Frequently Asked Questions (FAQ) for PostgreSQL
- Last updated: Tue Feb 12 12:18:09 EST 2002
+ Last updated: Thu Feb 14 12:14:47 EST 2002
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
Why?
3.4) When I try to start postmaster, I get IpcSemaphoreCreate errors.
Why?
- 3.5) How do I prevent other hosts from accessing my PostgreSQL
- database?
- 3.6) Why can't I connect to my database from another machine?
- 3.7) How do I tune the database engine for better performance?
- 3.8) What debugging features are available?
- 3.9) Why do I get "Sorry, too many clients" when trying to connect?
- 3.10) What are the pg_sorttempNNN.NN files in my database directory?
+ 3.5) How do I control connections from other hosts?
+ 3.6) How do I tune the database engine for better performance?
+ 3.7) What debugging features are available?
+ 3.8) Why do I get "Sorry, too many clients" when trying to connect?
+ 3.9) What are the pg_sorttempNNN.NN files in my database directory?
Operational Questions
Administrator's Guide for more detailed information about shared
memory and semaphores.
- 3.5) How do I prevent other hosts from accessing my PostgreSQL database?
+ 3.5) How do I control connections from other hosts?
By default, PostgreSQL only allows connections from the local machine
using Unix domain sockets. Other machines will not be able to connect
authentication by modifying the file $PGDATA/pg_hba.conf accordingly.
This will allow TCP/IP connections.
- 3.6) Why can't I connect to my database from another machine?
-
- The default configuration allows only Unix domain socket connections
- from the local machine. To enable TCP/IP connections, make sure
- postmaster has been started with the -i option, and add an appropriate
- host entry to the file pgsql/data/pg_hba.conf.
-
- 3.7) How do I tune the database engine for better performance?
+ 3.6) How do I tune the database engine for better performance?
Certainly, indexes can speed up queries. The EXPLAIN command allows
you to see how PostgreSQL is interpreting your query, and which
You can also use the CLUSTER command to group data in tables to match
an index. See the CLUSTER manual page for more details.
- 3.8) What debugging features are available?
+ 3.7) What debugging features are available?
PostgreSQL has several features that report status information that
can be valuable for debugging purposes.
pgsql/data/base/dbname directory. The client profile file will be put
in the client's current directory.
- 3.9) Why do I get "Sorry, too many clients" when trying to connect?
+ 3.8) Why do I get "Sorry, too many clients" when trying to connect?
You need to increase postmaster's limit on how many concurrent backend
processes it can start.
was 64, and changing it required a rebuild after altering the
MaxBackendId constant in include/storage/sinvaladt.h.
- 3.10) What are the pg_sorttempNNN.NN files in my database directory?
+ 3.9) What are the pg_sorttempNNN.NN files in my database directory?
They are temporary files generated by the query executor. For example,
if a sort needs to be done to satisfy an ORDER BY, and the sort
4.6) How much database disk space is required to store data from a typical
text file?
- A PostgreSQL database may need six-and-a-half times the disk space
- required to store the data in a flat file.
-
- Consider a file of 300,000 lines with two integers on each line. The
- flat file is 2.4 MB. The size of the PostgreSQL database file
- containing this data can be estimated at 14 MB:
+ A PostgreSQL database may require up to five times the disk space to
+ store data from a text file.
+
+ As an example, consider a file of 100,000 lines with an integer and
+ text description on each line. Suppose the text string avergages
+ twenty characters in length. The flat file would be 2.8 MB. The size
+ of the PostgreSQL database file containing this data can be estimated
+ as 6.6 MB:
36 bytes: each row header (approximate)
- + 8 bytes: two int fields @ 4 bytes each
+ 26 bytes: two int fields @ 4 bytes each
+ 4 bytes: pointer on page to tuple
----------------------------------------
- 48 bytes per row
+ 66 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
+ ------------------- = 124 rows per database page (rounded down)
+ 66 bytes per row
- 300000 data rows
- -------------------- = 1755 database pages
- 171 rows per page
+ 100000 data rows
+ -------------------- = 807 database pages (rounded up)
+ 124 rows per page
-1755 database pages * 8192 bytes per page = 14,376,960 bytes (14 MB)
+807 database pages * 8192 bytes per page = 6,610,944 bytes (6.6 MB)
Indexes do not require as much overhead, but do contain the data that
is being indexed, so they can be large also.
alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
- <P>Last updated: Tue Feb 12 12:18:09 EST 2002</P>
+ <P>Last updated: Thu Feb 14 12:14:47 EST 2002</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
get <I>IpcMemoryCreate</I> errors. Why?<BR>
<A href="#3.4">3.4</A>) When I try to start <I>postmaster</I>, I
get <I>IpcSemaphoreCreate</I> errors. Why?<BR>
- <A href="#3.5">3.5</A>) How do I prevent other hosts from
- accessing my PostgreSQL database?<BR>
- <A href="#3.6">3.6</A>) Why can't I connect to my database from
- another machine?<BR>
- <A href="#3.7">3.7</A>) How do I tune the database engine for
+ <A href="#3.5">3.5</A>) How do I control connections from other hosts?<BR>
+ <A href="#3.6">3.6</A>) How do I tune the database engine for
better performance?<BR>
- <A href="#3.8">3.8</A>) What debugging features are available?<BR>
- <A href="#3.9">3.9</A>) Why do I get <I>"Sorry, too many
+ <A href="#3.7">3.7</A>) What debugging features are available?<BR>
+ <A href="#3.8">3.8</A>) Why do I get <I>"Sorry, too many
clients"</I> when trying to connect?<BR>
- <A href="#3.10">3.10</A>) What are the <I>pg_sorttempNNN.NN</I>
+ <A href="#3.9">3.9</A>) What are the <I>pg_sorttempNNN.NN</I>
files in my database directory?<BR>
PostgreSQL Administrator's Guide for more detailed information
about shared memory and semaphores.</P>
- <H4><A name="3.5">3.5</A>) How do I prevent other hosts from
- accessing my PostgreSQL database?</H4>
+ <H4><A name="3.5">3.5</A>) How do I control connections from other
+ hosts?</H4>
<P>By default, PostgreSQL only allows connections from the local
machine using Unix domain sockets. Other machines will not be able
<I>$PGDATA/pg_hba.conf</I> accordingly. This will allow TCP/IP
connections.</P>
- <H4><A name="3.6">3.6</A>) Why can't I connect to my database from
- another machine?</H4>
-
- <P>The default configuration allows only Unix domain socket
- connections from the local machine. To enable TCP/IP connections,
- make sure <I>postmaster</I> has been started with the <I>-i</I>
- option, and add an appropriate host entry to the file
- <I>pgsql/data/pg_hba.conf</I>.</P>
-
- <H4><A name="3.7">3.7</A>) How do I tune the database engine for
+ <H4><A name="3.6">3.6</A>) How do I tune the database engine for
better performance?</H4>
<P>Certainly, indexes can speed up queries. The
data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
manual page for more details.</P>
- <H4><A name="3.8">3.8</A>) What debugging features are
+ <H4><A name="3.7">3.7</A>) What debugging features are
available?</H4>
<P>PostgreSQL has several features that report status information
in the <I>pgsql/data/base/dbname</I> directory. The client profile
file will be put in the client's current directory.</P>
- <H4><A name="3.9">3.9</A>) Why do I get <I>"Sorry, too many
+ <H4><A name="3.8">3.8</A>) Why do I get <I>"Sorry, too many
clients"</I> when trying to connect?</H4>
<P>You need to increase <I>postmaster</I>'s limit on how many
the MaxBackendId constant in
<I>include/storage/sinvaladt.h</I>.</P>
- <H4><A name="3.10">3.10</A>) What are the <I>pg_sorttempNNN.NN</I>
+ <H4><A name="3.9">3.9</A>) What are the <I>pg_sorttempNNN.NN</I>
files in my database directory?</H4>
<P>They are temporary files generated by the query executor. For
<H4><A name="4.6">4.6</A>) How much database disk space is required
to store data from a typical text file?</H4>
- <P>A PostgreSQL database may need six-and-a-half times the disk
- space required to store the data in a flat file.</P>
+ <P>A PostgreSQL database may require up to five times the disk space
+ to store data from a text file.</P>
- <P>Consider a file of 300,000 lines with two integers on each line.
- The flat file is 2.4 MB. The size of the PostgreSQL database file
- containing this data can be estimated at 14 MB:</P>
+ <P>As an example, consider a file of 100,000 lines with an integer
+ and text description on each line. Suppose the text string avergages
+ twenty characters in length. The flat file would be 2.8 MB. The size
+ of the PostgreSQL database file containing this data can be
+ estimated as 6.6 MB:</P>
<PRE>
36 bytes: each row header (approximate)
- + 8 bytes: two int fields @ 4 bytes each
+ 26 bytes: two int fields @ 4 bytes each
+ 4 bytes: pointer on page to tuple
----------------------------------------
- 48 bytes per row
+ 66 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
+ ------------------- = 124 rows per database page (rounded down)
+ 66 bytes per row
- 300000 data rows
- -------------------- = 1755 database pages
- 171 rows per page
+ 100000 data rows
+ -------------------- = 807 database pages (rounded up)
+ 124 rows per page
-1755 database pages * 8192 bytes per page = 14,376,960 bytes (14 MB)
+807 database pages * 8192 bytes per page = 6,610,944 bytes (6.6 MB)
</PRE>
<P>Indexes do not require as much overhead, but do contain the data
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