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215 lines
7.2 KiB
Plaintext
215 lines
7.2 KiB
Plaintext
<!-- $PostgreSQL: pgsql/doc/src/sgml/auto-explain.sgml,v 1.8.2.1 2010/07/29 19:34:36 petere Exp $ -->
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<sect1 id="auto-explain">
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<title>auto_explain</title>
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<indexterm zone="auto-explain">
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<primary>auto_explain</primary>
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</indexterm>
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<para>
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The <filename>auto_explain</filename> module provides a means for
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logging execution plans of slow statements automatically, without
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having to run <xref linkend="sql-explain">
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by hand. This is especially helpful for tracking down un-optimized queries
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in large applications.
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</para>
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<para>
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The module provides no SQL-accessible functions. To use it, simply
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load it into the server. You can load it into an individual session:
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<programlisting>
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LOAD 'auto_explain';
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</programlisting>
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(You must be superuser to do that.) More typical usage is to preload
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it into all sessions by including <literal>auto_explain</> in
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<xref linkend="guc-shared-preload-libraries"> in
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<filename>postgresql.conf</>. Then you can track unexpectedly slow queries
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no matter when they happen. Of course there is a price in overhead for
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that.
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</para>
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<sect2>
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<title>Configuration parameters</title>
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<para>
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There are several configuration parameters that control the behavior of
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<filename>auto_explain</filename>. Note that the default behavior is
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to do nothing, so you must set at least
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<varname>auto_explain.log_min_duration</varname> if you want any results.
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</para>
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<variablelist>
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<varlistentry>
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<term>
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<varname>auto_explain.log_min_duration</varname> (<type>integer</type>)
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</term>
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<indexterm>
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<primary><varname>auto_explain.log_min_duration</> configuration parameter</primary>
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</indexterm>
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<listitem>
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<para>
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<varname>auto_explain.log_min_duration</varname> is the minimum statement
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execution time, in milliseconds, that will cause the statement's plan to
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be logged. Setting this to zero logs all plans. Minus-one (the default)
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disables logging of plans. For example, if you set it to
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<literal>250ms</literal> then all statements that run 250ms or longer
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will be logged. Only superusers can change this setting.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>auto_explain.log_analyze</varname> (<type>boolean</type>)
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</term>
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<indexterm>
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<primary><varname>auto_explain.log_analyze</> configuration parameter</primary>
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</indexterm>
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<listitem>
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<para>
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<varname>auto_explain.log_analyze</varname> causes <command>EXPLAIN ANALYZE</>
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output, rather than just <command>EXPLAIN</> output, to be printed
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when an execution plan is logged. This parameter is off by default.
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Only superusers can change this setting.
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</para>
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<note>
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<para>
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When this parameter is on, per-plan-node timing occurs for all
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statements executed, whether or not they run long enough to actually
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get logged. This can have extremely negative impact on performance.
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</para>
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</note>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>auto_explain.log_verbose</varname> (<type>boolean</type>)
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</term>
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<indexterm>
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<primary><varname>auto_explain.log_verbose</> configuration parameter</primary>
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</indexterm>
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<listitem>
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<para>
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<varname>auto_explain.log_verbose</varname> causes <command>EXPLAIN VERBOSE</>
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output, rather than just <command>EXPLAIN</> output, to be printed
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when an execution plan is logged. This parameter is off by default.
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Only superusers can change this setting.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>auto_explain.log_buffers</varname> (<type>boolean</type>)
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</term>
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<indexterm>
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<primary><varname>auto_explain.log_buffers</> configuration parameter</primary>
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</indexterm>
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<listitem>
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<para>
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<varname>auto_explain.log_buffers</varname> causes <command>EXPLAIN
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(ANALYZE, BUFFERS)</> output, rather than just <command>EXPLAIN</>
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output, to be printed when an execution plan is logged. This parameter is
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off by default. Only superusers can change this setting. This
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parameter has no effect unless <varname>auto_explain.log_analyze</>
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parameter is set.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>auto_explain.log_format</varname> (<type>enum</type>)
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</term>
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<indexterm>
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<primary><varname>auto_explain.log_format</> configuration parameter</primary>
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</indexterm>
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<listitem>
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<para>
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<varname>auto_explain.log_format</varname> selects the
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<command>EXPLAIN</> output format to be used.
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The allowed values are <literal>text</literal>, <literal>xml</literal>,
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<literal>json</literal>, and <literal>yaml</literal>. The default is text.
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Only superusers can change this setting.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>auto_explain.log_nested_statements</varname> (<type>boolean</type>)
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</term>
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<indexterm>
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<primary><varname>auto_explain.log_nested_statements</> configuration parameter</primary>
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</indexterm>
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<listitem>
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<para>
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<varname>auto_explain.log_nested_statements</varname> causes nested
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statements (statements executed inside a function) to be considered
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for logging. When it is off, only top-level query plans are logged. This
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parameter is off by default. Only superusers can change this setting.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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In order to set these parameters in your <filename>postgresql.conf</> file,
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you will need to add <literal>auto_explain</> to
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<xref linkend="guc-custom-variable-classes">. Typical usage might be:
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</para>
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<programlisting>
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# postgresql.conf
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shared_preload_libraries = 'auto_explain'
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custom_variable_classes = 'auto_explain'
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auto_explain.log_min_duration = '3s'
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</programlisting>
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</sect2>
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<sect2>
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<title>Example</title>
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<programlisting>
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postgres=# LOAD 'auto_explain';
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postgres=# SET auto_explain.log_min_duration = 0;
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postgres=# SELECT count(*)
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FROM pg_class, pg_index
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WHERE oid = indrelid AND indisunique;
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</programlisting>
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<para>
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This might produce log output such as:
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</para>
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<screen><![CDATA[
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LOG: duration: 3.651 ms plan:
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Query Text: SELECT count(*)
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FROM pg_class, pg_index
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WHERE oid = indrelid AND indisunique;
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Aggregate (cost=16.79..16.80 rows=1 width=0) (actual time=3.626..3.627 rows=1 loops=1)
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-> Hash Join (cost=4.17..16.55 rows=92 width=0) (actual time=3.349..3.594 rows=92 loops=1)
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Hash Cond: (pg_class.oid = pg_index.indrelid)
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-> Seq Scan on pg_class (cost=0.00..9.55 rows=255 width=4) (actual time=0.016..0.140 rows=255 loops=1)
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-> Hash (cost=3.02..3.02 rows=92 width=4) (actual time=3.238..3.238 rows=92 loops=1)
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Buckets: 1024 Batches: 1 Memory Usage: 4kB
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-> Seq Scan on pg_index (cost=0.00..3.02 rows=92 width=4) (actual time=0.008..3.187 rows=92 loops=1)
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Filter: indisunique
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]]></screen>
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</sect2>
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<sect2>
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<title>Author</title>
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<para>
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Takahiro Itagaki <email>itagaki.takahiro@oss.ntt.co.jp</email>
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</para>
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</sect2>
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</sect1>
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