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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
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>auto_explain</TITLE
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><A
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>PostgreSQL 9.1.15 Documentation</A
></TH
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>Appendix F. Additional Supplied Modules</TD
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AUTO-EXPLAIN"
>F.3. auto_explain</A
></H1
><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"
>EXPLAIN</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="AEN136088"
>F.3.1. 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_buffers</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> <TT
CLASS="VARNAME"
>auto_explain.log_buffers</TT
> causes <TT
CLASS="COMMAND"
>EXPLAIN
(ANALYZE, BUFFERS)</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. This
parameter has no effect unless <TT
CLASS="VARNAME"
>auto_explain.log_analyze</TT
>
parameter is set.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>auto_explain.log_format</TT
> (<TT
CLASS="TYPE"
>enum</TT
>)</DT
><DD
><P
> <TT
CLASS="VARNAME"
>auto_explain.log_format</TT
> selects the
<TT
CLASS="COMMAND"
>EXPLAIN</TT
> output format to be used.
The allowed values are <TT
CLASS="LITERAL"
>text</TT
>, <TT
CLASS="LITERAL"
>xml</TT
>,
<TT
CLASS="LITERAL"
>json</TT
>, and <TT
CLASS="LITERAL"
>yaml</TT
>. The default is text.
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="AEN136174"
>F.3.2. 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="SCREEN"
>LOG: duration: 3.651 ms plan:
Query Text: SELECT count(*)
FROM pg_class, pg_index
WHERE oid = indrelid AND indisunique;
Aggregate (cost=16.79..16.80 rows=1 width=0) (actual time=3.626..3.627 rows=1 loops=1)
-> Hash Join (cost=4.17..16.55 rows=92 width=0) (actual time=3.349..3.594 rows=92 loops=1)
Hash Cond: (pg_class.oid = pg_index.indrelid)
-> Seq Scan on pg_class (cost=0.00..9.55 rows=255 width=4) (actual time=0.016..0.140 rows=255 loops=1)
-> Hash (cost=3.02..3.02 rows=92 width=4) (actual time=3.238..3.238 rows=92 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 4kB
-> Seq Scan on pg_index (cost=0.00..3.02 rows=92 width=4) (actual time=0.008..3.187 rows=92 loops=1)
Filter: indisunique</PRE
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN136179"
>F.3.3. 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
>
</P
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