108 lines
2.9 KiB
Plaintext
108 lines
2.9 KiB
Plaintext
<!-- doc/src/sgml/pgsurgery.sgml -->
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<sect1 id="pgsurgery" xreflabel="pg_surgery">
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<title>pg_surgery — perform low-level surgery on relation data</title>
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<indexterm zone="pgsurgery">
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<primary>pg_surgery</primary>
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</indexterm>
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<para>
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The <filename>pg_surgery</filename> module provides various functions to
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perform surgery on a damaged relation. These functions are unsafe by design
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and using them may corrupt (or further corrupt) your database. For example,
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these functions can easily be used to make a table inconsistent with its
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own indexes, to cause <literal>UNIQUE</literal> or
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<literal>FOREIGN KEY</literal> constraint violations, or even to make
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tuples visible which, when read, will cause a database server crash.
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They should be used with great caution and only as a last resort.
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</para>
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<sect2 id="pgsurgery-funcs">
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<title>Functions</title>
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<variablelist>
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<varlistentry>
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<term>
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<function>heap_force_kill(regclass, tid[]) returns void</function>
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</term>
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<listitem>
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<para>
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<function>heap_force_kill</function> marks <quote>used</quote> line
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pointers as <quote>dead</quote> without examining the tuples. The
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intended use of this function is to forcibly remove tuples that are not
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otherwise accessible. For example:
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<programlisting>
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test=> select * from t1 where ctid = '(0, 1)';
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ERROR: could not access status of transaction 4007513275
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DETAIL: Could not open file "pg_xact/0EED": No such file or directory.
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test=# select heap_force_kill('t1'::regclass, ARRAY['(0, 1)']::tid[]);
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heap_force_kill
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-----------------
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(1 row)
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test=# select * from t1 where ctid = '(0, 1)';
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(0 rows)
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</programlisting>
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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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<function>heap_force_freeze(regclass, tid[]) returns void</function>
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</term>
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<listitem>
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<para>
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<function>heap_force_freeze</function> marks tuples as frozen without
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examining the tuple data. The intended use of this function is to
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make accessible tuples which are inaccessible due to corrupted
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visibility information, or which prevent the table from being
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successfully vacuumed due to corrupted visibility information.
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For example:
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<programlisting>
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test=> vacuum t1;
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ERROR: found xmin 507 from before relfrozenxid 515
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CONTEXT: while scanning block 0 of relation "public.t1"
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test=# select ctid from t1 where xmin = 507;
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ctid
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-------
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(0,3)
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(1 row)
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test=# select heap_force_freeze('t1'::regclass, ARRAY['(0, 3)']::tid[]);
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heap_force_freeze
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-------------------
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(1 row)
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test=# select ctid from t1 where xmin = 2;
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ctid
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-------
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(0,3)
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(1 row)
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</programlisting>
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect2>
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<sect2 id="pgsurgery-authors">
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<title>Authors</title>
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<para>
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Ashutosh Sharma <email>ashu.coek88@gmail.com</email>
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</para>
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</sect2>
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</sect1>
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