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>Appendix F. Additional Supplied Modules</TD
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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="AEN146524"
>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.
Turning off <TT
CLASS="VARNAME"
>auto_explain.log_timing</TT
> ameliorates the
performance cost, at the price of obtaining less information.
</P
></BLOCKQUOTE
></DIV
></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
> controls whether buffer
usage statistics are printed when an execution plan is logged; it's
equivalent to the <TT
CLASS="LITERAL"
>BUFFERS</TT
> option of <TT
CLASS="COMMAND"
>EXPLAIN</TT
>.
This parameter has no effect
unless <TT
CLASS="VARNAME"
>auto_explain.log_analyze</TT
> is enabled.
This parameter is off by default.
Only superusers can change this setting.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>auto_explain.log_timing</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> <TT
CLASS="VARNAME"
>auto_explain.log_timing</TT
> controls whether per-node
timing information is printed when an execution plan is logged; it's
equivalent to the <TT
CLASS="LITERAL"
>TIMING</TT
> option of <TT
CLASS="COMMAND"
>EXPLAIN</TT
>.
The overhead of repeatedly reading the system clock can slow down
queries significantly on some systems, so it may be useful to set this
parameter to off when only actual row counts, and not exact times, are
needed.
This parameter has no effect
unless <TT
CLASS="VARNAME"
>auto_explain.log_analyze</TT
> is enabled.
This parameter is on by default.
Only superusers can change this setting.
</P
></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
> controls whether verbose
details are printed when an execution plan is logged; it's
equivalent to the <TT
CLASS="LITERAL"
>VERBOSE</TT
> option of <TT
CLASS="COMMAND"
>EXPLAIN</TT
>.
This parameter is off by default.
Only superusers can change this setting.
</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 ordinary usage, these parameters are set
in <TT
CLASS="FILENAME"
>postgresql.conf</TT
>, although superusers can alter them
on-the-fly within their own sessions.
Typical usage might be:
</P
><PRE
CLASS="PROGRAMLISTING"
># postgresql.conf
shared_preload_libraries = 'auto_explain'
auto_explain.log_min_duration = '3s'</PRE
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN146622"
>F.3.2. Example</A
></H2
><PRE
CLASS="PROGRAMLISTING"
>postgres=# LOAD 'auto_explain';
postgres=# SET auto_explain.log_min_duration = 0;
postgres=# SET auto_explain.log_analyze = true;
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="AEN146627"
>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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