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>PostgreSQL 8.4.12 Documentation</TH
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>Appendix F. Additional Supplied Modules</TD
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><H1
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><A
NAME="AUTO-EXPLAIN"
>F.2. auto_explain</A
></H1
><A
NAME="AEN114320"
></A
><P
> The <TT
CLASS="FILENAME"
>auto_explain</TT
> module provides a means for
logging execution plans of slow statements automatically, without
having to run <A
HREF="sql-explain.html"
><I
>EXPLAIN</I
></A
>
by hand. This is especially helpful for tracking down un-optimized queries
in large applications.
</P
><P
> The module provides no SQL-accessible functions. To use it, simply
load it into the server. You can load it into an individual session:
</P><PRE
CLASS="PROGRAMLISTING"
>LOAD 'auto_explain';
</PRE
><P>
(You must be superuser to do that.) More typical usage is to preload
it into all sessions by including <TT
CLASS="LITERAL"
>auto_explain</TT
> in
<A
HREF="runtime-config-resource.html#GUC-SHARED-PRELOAD-LIBRARIES"
>shared_preload_libraries</A
> in
<TT
CLASS="FILENAME"
>postgresql.conf</TT
>. Then you can track unexpectedly slow queries
no matter when they happen. Of course there is a price in overhead for
that.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN114330"
>F.2.1. Configuration parameters</A
></H2
><P
> There are several configuration parameters that control the behavior of
<TT
CLASS="FILENAME"
>auto_explain</TT
>. Note that the default behavior is
to do nothing, so you must set at least
<TT
CLASS="VARNAME"
>auto_explain.log_min_duration</TT
> if you want any results.
</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="VARNAME"
>auto_explain.log_min_duration</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> <TT
CLASS="VARNAME"
>auto_explain.log_min_duration</TT
> is the minimum statement
execution time, in milliseconds, that will cause the statement's plan to
be logged. Setting this to zero logs all plans. Minus-one (the default)
disables logging of plans. For example, if you set it to
<TT
CLASS="LITERAL"
>250ms</TT
> then all statements that run 250ms or longer
will be logged. Only superusers can change this setting.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>auto_explain.log_analyze</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> <TT
CLASS="VARNAME"
>auto_explain.log_analyze</TT
> causes <TT
CLASS="COMMAND"
>EXPLAIN ANALYZE</TT
>
output, rather than just <TT
CLASS="COMMAND"
>EXPLAIN</TT
> output, to be printed
when an execution plan is logged. This parameter is off by default.
Only superusers can change this setting.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> When this parameter is on, per-plan-node timing occurs for all
statements executed, whether or not they run long enough to actually
get logged. This can have an extremely negative impact on performance.
</P
></BLOCKQUOTE
></DIV
></DD
><DT
><TT
CLASS="VARNAME"
>auto_explain.log_verbose</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> <TT
CLASS="VARNAME"
>auto_explain.log_verbose</TT
> causes <TT
CLASS="COMMAND"
>EXPLAIN VERBOSE</TT
>
output, rather than just <TT
CLASS="COMMAND"
>EXPLAIN</TT
> output, to be printed
when an execution plan is logged. This parameter is off by default.
Only superusers can change this setting.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>auto_explain.log_nested_statements</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> <TT
CLASS="VARNAME"
>auto_explain.log_nested_statements</TT
> causes nested
statements (statements executed inside a function) to be considered
for logging. When it is off, only top-level query plans are logged. This
parameter is off by default. Only superusers can change this setting.
</P
></DD
></DL
></DIV
><P
> In order to set these parameters in your <TT
CLASS="FILENAME"
>postgresql.conf</TT
> file,
you will need to add <TT
CLASS="LITERAL"
>auto_explain</TT
> to
<A
HREF="runtime-config-custom.html#GUC-CUSTOM-VARIABLE-CLASSES"
>custom_variable_classes</A
>. Typical usage might be:
</P
><PRE
CLASS="PROGRAMLISTING"
># postgresql.conf
shared_preload_libraries = 'auto_explain'
custom_variable_classes = 'auto_explain'
auto_explain.log_min_duration = '3s'
</PRE
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN114388"
>F.2.2. Example</A
></H2
><PRE
CLASS="PROGRAMLISTING"
> postgres=# LOAD 'auto_explain';
postgres=# SET auto_explain.log_min_duration = 0;
postgres=# SELECT count(*)
FROM pg_class, pg_index
WHERE oid = indrelid AND indisunique;
</PRE
><P
> This might produce log output such as:
</P
><PRE
CLASS="PROGRAMLISTING"
> LOG: duration: 0.986 ms plan:
Aggregate (cost=14.90..14.91 rows=1 width=0)
-> Hash Join (cost=3.91..14.70 rows=81 width=0)
Hash Cond: (pg_class.oid = pg_index.indrelid)
-> Seq Scan on pg_class (cost=0.00..8.27 rows=227 width=4)
-> Hash (cost=2.90..2.90 rows=81 width=4)
-> Seq Scan on pg_index (cost=0.00..2.90 rows=81 width=4)
Filter: indisunique
STATEMENT: SELECT count(*)
FROM pg_class, pg_index
WHERE oid = indrelid AND indisunique;
</PRE
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN114393"
>F.2.3. Author</A
></H2
><P
> Takahiro Itagaki <CODE
CLASS="EMAIL"
><<A
HREF="mailto:itagaki.takahiro@oss.ntt.co.jp"
>itagaki.takahiro@oss.ntt.co.jp</A
>></CODE
>
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