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312061cbc7
This doc patch (master hash 66bc9d2d3e) was decided to be too
significant for backpatching, so reverted in all but master. Also fix
SGML file header comment in master.
Reported-by: Peter Eisentraut
Discussion: https://postgr.es/m/c6304b19-6ff7-f3af-0148-cf7aa7e2fbfd@enterprisedb.com
Backpatch-through: 11
151 lines
4.2 KiB
Plaintext
151 lines
4.2 KiB
Plaintext
<!-- doc/src/sgml/pgrowlocks.sgml -->
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<sect1 id="pgrowlocks" xreflabel="pgrowlocks">
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<title>pgrowlocks</title>
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<indexterm zone="pgrowlocks">
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<primary>pgrowlocks</primary>
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</indexterm>
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<para>
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The <filename>pgrowlocks</filename> module provides a function to show row
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locking information for a specified table.
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</para>
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<para>
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By default use is restricted to superusers, members of the
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<literal>pg_stat_scan_tables</literal> role, and users with
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<literal>SELECT</literal> permissions on the table.
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</para>
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<sect2>
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<title>Overview</title>
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<indexterm>
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<primary>pgrowlocks</primary>
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</indexterm>
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<synopsis>
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pgrowlocks(text) returns setof record
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</synopsis>
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<para>
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The parameter is the name of a table. The result is a set of records,
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with one row for each locked row within the table. The output columns
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are shown in <xref linkend="pgrowlocks-columns"/>.
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</para>
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<table id="pgrowlocks-columns">
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<title><function>pgrowlocks</function> Output Columns</title>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Name</entry>
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<entry>Type</entry>
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<entry>Description</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><structfield>locked_row</structfield></entry>
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<entry><type>tid</type></entry>
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<entry>Tuple ID (TID) of locked row</entry>
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</row>
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<row>
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<entry><structfield>locker</structfield></entry>
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<entry><type>xid</type></entry>
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<entry>Transaction ID of locker, or multixact ID if multitransaction</entry>
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</row>
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<row>
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<entry><structfield>multi</structfield></entry>
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<entry><type>boolean</type></entry>
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<entry>True if locker is a multitransaction</entry>
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</row>
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<row>
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<entry><structfield>xids</structfield></entry>
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<entry><type>xid[]</type></entry>
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<entry>Transaction IDs of lockers (more than one if multitransaction)</entry>
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</row>
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<row>
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<entry><structfield>modes</structfield></entry>
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<entry><type>text[]</type></entry>
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<entry>Lock mode of lockers (more than one if multitransaction),
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an array of <literal>Key Share</literal>, <literal>Share</literal>,
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<literal>For No Key Update</literal>, <literal>No Key Update</literal>,
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<literal>For Update</literal>, <literal>Update</literal>.</entry>
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</row>
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<row>
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<entry><structfield>pids</structfield></entry>
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<entry><type>integer[]</type></entry>
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<entry>Process IDs of locking backends (more than one if multitransaction)</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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<function>pgrowlocks</function> takes <literal>AccessShareLock</literal> for the
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target table and reads each row one by one to collect the row locking
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information. This is not very speedy for a large table. Note that:
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</para>
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<orderedlist>
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<listitem>
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<para>
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If an <literal>ACCESS EXCLUSIVE</literal> lock is taken on the table,
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<function>pgrowlocks</function> will be blocked.
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</para>
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</listitem>
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<listitem>
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<para>
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<function>pgrowlocks</function> is not guaranteed to produce a
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self-consistent snapshot. It is possible that a new row lock is taken,
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or an old lock is freed, during its execution.
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</para>
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</listitem>
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</orderedlist>
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<para>
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<function>pgrowlocks</function> does not show the contents of locked
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rows. If you want to take a look at the row contents at the same time, you
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could do something like this:
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<programlisting>
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SELECT * FROM accounts AS a, pgrowlocks('accounts') AS p
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WHERE p.locked_row = a.ctid;
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</programlisting>
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Be aware however that such a query will be very inefficient.
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</para>
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</sect2>
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<sect2>
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<title>Sample Output</title>
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<screen>
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=# SELECT * FROM pgrowlocks('t1');
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locked_row | locker | multi | xids | modes | pids
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------------+--------+-------+-------+----------------+--------
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(0,1) | 609 | f | {609} | {"For Share"} | {3161}
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(0,2) | 609 | f | {609} | {"For Share"} | {3161}
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(0,3) | 607 | f | {607} | {"For Update"} | {3107}
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(0,4) | 607 | f | {607} | {"For Update"} | {3107}
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(4 rows)
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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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Tatsuo Ishii
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</para>
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</sect2>
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</sect1>
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