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Some sections of the documentation used "exclusive lock" to describe that an ACCESS EXCLUSIVE lock is taken during a given operation. This can be confusing to the reader as ACCESS SHARE is allowed with an EXCLUSIVE lock is used, but that would not be the case with what is described on those parts of the documentation. Author: Greg Rychlewski Discussion: https://postgr.es/m/CAKemG7VptD=7fNWckFMsMVZL_zzvgDO6v2yVmQ+ZiBfc_06kCQ@mail.gmail.com Backpatch-through: 9.6
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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