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Update FAQ's for release.

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Bruce Momjian
1999-06-05 03:43:07 +00:00
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Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Sat Oct 24 00:12:23 EDT 1998
Last updated: Fri Jun 4 23:30:19 EDT 1999
Current maintainer: Bruce Momjian (maillist@candle.pha.pa.us)
@ -9,13 +9,10 @@
postgreSQL Web site, http://postgreSQL.org.
Linux-specific questions are answered in
http://postgreSQL.org/docs/faq-linux.shtml.
http://postgreSQL.org/docs/faq-linux.html.
Irix-specific questions are answered in
http://postgreSQL.org/docs/faq-irix.shtml.
HPUX-specific questions are answered in
http://postgreSQL.org/docs/faq-hpux.shtml.
http://postgreSQL.org/docs/faq-irix.html.
_________________________________________________________________
General questions
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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 64 concurrent backends?
2.14) What non-unix ports are available?
2.13) When I try to start the postmaster, I get IpcSemaphoreCreate
errors. Why?
2.14) I get 'Sorry, too many clients' when trying to connect. Why?
2.15) What non-unix ports are available?
Operational questions
@ -70,9 +69,8 @@
3.4) What is the exact difference between binary cursors and normal
cursors?
3.5) What is an R-tree index and what is it used for?
3.6) What is the maximum size for a tuple?
3.7) I defined indices but my queries don't seem to make use of them.
Why?
3.6) What is the maximum size for a row, table, database?
3.7) My queries are slow or don't make use of the indexes. Why?
3.8) How do I do regular expression searches? case-insensitive regexp
searching?
3.9) I experienced a server crash during a vacuum. How do I remove the
@ -240,11 +238,11 @@ Section 1: General Questions
http://postgreSQL.org
There also an IRC channel on EFNet, channel #PostgreSQL. I use the
unix command irc -c '#PostgreSQL' "$USER" irc.ais.net
unix command irc -c '#PostgreSQL' "$USER" irc.phoenix.net
1.6) Latest release of PostgreSQL
The latest release of PostgreSQL is version 6.4.
The latest release of PostgreSQL is version 6.5.
We plan to have major releases every four months.
@ -274,13 +272,12 @@ Section 1: General Questions
1.10) Does PostgreSQL work with databases from earlier versions of
PostgreSQL?
Upgrading to 6.4 from release 6.3.* can be accomplished using the new
pg_upgrade utility. Those upgrading from earlier releases require a
dump and restore.
Upgrading to 6.5 can not use the pg_upgrade utility. Those upgrading
from earlier releases require a dump and restore.
Those ugrading from versions earlier than 1.09 must upgrade to 1.09
Those upgrading 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 6.4.
load it into 6.5.
1.11) Are there ODBC drivers for PostgreSQL?
@ -309,14 +306,14 @@ Section 1: General Questions
PHP is great for simple stuff, but for more complex stuff, some still
use the perl interface and CGI.pm.
An WWW gatway based on WDB using perl can be downloaded from
An WWW gateway 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?
We have a nice graphical user interface called pgaccess, which is
shipped as part of the distribtion. Pgaccess also has a report
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,
@ -365,8 +362,9 @@ Section 2: Installation Questions
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.
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.
2.4) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
@ -384,8 +382,9 @@ Section 2: Installation Questions
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.
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.
2.7) I have changed a source file, but a recompile does not see the change?
@ -420,7 +419,7 @@ Section 2: Installation Questions
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 their in their own transaction.
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.
@ -436,7 +435,7 @@ Section 2: Installation Questions
You can also use the postgres -S option to increase the maximum amount
of memory used by each backend process for temporary sorts. Each
buffer is 1K and the defualt is 512 buffers.
buffer is 1K and the default is 512 buffers.
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.
@ -474,7 +473,7 @@ Section 2: Installation Questions
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
The postgres program has a -s, -A, -t options that can be very useful
for debugging and performance measurements.
You can also compile with profiling to see what functions are taking
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pgsql/data/base/dbname directory. The client profile file will be put
in the current directory.
2.13) How do I enable more than 64 concurrent backends?
2.13) When I try to start the postmaster, I get IpcSemaphoreCreate errors.
Why?
Edit include/storage/sinvaladt.h, and change the value of
MaxBackendId. In the future, we plan to make this a configurable
prameter.
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.
2.14) What non-unix ports are available?
If the error message is something else, you might not have semaphore
support configured in your kernel at all.
2.14) I get 'Sorry, too many clients' when trying to connect. Why?
You need to increase the postmaster's limit on how many concurrent
backend processes it can start.
In Postgres 6.5, the default limit is 32 processes. You can increase
it by restarting the postmaster with a suitable -N value. With the
default configuration you can set -N as large as 1024; if you need
more, increase MAXBACKENDS in include/config.h and rebuild. You can
set the default value of -N at configuration time, if you like, using
configure's --with-maxbackends switch.
Note that if you make -N larger than 32, you should consider
increasing -B beyond its default of 64. For large numbers of backend
processes, you are also likely to find that you need to increase
various Unix kernel configuration parameters. Things to check include
the maximum size of shared memory blocks, SHMMAX, the maximum number
of semaphores, SEMMNS and SEMMNI, the maximum number of processes,
NPROC, the maximum number of processes per user, MAXUPRC, and the
maximum number of open files, NFILE and NINODE. The reason that
Postgres has a limit on the number of allowed backend processes is so
that you can ensure that your system won't run out of resources.
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.
2.15) 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 win32.mak is included in the distributiion for making a Win32
A file win32.mak is included in the distribution for making a Win32
libpq library and psql.
Someone is attempting to port our PostgreSQL database server to
Windows NT using the Cygnus Unix/NT porting library. He has gotten it
compiled, but initdb is currently failing.
The database server is now working on Windows NT using the Cygnus
Unix/NT porting library. The only feature missing is dynamic loading
of user-defined functions/types. See
http://www.askesis.nl/AskesisPostgresIndex.html for more information.
There is another port using U/Win at
http://surya.wipro.com/uwin/ported.html.
_________________________________________________________________
Section 3: PostgreSQL Features
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extending R-trees require a bit of work and we don't currently have
any documentation on how to do it.
3.6) What is the maximum size for a tuple?
3.6) What is the maximum size for a row, table, database?
Tuples are limited to 8K bytes. Taking into account system attributes
Rows 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
Rows do not cross 8k boundaries so a 5k row will require 8k of
storage.
3.7) I defined indices but my queries don't seem to make use of them. Why?
Table and database sizes are unlimited. There are many databases that
are tens of gigabytes, and probably some that are hundreds of
gigabytes.
3.7) 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 sequentail
scan would be faster. For column-specific optimization statistics, use
vacuum analyze.
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, and vacuum analyze must be run to
collect them periodically.
Indexes are not used for order by operations.
When using wildcard operators like LIKE or ~, indices can only be used
if the beginning of the search is anchored to the start of the string.
So, to use indices, LIKE searches can should not begin with %, and
~(regular expression searches) should start with ^.
When using wild-card operators like LIKE or ~, indices can only be
used if the beginning of the search is anchored to the start of the
string. So, to use indices, LIKE searches can should not begin with %,
and ~(regular expression searches) should start with ^.
3.8) How do I do regular expression searches? case-insensitive regexp
searching?
@ -606,7 +654,7 @@ 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 tuple length
TEXT text length limited only by maximum row length
BYTEA bytea variable-length array of bytes
You need to use the internal name when doing internal operations.
@ -681,12 +729,15 @@ BYTEA bytea variable-length array of bytes
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 (mycol int);
INSERT INTO new_table SELECT oid, mycol FROM old_table;
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 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.
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.
3.18) What is the meaning of some of the terms used in PostgreSQL?
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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.
You are pfree'ing something that was not palloc'ed. Beware of mixing
malloc/free and palloc/pfree.
4.3) I've written some nifty new types and functions for PostgreSQL.