (maillist@candle.pha.pa.us)
@ -16,9 +16,6 @@
http://postgreSQL.org/docs/FAQ-irix.shtml. http://postgreSQL.org/docs/FAQ-irix.shtml.
Changes in this version (* = modified, + = new): 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: Questions answered:
@ -32,12 +29,16 @@ Questions answered:
1.5) Support for PostgreSQL 1.5) Support for PostgreSQL
1.6) Latest release of PostgreSQL 1.6) Latest release of PostgreSQL
1.7) Is there a commercial version of PostgreSQL? 1.7) Is there a commercial version of PostgreSQL?
1.9) What version of SQL does PostgreSQL use? 1.8) What version of SQL does PostgreSQL use?
1.9) What documentation is available for PostgreSQL?
1.10) Does PostgreSQL work with databases from earlier versions of 1.10) Does PostgreSQL work with databases from earlier versions of
postgres? postgres?
1.11) How many people use PostgreSQL? 1.11) Are there ODBC drivers for PostgreSQL?
1.12) What tools are available for hooking postgres to Web pages?
1.13) Does PostgreSQL have a graphical user interface? A report
generator? A embedded query language interface?
2) Installation questions 2) Installation/Configuration questions
2.1) initdb doesn't run 2.1) initdb doesn't run
2.2) when I start up the postmaster, I get "FindBackend: could not 2.2) when I start up the postmaster, I get "FindBackend: could not
@ -52,60 +53,43 @@ Questions answered:
2.6) When I try to start the postmaster, I get IpcMemoryCreate errors. 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 2.7) I have changed a source file, but a recompile does not see the
change? change?
2.8) How do I prevent other hosts from accessing my PostgreSQL
2.9) I can't access the database as the 'root' user.
2.10) All my servers crash under concurrent table access. Why?
2.11) How do I tune the database engine for better performance?
2.12) What debugging features are available in PostgreSQL?
2.13) How do I enable more than 32 concurrent backends?
3) Operational questions 3) Operational questions
3.1) How do I specify a KEY or other constraints on a column? 3.1) Does PostgreSQL support nested subqueries?
3.2) Does PostgreSQL support nested subqueries? 3.2) I've having a lot of problems using rules.
3.3) How do I define a unique indices? 3.3) I can't seem to write into the middle of large objects reliably.
3.4) I've having a lot of problems using rules. 3.4) How can I write client applications to PostgreSQL?
3.5) I can't seem to write into the middle of large objects reliably. 3.5) How do I set up a pg_group?
3.6) Does PostgreSQL have a graphical user interface? A report 3.6) What is the exact difference between binary cursors and normal
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? cursors?
3.11) Why doesn't the != operator work? 3.7) What is a R-tree index and what is it used for?
3.12) What is a R-tree index and what is it used for? 3.8) What is the maximum size for a tuple?
3.13) What is the maximum size for a tuple? 3.9) I defined indices but my queries don't seem to make use of them.
3.14) I defined indices but my queries don't seem to make use of them.
Why? Why?
3.15) Are there ODBC drivers for PostgreSQL? 3.10) How do I do regular expression searches? case-insensitive regexp
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? searching?
3.18) I can't access the database as the 'root' user. 3.11) I experienced a server crash during a vacuum. How do I remove
3.19) I experienced a server crash during a vacuum. How do I remove
the lock file? the lock file?
3.20) What is the difference between the various character types? 3.12) What is the difference between the various character types?
3.21) In a query, how do I detect if a field is NULL? 3.13) 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? 3.14) How do I see how the query optimizer is evaluating my query?
3.23) How do I create a serial field? 3.15) How do I create a serial field?
3.24) How do I create a multi-column index? 3.16) What are the pg_psort.XXX files in my database directory?
3.25) What are the temp_XXX files in my database directory? 3.17) Why can't I connect to my database from another machine?
3.26) Why are my table files not getting any smaller after a delete? 3.18) How do I find out what indexes or operations are defined in the
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? database?
3.31) Why do statements require an extra character at the end? Why 3.19) What is the time-warp feature and how does it relate to vacuum?
does 'createuser' return 'unexpected last match in input()'? Why does 3.20) What is an oid? What is a tid?
pg_dump fail? 3.21) What is the meaning of some of the terms used in Postgres?
3.32) All my servers crash under concurrent table access. Why? 3.22) What is Genetic Query Optimization?
3.33) What tools are available for hooking postgres to Web pages? 3.23) How do you remove a column from a table?
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) Questions about extending PostgreSQL
@ -260,10 +244,7 @@ Section 1: General Questions
on July 22, 1997. For information about what is new in 6.1.1, see our on July 22, 1997. For information about what is new in 6.1.1, see our
TODO list on our WWW page. TODO list on our WWW page.
We expect a 7.0 release in several months that will remove time-travel We expect a 6.2 release at the end of September.
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? 1.7) Is there a commercial version of PostgreSQL?
@ -298,10 +279,6 @@ Section 1: General Questions
1.10) Does PostgreSQL work with databases from earlier versions of postgres? 1.10) Does PostgreSQL work with databases from earlier versions of postgres?
PostgreSQL v1.09 is compatible with databases created with v1.01. 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 to 6.1 requires a dump and restore from previous releases.
@ -314,13 +291,49 @@ Section 1: General Questions
first without a dump/reload, then dump the data from 1.09, and then first without a dump/reload, then dump the data from 1.09, and then
load it into 6.1.1. load it into 6.1.1.
1.11) How many people use PostgreSQL? 1.11) Are there ODBC drivers for PostgreSQL?
Since we don't have any licensing or registration scheme, it's There are two ODBC drivers available, PostODBC and OpenLink ODBC.
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.
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 very popular. 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.
1.12) 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
1.13) 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.
_________________________________________________________________ _________________________________________________________________
Section 2: Installation Questions Section 2: Installation Questions
@ -370,62 +383,115 @@ Section 2: Installation Questions
The Makefiles do not have the proper dependencies for include files. 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'.
2.8) How do I prevent other hosts from accessing my PostgreSQL backend?
Use host-based authentication by modifying the file $PGDATA/pg_hba
accordingly.
2.9) 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.
2.10) All my servers crash under concurrent table access. Why?
This problem can be caused by a kernel that is not configured to
support semaphores.
2.11) 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.
2.12) 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 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. Be warned that a
debug level of three generates large log files.
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.
You can also compile with profiling to see what functions are taking
execution time.
2.13) 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 3: PostgreSQL Features Section 3: PostgreSQL Features
3.1) How do I specify a KEY or other constraints on a column? 3.1) Does PostgreSQL support nested subqueries?
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 Subqueries are not implemented, but they can be simulated using sql
functions. functions.
3.3) How do I define a unique indices? 3.2) I've having a lot of problems using rules.
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 Currently, the rule system in PostgreSQL is mostly broken. It works
enough to support the view mechanism, but that's about it. Use enough to support the view mechanism, but that's about it. Use
PostgreSQL rules at your own peril. PostgreSQL rules at your own peril.
3.5) I can't seem to write into the middle of large objects reliably. 3.3) I can't seem to write into the middle of large objects reliably.
The Inversion large object system in PostgreSQL is also mostly broken. The Inversion large object system in PostgreSQL is also mostly broken.
It works well enough for storing large wads of data and reading them It works well enough for storing large wads of data and reading them
back out, but the implementation has some underlying problems. Use back out, but the implementation has some underlying problems. Use
PostgreSQL large objects at your own peril. PostgreSQL large objects at your own peril.
3.6) Does PostgreSQL have a graphical user interface? A report generator? A 3.4) How can I write client applications to PostgreSQL?
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 PostgreSQL supports a C-callable library interface called libpq as
well as a Tcl-based library interface called libtcl. well as many others. See the /src/interfaces directory.
Others have contributed a perl interface and a WWW gateway to Others have contributed a perl interface and a WWW gateway to
PostgreSQL. See the PostgreSQL home pages for more details. PostgreSQL. See the PostgreSQL home pages for more details.
3.8) How do I prevent other hosts from accessing my PostgreSQL backend? 3.5) How do I set up a pg_group?
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 Currently, there is no easy interface to set up user groups. You have
to explicitly insert/update the pg_group table. For example: to explicitly insert/update the pg_group table. For example:
@ -446,7 +512,7 @@ Section 3: PostgreSQL Features
* grolist: the list of pg_user id's that belong in the group. This * grolist: the list of pg_user id's that belong in the group. This
is an int4[]. is an int4[].
3.10) What is the exact difference between binary cursors and normal cursors? 3.6) What is the exact difference between binary cursors and normal cursors?
Normal cursors return data back in ASCII format. Since data is stored Normal cursors return data back in ASCII format. Since data is stored
natively in binary format, the system must do a conversion to produce natively in binary format, the system must do a conversion to produce
@ -467,14 +533,7 @@ Section 3: PostgreSQL Features
then getting it back in ASCII will save you some effort on the client then getting it back in ASCII will save you some effort on the client
side. side.
3.11) Why doesn't the != operator work? 3.7) What is a R-tree index and what is it used for?
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 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 handle range searches. A B-tree index only handles range searches in a
@ -492,7 +551,12 @@ Section 3: PostgreSQL Features
You can also find this paper in Stonebraker's "Readings in Database You can also find this paper in Stonebraker's "Readings in Database
Systems" Systems"
3.13) What is the maximum size for a tuple? 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.
3.8) What is the maximum size for a tuple?
Tuples are limited to 8K bytes. Taking into account system attributes 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 and other overhead, one should stay well shy of 8,000 bytes to be on
@ -502,13 +566,14 @@ Section 3: PostgreSQL Features
Tuples do not cross 8k boundaries so a 5k tuple will require 8k of Tuples do not cross 8k boundaries so a 5k tuple will require 8k of
storage. storage.
3.14) I defined indices but my queries don't seem to make use of them. Why? 3.9) 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 PostgreSQL does not automatically maintain statistics. One has to make
an explicit 'vacuum' call to update the statistics. After statistics an explicit 'vacuum' call to update the statistics. After statistics
are updated, the optimizer has a better shot at using indices. Note are updated, the optimizer has a better shot at using indices. Note
that the optimizer is limited and does not use indices in some that the optimizer is limited and does not use indices in some
circumstances (such as OR clauses). circumstances (such as OR clauses). For column-specific optimization
statistics, use 'vacuum analyze'.
If the system still does not see the index, it is probably because you 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 have created an index on a field with the improper *_ops type. For
@ -522,36 +587,7 @@ Section 3: PostgreSQL Features
Indexes not used for ORDER BY operations. Indexes not used for ORDER BY operations.
3.15) Are there ODBC drivers for PostgreSQL? 3.10) How do I do regular expression searches? case-insensitive regexp
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? searching?
PostgreSQL supports the SQL LIKE syntax as well as more general PostgreSQL supports the SQL LIKE syntax as well as more general
@ -559,14 +595,7 @@ Section 3: PostgreSQL Features
negated regexp operator. ~* and !~* are the case-insensitive regular negated regexp operator. ~* and !~* are the case-insensitive regular
expression operators. expression operators.
3.18) I can't access the database as the 'root' user. 3.11) I experienced a server crash during a vacuum. How do I remove the lock
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? file?
If the server crashes during a vacuum command, chances are it will If the server crashes during a vacuum command, chances are it will
@ -580,7 +609,7 @@ Section 3: PostgreSQL Features
file called "pg_vlock" in your database directory (which is file called "pg_vlock" in your database directory (which is
$PGDATA/base/<dbName>) $PGDATA/base/<dbName>)
3.20) What is the difference between the various character types? 3.12) What is the difference between the various character types?
Type Internal Name Notes Type Internal Name Notes
-------------------------------------------------- --------------------------------------------------
@ -603,25 +632,25 @@ BYTEA bytea variable-length array of bytes
in the field. TEXT and BYTEA are the only character types that have in the field. TEXT and BYTEA are the only character types that have
variable length on the disk. variable length on the disk.
3.21) In a query, how do I detect if a field is NULL? 3.13) In a query, how do I detect if a field is NULL?
PostgreSQL has two builtin keywords, "isnull" and "notnull" (note no You test the column with IS NULL and IS NOT NULL.
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? 3.14) How do I see how the query optimizer is evaluating my query?
Place the word 'EXPLAIN' at the beginning of the query, for example: Place the word 'EXPLAIN' at the beginning of the query, for example:
EXPLAIN SELECT * FROM table1 WHERE age = 23; EXPLAIN SELECT * FROM table1 WHERE age = 23;
3.23) How do I create a serial field? 3.15) How do I create a serial field?
Postgres does not allow the user to specifiy a user column as type 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. 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 However, if you need to dump and reload the database, you need to use
using postgres version 1.07 or later or 6.* with pgdump's -o option or pgdump's -o option or COPY's WITH OIDS option to preserver the oids.
COPY's WITH OIDS option to preserver the oids.
We also have a SEQUENCE function that is similar to SERIAL. See the
create_sequence manual page.
Another valid way of doing this is to create a function: Another valid way of doing this is to create a function:
@ -641,92 +670,28 @@ BYTEA bytea variable-length array of bytes
could select the same new id. This statement should be performed could select the same new id. This statement should be performed
within a transaction. within a transaction.
Sequences were implemented in 6.1 3.16) What are the pg_psort.XXX files in my database directory?
3.24) How do I create a multi-column index? They are temporary sort files generated by the query executor. For
example, if a sort needs to be done to satisfy an ORDER BY, some temp
In 6.0, you can not directly create a multi-column index using create files are generated as a result of the sort.
index. You need to define a function which acts on the multiple
columns, then use create index with that function.
This feature is available in 6.1 and later releases.
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 If you have no transactions or sorts running at the time, it is safe
to delete the temp_ files. to delete the pg_psort.XXX files.
3.26) Why are my table files not getting any smaller after a delete? 3.17) Why can't I connect to my database from another machine?
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 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. 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 3.18) How do I find out what indexes or operations are defined in the
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? database?
Run the file pgsql/src/tutorial/syscat.source. It illustrates many of Run the file pgsql/src/tutorial/syscat.source. It illustrates many of
the 'select's needed to get information out of the database system the 'select's needed to get information out of the database system
tables. tables.
3.31) Why do statements require an extra character at the end? Why does 3.19) What is the time-warp feature and how does it relate to vacuum?
'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 PostgreSQL handles data changes differently than most database
systems. When a row is changed in a table, the original row is marked systems. When a row is changed in a table, the original row is marked
@ -755,61 +720,7 @@ BYTEA bytea variable-length array of bytes
The time-travel feature will be removed in 7.0. The time-travel feature will be removed in 7.0.
3.35) How do I tune the database engine for better performance? 3.20) What is an oid? What is a tid?
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 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 row that is created in Postgres gets a unique oid. All oids generated
@ -829,7 +740,7 @@ BYTEA bytea variable-length array of bytes
are used by index entries to point to physical rows. They can not be are used by index entries to point to physical rows. They can not be
accessed through sql. accessed through sql.
3.38) What is the meaning of some of the terms used in Postgres? 3.21) What is the meaning of some of the terms used in Postgres?
Some of the source code and older documentation use terms that have Some of the source code and older documentation use terms that have
more common usage. Here are some: more common usage. Here are some:
@ -845,7 +756,7 @@ BYTEA bytea variable-length array of bytes
Please let me know if you think of any more. Please let me know if you think of any more.
3.39) What is Genetic Query Optimization? 3.22) What is Genetic Query Optimization?
The GEQO module in PostgreSQL is intended to solve the query The GEQO module in PostgreSQL is intended to solve the query
optimization problem of joining many tables by means of a Genetic optimization problem of joining many tables by means of a Genetic
@ -854,17 +765,15 @@ BYTEA bytea variable-length array of bytes
For further information see README.GEQO <utesch@aut.tu-freiberg.de>. For further information see README.GEQO <utesch@aut.tu-freiberg.de>.
3.40) I am running Solaris and my dates display wrong. Why? 3.23) How do you remove a column from a table?
There was a bug in 6.0 that caused this problem under Solaris with -O2 We do not support ALTER TABLE DROP COLUMN, but do this:
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.
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;
_________________________________________________________________ _________________________________________________________________
Section 4: Extending PostgreSQL Section 4: Extending PostgreSQL
@ -896,7 +805,6 @@ Section 4: Extending PostgreSQL
This requires extreme wizardry, so extreme that the authors have not 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 ever tried it, though in principle it can be done. The short answer is
... you can't. This capability is forthcoming in the future. ... you can't. This capability is forthcoming in the future.
_________________________________________________________________ _________________________________________________________________
Section 5: Bugs Section 5: Bugs