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> just that certain commands do. TRUNCATE isn't shown. Patch against HEAD to add TRUNCATE to the list of commands that aquire ACCESS EXCLUSIVE. Jim C. Nasby, Sr.
1024 lines
38 KiB
Plaintext
1024 lines
38 KiB
Plaintext
<!--
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$PostgreSQL: pgsql/doc/src/sgml/mvcc.sgml,v 2.54 2006/03/04 04:41:36 momjian Exp $
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-->
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<chapter id="mvcc">
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<title>Concurrency Control</title>
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<indexterm>
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<primary>concurrency</primary>
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</indexterm>
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<para>
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This chapter describes the behavior of the
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<productname>PostgreSQL</productname> database system when two or
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more sessions try to access the same data at the same time. The
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goals in that situation are to allow efficient access for all
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sessions while maintaining strict data integrity. Every developer
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of database applications should be familiar with the topics covered
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in this chapter.
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</para>
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<sect1 id="mvcc-intro">
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<title>Introduction</title>
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<indexterm>
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<primary>MVCC</primary>
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</indexterm>
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<para>
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Unlike traditional database systems which use locks for concurrency control,
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<productname>PostgreSQL</productname>
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maintains data consistency by using a multiversion model
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(Multiversion Concurrency Control, <acronym>MVCC</acronym>).
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This means that while querying a database each transaction sees
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a snapshot of data (a <firstterm>database version</firstterm>)
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as it was some
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time ago, regardless of the current state of the underlying data.
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This protects the transaction from viewing inconsistent data that
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could be caused by (other) concurrent transaction updates on the same
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data rows, providing <firstterm>transaction isolation</firstterm>
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for each database session.
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</para>
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<para>
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The main advantage to using the <acronym>MVCC</acronym> model of
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concurrency control rather than locking is that in
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<acronym>MVCC</acronym> locks acquired for querying (reading) data
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do not conflict with locks acquired for writing data, and so
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reading never blocks writing and writing never blocks reading.
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</para>
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<para>
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Table- and row-level locking facilities are also available in
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<productname>PostgreSQL</productname> for applications that cannot
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adapt easily to <acronym>MVCC</acronym> behavior. However, proper
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use of <acronym>MVCC</acronym> will generally provide better
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performance than locks.
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</para>
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</sect1>
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<sect1 id="transaction-iso">
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<title>Transaction Isolation</title>
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<indexterm>
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<primary>transaction isolation</primary>
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</indexterm>
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<para>
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The <acronym>SQL</acronym> standard defines four levels of
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transaction isolation in terms of three phenomena that must be
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prevented between concurrent transactions. These undesirable
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phenomena are:
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<variablelist>
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<varlistentry>
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<term>
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dirty read
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<indexterm><primary>dirty read</primary></indexterm>
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</term>
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<listitem>
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<para>
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A transaction reads data written by a concurrent uncommitted transaction.
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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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nonrepeatable read
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<indexterm><primary>nonrepeatable read</primary></indexterm>
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</term>
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<listitem>
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<para>
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A transaction re-reads data it has previously read and finds that data
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has been modified by another transaction (that committed since the
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initial read).
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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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phantom read
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<indexterm><primary>phantom read</primary></indexterm>
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</term>
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<listitem>
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<para>
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A transaction re-executes a query returning a set of rows that satisfy a
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search condition and finds that the set of rows satisfying the condition
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has changed due to another recently-committed transaction.
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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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<para>
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<indexterm>
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<primary>transaction isolation level</primary>
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</indexterm>
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The four transaction isolation levels and the corresponding
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behaviors are described in <xref linkend="mvcc-isolevel-table">.
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</para>
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<table tocentry="1" id="mvcc-isolevel-table">
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<title><acronym>SQL</acronym> Transaction Isolation Levels</title>
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<tgroup cols="4">
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<thead>
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<row>
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<entry>
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Isolation Level
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</entry>
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<entry>
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Dirty Read
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</entry>
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<entry>
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Nonrepeatable Read
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</entry>
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<entry>
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Phantom Read
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</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>
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Read uncommitted
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</entry>
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<entry>
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Possible
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</entry>
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<entry>
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Possible
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</entry>
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<entry>
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Possible
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</entry>
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</row>
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<row>
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<entry>
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Read committed
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</entry>
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<entry>
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Not possible
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</entry>
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<entry>
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Possible
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</entry>
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<entry>
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Possible
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</entry>
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</row>
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<row>
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<entry>
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Repeatable read
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</entry>
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<entry>
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Not possible
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</entry>
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<entry>
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Not possible
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</entry>
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<entry>
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Possible
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</entry>
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</row>
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<row>
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<entry>
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Serializable
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</entry>
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<entry>
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Not possible
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</entry>
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<entry>
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Not possible
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</entry>
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<entry>
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Not possible
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</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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In <productname>PostgreSQL</productname>, you can request any of the
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four standard transaction isolation levels. But internally, there are
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only two distinct isolation levels, which correspond to the levels Read
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Committed and Serializable. When you select the level Read
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Uncommitted you really get Read Committed, and when you select
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Repeatable Read you really get Serializable, so the actual
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isolation level may be stricter than what you select. This is
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permitted by the SQL standard: the four isolation levels only
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define which phenomena must not happen, they do not define which
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phenomena must happen. The reason that <productname>PostgreSQL</>
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only provides two isolation levels is that this is the only
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sensible way to map the standard isolation levels to the multiversion
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concurrency control architecture. The behavior of the available
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isolation levels is detailed in the following subsections.
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</para>
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<para>
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To set the transaction isolation level of a transaction, use the
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command <xref linkend="sql-set-transaction" endterm="sql-set-transaction-title">.
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</para>
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<sect2 id="xact-read-committed">
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<title>Read Committed Isolation Level</title>
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<indexterm>
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<primary>transaction isolation level</primary>
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<secondary>read committed</secondary>
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</indexterm>
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<para>
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<firstterm>Read Committed</firstterm>
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is the default isolation level in <productname>PostgreSQL</productname>.
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When a transaction runs on this isolation level,
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a <command>SELECT</command> query sees only data committed before the
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query began; it never sees either uncommitted data or changes committed
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during query execution by concurrent transactions. (However, the
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<command>SELECT</command> does see the effects of previous updates
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executed within its own transaction, even though they are not yet
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committed.) In effect, a <command>SELECT</command> query
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sees a snapshot of the database as of the instant that that query
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begins to run. Notice that two successive <command>SELECT</command> commands can
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see different data, even though they are within a single transaction, if
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other transactions
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commit changes during execution of the first <command>SELECT</command>.
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</para>
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<para>
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<command>UPDATE</command>, <command>DELETE</command>, <command>SELECT
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FOR UPDATE</command>, and <command>SELECT FOR SHARE</command> commands
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behave the same as <command>SELECT</command>
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in terms of searching for target rows: they will only find target rows
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that were committed as of the command start time. However, such a target
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row may have already been updated (or deleted or locked) by
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another concurrent transaction by the time it is found. In this case, the
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would-be updater will wait for the first updating transaction to commit or
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roll back (if it is still in progress). If the first updater rolls back,
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then its effects are negated and the second updater can proceed with
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updating the originally found row. If the first updater commits, the
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second updater will ignore the row if the first updater deleted it,
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otherwise it will attempt to apply its operation to the updated version of
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the row. The search condition of the command (the <literal>WHERE</> clause) is
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re-evaluated to see if the updated version of the row still matches the
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search condition. If so, the second updater proceeds with its operation,
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starting from the updated version of the row. (In the case of
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<command>SELECT FOR UPDATE</command> and <command>SELECT FOR
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SHARE</command>, that means it is the updated version of the row that is
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locked and returned to the client.)
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</para>
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<para>
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Because of the above rule, it is possible for an updating command to see an
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inconsistent snapshot: it can see the effects of concurrent updating
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commands that affected the same rows it is trying to update, but it
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does not see effects of those commands on other rows in the database.
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This behavior makes Read Committed mode unsuitable for commands that
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involve complex search conditions. However, it is just right for simpler
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cases. For example, consider updating bank balances with transactions
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like
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<screen>
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BEGIN;
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UPDATE accounts SET balance = balance + 100.00 WHERE acctnum = 12345;
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UPDATE accounts SET balance = balance - 100.00 WHERE acctnum = 7534;
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COMMIT;
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</screen>
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If two such transactions concurrently try to change the balance of account
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12345, we clearly want the second transaction to start from the updated
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version of the account's row. Because each command is affecting only a
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predetermined row, letting it see the updated version of the row does
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not create any troublesome inconsistency.
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</para>
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<para>
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Since in Read Committed mode each new command starts with a new snapshot
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that includes all transactions committed up to that instant, subsequent
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commands in the same transaction will see the effects of the committed
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concurrent transaction in any case. The point at issue here is whether
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or not within a <emphasis>single</> command we see an absolutely consistent
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view of the database.
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</para>
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<para>
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The partial transaction isolation provided by Read Committed mode is
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adequate for many applications, and this mode is fast and simple to use.
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However, for applications that do complex queries and updates, it may
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be necessary to guarantee a more rigorously consistent view of the
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database than the Read Committed mode provides.
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</para>
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</sect2>
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<sect2 id="xact-serializable">
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<title>Serializable Isolation Level</title>
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<indexterm>
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<primary>transaction isolation level</primary>
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<secondary>serializable</secondary>
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</indexterm>
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<para>
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The level <firstterm>Serializable</firstterm> provides the strictest transaction
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isolation. This level emulates serial transaction execution,
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as if transactions had been executed one after another, serially,
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rather than concurrently. However, applications using this level must
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be prepared to retry transactions due to serialization failures.
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</para>
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<para>
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When a transaction is on the serializable level,
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a <command>SELECT</command> query sees only data committed before the
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transaction began; it never sees either uncommitted data or changes
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committed
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during transaction execution by concurrent transactions. (However, the
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<command>SELECT</command> does see the effects of previous updates
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executed within its own transaction, even though they are not yet
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committed.) This is different from Read Committed in that the
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<command>SELECT</command>
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sees a snapshot as of the start of the transaction, not as of the start
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of the current query within the transaction. Thus, successive
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<command>SELECT</command> commands within a single transaction always see the same
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data.
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</para>
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<para>
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<command>UPDATE</command>, <command>DELETE</command>, <command>SELECT
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FOR UPDATE</command>, and <command>SELECT FOR SHARE</command> commands
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behave the same as <command>SELECT</command>
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in terms of searching for target rows: they will only find target rows
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that were committed as of the transaction start time. However, such a
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target
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row may have already been updated (or deleted or locked) by
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another concurrent transaction by the time it is found. In this case, the
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serializable transaction will wait for the first updating transaction to commit or
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roll back (if it is still in progress). If the first updater rolls back,
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then its effects are negated and the serializable transaction can proceed
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with updating the originally found row. But if the first updater commits
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(and actually updated or deleted the row, not just locked it)
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then the serializable transaction will be rolled back with the message
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<screen>
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ERROR: could not serialize access due to concurrent update
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</screen>
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because a serializable transaction cannot modify or lock rows changed by
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other transactions after the serializable transaction began.
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</para>
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<para>
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When the application receives this error message, it should abort
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the current transaction and then retry the whole transaction from
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the beginning. The second time through, the transaction sees the
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previously-committed change as part of its initial view of the database,
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so there is no logical conflict in using the new version of the row
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as the starting point for the new transaction's update.
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</para>
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<para>
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Note that only updating transactions may need to be retried; read-only
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transactions will never have serialization conflicts.
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</para>
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<para>
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The Serializable mode provides a rigorous guarantee that each
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transaction sees a wholly consistent view of the database. However,
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the application has to be prepared to retry transactions when concurrent
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updates make it impossible to sustain the illusion of serial execution.
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Since the cost of redoing complex transactions may be significant,
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this mode is recommended only when updating transactions contain logic
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sufficiently complex that they may give wrong answers in Read
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Committed mode. Most commonly, Serializable mode is necessary when
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a transaction executes several successive commands that must see
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identical views of the database.
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</para>
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<sect3 id="mvcc-serializability">
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<title>Serializable Isolation versus True Serializability</title>
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<indexterm>
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<primary>serializability</primary>
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</indexterm>
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<indexterm>
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<primary>predicate locking</primary>
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</indexterm>
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<para>
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The intuitive meaning (and mathematical definition) of
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<quote>serializable</> execution is that any two successfully committed
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concurrent transactions will appear to have executed strictly serially,
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one after the other — although which one appeared to occur first may
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not be predictable in advance. It is important to realize that forbidding
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the undesirable behaviors listed in <xref linkend="mvcc-isolevel-table">
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is not sufficient to guarantee true serializability, and in fact
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<productname>PostgreSQL</productname>'s Serializable mode <emphasis>does
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not guarantee serializable execution in this sense</>. As an example,
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consider a table <structname>mytab</>, initially containing
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<screen>
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class | value
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-------+-------
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1 | 10
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1 | 20
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2 | 100
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2 | 200
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</screen>
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Suppose that serializable transaction A computes
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<screen>
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SELECT SUM(value) FROM mytab WHERE class = 1;
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</screen>
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and then inserts the result (30) as the <structfield>value</> in a
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new row with <structfield>class</> = 2. Concurrently, serializable
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transaction B computes
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<screen>
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SELECT SUM(value) FROM mytab WHERE class = 2;
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</screen>
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and obtains the result 300, which it inserts in a new row with
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<structfield>class</> = 1. Then both transactions commit. None of
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the listed undesirable behaviors have occurred, yet we have a result
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that could not have occurred in either order serially. If A had
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executed before B, B would have computed the sum 330, not 300, and
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similarly the other order would have resulted in a different sum
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computed by A.
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</para>
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<para>
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To guarantee true mathematical serializability, it is necessary for
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a database system to enforce <firstterm>predicate locking</>, which
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means that a transaction cannot insert or modify a row that would
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have matched the <literal>WHERE</> condition of a query in another concurrent
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transaction. For example, once transaction A has executed the query
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<literal>SELECT ... WHERE class = 1</>, a predicate-locking system
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would forbid transaction B from inserting any new row with class 1
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until A has committed.
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<footnote>
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<para>
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Essentially, a predicate-locking system prevents phantom reads
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by restricting what is written, whereas MVCC prevents them by
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restricting what is read.
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</para>
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</footnote>
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Such a locking system is complex to
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implement and extremely expensive in execution, since every session must
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be aware of the details of every query executed by every concurrent
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transaction. And this large expense is mostly wasted, since in
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practice most applications do not do the sorts of things that could
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result in problems. (Certainly the example above is rather contrived
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and unlikely to represent real software.) Accordingly,
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<productname>PostgreSQL</productname> does not implement predicate
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locking, and so far as we are aware no other production DBMS does either.
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</para>
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<para>
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In those cases where the possibility of nonserializable execution
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is a real hazard, problems can be prevented by appropriate use of
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explicit locking. Further discussion appears in the following
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sections.
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</para>
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</sect3>
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</sect2>
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</sect1>
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<sect1 id="explicit-locking">
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<title>Explicit Locking</title>
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|
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<indexterm>
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<primary>lock</primary>
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</indexterm>
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|
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<para>
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<productname>PostgreSQL</productname> provides various lock modes
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to control concurrent access to data in tables. These modes can
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be used for application-controlled locking in situations where
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<acronym>MVCC</acronym> does not give the desired behavior. Also,
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most <productname>PostgreSQL</productname> commands automatically
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acquire locks of appropriate modes to ensure that referenced
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tables are not dropped or modified in incompatible ways while the
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command executes. (For example, <command>ALTER TABLE</> cannot be
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executed concurrently with other operations on the same table.)
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</para>
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<para>
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To examine a list of the currently outstanding locks in a database
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server, use the <structname>pg_locks</structname> system view
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(<xref linkend="view-pg-locks">). For more
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|
information on monitoring the status of the lock manager
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|
subsystem, refer to <xref linkend="monitoring">.
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</para>
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<sect2 id="locking-tables">
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<title>Table-Level Locks</title>
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|
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<indexterm zone="locking-tables">
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<primary>LOCK</primary>
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</indexterm>
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<para>
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The list below shows the available lock modes and the contexts in
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which they are used automatically by
|
|
<productname>PostgreSQL</productname>. You can also acquire any
|
|
of these locks explicitly with the command <xref
|
|
linkend="sql-lock" endterm="sql-lock-title">.
|
|
Remember that all of these lock modes are table-level locks,
|
|
even if the name contains the word
|
|
<quote>row</quote>; the names of the lock modes are historical.
|
|
To some extent the names reflect the typical usage of each lock
|
|
mode — but the semantics are all the same. The only real difference
|
|
between one lock mode and another is the set of lock modes with
|
|
which each conflicts. Two transactions cannot hold locks of conflicting
|
|
modes on the same table at the same time. (However, a transaction
|
|
never conflicts with itself. For example, it may acquire
|
|
<literal>ACCESS EXCLUSIVE</literal> lock and later acquire
|
|
<literal>ACCESS SHARE</literal> lock on the same table.) Non-conflicting
|
|
lock modes may be held concurrently by many transactions. Notice in
|
|
particular that some lock modes are self-conflicting (for example,
|
|
an <literal>ACCESS EXCLUSIVE</literal> lock cannot be held by more than one
|
|
transaction at a time) while others are not self-conflicting (for example,
|
|
an <literal>ACCESS SHARE</literal> lock can be held by multiple transactions).
|
|
Once acquired, a lock is held till end of transaction.
|
|
</para>
|
|
|
|
<variablelist>
|
|
<title>Table-level lock modes</title>
|
|
<varlistentry>
|
|
<term>
|
|
<literal>ACCESS SHARE</literal>
|
|
</term>
|
|
<listitem>
|
|
<para>
|
|
Conflicts with the <literal>ACCESS EXCLUSIVE</literal> lock
|
|
mode only.
|
|
</para>
|
|
|
|
<para>
|
|
The commands <command>SELECT</command> and
|
|
<command>ANALYZE</command> acquire a lock of this mode on
|
|
referenced tables. In general, any query that only reads a table
|
|
and does not modify it will acquire this lock mode.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term>
|
|
<literal>ROW SHARE</literal>
|
|
</term>
|
|
<listitem>
|
|
<para>
|
|
Conflicts with the <literal>EXCLUSIVE</literal> and
|
|
<literal>ACCESS EXCLUSIVE</literal> lock modes.
|
|
</para>
|
|
|
|
<para>
|
|
The <command>SELECT FOR UPDATE</command> and
|
|
<command>SELECT FOR SHARE</command> commands acquire a
|
|
lock of this mode on the target table(s) (in addition to
|
|
<literal>ACCESS SHARE</literal> locks on any other tables
|
|
that are referenced but not selected
|
|
<option>FOR UPDATE/FOR SHARE</option>).
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term>
|
|
<literal>ROW EXCLUSIVE</literal>
|
|
</term>
|
|
<listitem>
|
|
<para>
|
|
Conflicts with the <literal>SHARE</literal>, <literal>SHARE ROW
|
|
EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
|
|
<literal>ACCESS EXCLUSIVE</literal> lock modes.
|
|
</para>
|
|
|
|
<para>
|
|
The commands <command>UPDATE</command>,
|
|
<command>DELETE</command>, and <command>INSERT</command>
|
|
acquire this lock mode on the target table (in addition to
|
|
<literal>ACCESS SHARE</literal> locks on any other referenced
|
|
tables). In general, this lock mode will be acquired by any
|
|
command that modifies the data in a table.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term>
|
|
<literal>SHARE UPDATE EXCLUSIVE</literal>
|
|
</term>
|
|
<listitem>
|
|
<para>
|
|
Conflicts with the <literal>SHARE UPDATE EXCLUSIVE</literal>,
|
|
<literal>SHARE</literal>, <literal>SHARE ROW
|
|
EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
|
|
<literal>ACCESS EXCLUSIVE</literal> lock modes.
|
|
This mode protects a table against
|
|
concurrent schema changes and <command>VACUUM</> runs.
|
|
</para>
|
|
|
|
<para>
|
|
Acquired by <command>VACUUM</command> (without <option>FULL</option>).
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term>
|
|
<literal>SHARE</literal>
|
|
</term>
|
|
<listitem>
|
|
<para>
|
|
Conflicts with the <literal>ROW EXCLUSIVE</literal>,
|
|
<literal>SHARE UPDATE EXCLUSIVE</literal>, <literal>SHARE ROW
|
|
EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
|
|
<literal>ACCESS EXCLUSIVE</literal> lock modes.
|
|
This mode protects a table against concurrent data changes.
|
|
</para>
|
|
|
|
<para>
|
|
Acquired by <command>CREATE INDEX</command>.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term>
|
|
<literal>SHARE ROW EXCLUSIVE</literal>
|
|
</term>
|
|
<listitem>
|
|
<para>
|
|
Conflicts with the <literal>ROW EXCLUSIVE</literal>,
|
|
<literal>SHARE UPDATE EXCLUSIVE</literal>,
|
|
<literal>SHARE</literal>, <literal>SHARE ROW
|
|
EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
|
|
<literal>ACCESS EXCLUSIVE</literal> lock modes.
|
|
</para>
|
|
|
|
<para>
|
|
This lock mode is not automatically acquired by any
|
|
<productname>PostgreSQL</productname> command.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term>
|
|
<literal>EXCLUSIVE</literal>
|
|
</term>
|
|
<listitem>
|
|
<para>
|
|
Conflicts with the <literal>ROW SHARE</literal>, <literal>ROW
|
|
EXCLUSIVE</literal>, <literal>SHARE UPDATE
|
|
EXCLUSIVE</literal>, <literal>SHARE</literal>, <literal>SHARE
|
|
ROW EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
|
|
<literal>ACCESS EXCLUSIVE</literal> lock modes.
|
|
This mode allows only concurrent <literal>ACCESS SHARE</literal> locks,
|
|
i.e., only reads from the table can proceed in parallel with a
|
|
transaction holding this lock mode.
|
|
</para>
|
|
|
|
<para>
|
|
This lock mode is not automatically acquired on user tables by any
|
|
<productname>PostgreSQL</productname> command. However it is
|
|
acquired on certain system catalogs in some operations.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term>
|
|
<literal>ACCESS EXCLUSIVE</literal>
|
|
</term>
|
|
<listitem>
|
|
<para>
|
|
Conflicts with locks of all modes (<literal>ACCESS
|
|
SHARE</literal>, <literal>ROW SHARE</literal>, <literal>ROW
|
|
EXCLUSIVE</literal>, <literal>SHARE UPDATE
|
|
EXCLUSIVE</literal>, <literal>SHARE</literal>, <literal>SHARE
|
|
ROW EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
|
|
<literal>ACCESS EXCLUSIVE</literal>).
|
|
This mode guarantees that the
|
|
holder is the only transaction accessing the table in any way.
|
|
</para>
|
|
|
|
<para>
|
|
Acquired by the <command>ALTER TABLE</command>, <command>DROP
|
|
TABLE</command>, <command>TRUNCATE</command>, <command>REINDEX</command>,
|
|
<command>CLUSTER</command>, and <command>VACUUM FULL</command>
|
|
commands. This is also the default lock mode for <command>LOCK
|
|
TABLE</command> statements that do not specify a mode explicitly.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
|
|
<tip>
|
|
<para>
|
|
Only an <literal>ACCESS EXCLUSIVE</literal> lock blocks a
|
|
<command>SELECT</command> (without <option>FOR UPDATE/SHARE</option>)
|
|
statement.
|
|
</para>
|
|
</tip>
|
|
|
|
</sect2>
|
|
|
|
<sect2 id="locking-rows">
|
|
<title>Row-Level Locks</title>
|
|
|
|
<para>
|
|
In addition to table-level locks, there are row-level locks, which
|
|
can be exclusive or shared locks. An exclusive row-level lock on a
|
|
specific row is automatically acquired when the row is updated or
|
|
deleted. The lock is held until the transaction commits or rolls
|
|
back. Row-level locks do not affect data querying; they block
|
|
<emphasis>writers to the same row</emphasis> only.
|
|
</para>
|
|
|
|
<para>
|
|
To acquire an exclusive row-level lock on a row without actually
|
|
modifying the row, select the row with <command>SELECT FOR
|
|
UPDATE</command>. Note that once the row-level lock is acquired,
|
|
the transaction may update the row multiple times without
|
|
fear of conflicts.
|
|
</para>
|
|
|
|
<para>
|
|
To acquire a shared row-level lock on a row, select the row with
|
|
<command>SELECT FOR SHARE</command>. A shared lock does not prevent
|
|
other transactions from acquiring the same shared lock. However,
|
|
no transaction is allowed to update, delete, or exclusively lock a
|
|
row on which any other transaction holds a shared lock. Any attempt
|
|
to do so will block until the shared locks have been released.
|
|
</para>
|
|
|
|
<para>
|
|
<productname>PostgreSQL</productname> doesn't remember any
|
|
information about modified rows in memory, so it has no limit to
|
|
the number of rows locked at one time. However, locking a row
|
|
may cause a disk write; thus, for example, <command>SELECT FOR
|
|
UPDATE</command> will modify selected rows to mark them locked, and so
|
|
will result in disk writes.
|
|
</para>
|
|
|
|
<para>
|
|
In addition to table and row locks, page-level share/exclusive locks are
|
|
used to control read/write access to table pages in the shared buffer
|
|
pool. These locks are released immediately after a row is fetched or
|
|
updated. Application developers normally need not be concerned with
|
|
page-level locks, but we mention them for completeness.
|
|
</para>
|
|
|
|
</sect2>
|
|
|
|
<sect2 id="locking-deadlocks">
|
|
<title>Deadlocks</title>
|
|
|
|
<indexterm zone="locking-deadlocks">
|
|
<primary>deadlock</primary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
The use of explicit locking can increase the likelihood of
|
|
<firstterm>deadlocks</>, wherein two (or more) transactions each
|
|
hold locks that the other wants. For example, if transaction 1
|
|
acquires an exclusive lock on table A and then tries to acquire
|
|
an exclusive lock on table B, while transaction 2 has already
|
|
exclusive-locked table B and now wants an exclusive lock on table
|
|
A, then neither one can proceed.
|
|
<productname>PostgreSQL</productname> automatically detects
|
|
deadlock situations and resolves them by aborting one of the
|
|
transactions involved, allowing the other(s) to complete.
|
|
(Exactly which transaction will be aborted is difficult to
|
|
predict and should not be relied on.)
|
|
</para>
|
|
|
|
<para>
|
|
Note that deadlocks can also occur as the result of row-level
|
|
locks (and thus, they can occur even if explicit locking is not
|
|
used). Consider the case in which there are two concurrent
|
|
transactions modifying a table. The first transaction executes:
|
|
|
|
<screen>
|
|
UPDATE accounts SET balance = balance + 100.00 WHERE acctnum = 11111;
|
|
</screen>
|
|
|
|
This acquires a row-level lock on the row with the specified
|
|
account number. Then, the second transaction executes:
|
|
|
|
<screen>
|
|
UPDATE accounts SET balance = balance + 100.00 WHERE acctnum = 22222;
|
|
UPDATE accounts SET balance = balance - 100.00 WHERE acctnum = 11111;
|
|
</screen>
|
|
|
|
The first <command>UPDATE</command> statement successfully
|
|
acquires a row-level lock on the specified row, so it succeeds in
|
|
updating that row. However, the second <command>UPDATE</command>
|
|
statement finds that the row it is attempting to update has
|
|
already been locked, so it waits for the transaction that
|
|
acquired the lock to complete. Transaction two is now waiting on
|
|
transaction one to complete before it continues execution. Now,
|
|
transaction one executes:
|
|
|
|
<screen>
|
|
UPDATE accounts SET balance = balance - 100.00 WHERE acctnum = 22222;
|
|
</screen>
|
|
|
|
Transaction one attempts to acquire a row-level lock on the
|
|
specified row, but it cannot: transaction two already holds such
|
|
a lock. So it waits for transaction two to complete. Thus,
|
|
transaction one is blocked on transaction two, and transaction
|
|
two is blocked on transaction one: a deadlock
|
|
condition. <productname>PostgreSQL</productname> will detect this
|
|
situation and abort one of the transactions.
|
|
</para>
|
|
|
|
<para>
|
|
The best defense against deadlocks is generally to avoid them by
|
|
being certain that all applications using a database acquire
|
|
locks on multiple objects in a consistent order. In the example
|
|
above, if both transactions
|
|
had updated the rows in the same order, no deadlock would have
|
|
occurred. One should also ensure that the first lock acquired on
|
|
an object in a transaction is the highest mode that will be
|
|
needed for that object. If it is not feasible to verify this in
|
|
advance, then deadlocks may be handled on-the-fly by retrying
|
|
transactions that are aborted due to deadlock.
|
|
</para>
|
|
|
|
<para>
|
|
So long as no deadlock situation is detected, a transaction seeking
|
|
either a table-level or row-level lock will wait indefinitely for
|
|
conflicting locks to be released. This means it is a bad idea for
|
|
applications to hold transactions open for long periods of time
|
|
(e.g., while waiting for user input).
|
|
</para>
|
|
</sect2>
|
|
</sect1>
|
|
|
|
<sect1 id="applevel-consistency">
|
|
<title>Data Consistency Checks at the Application Level</title>
|
|
|
|
<para>
|
|
Because readers in <productname>PostgreSQL</productname>
|
|
do not lock data, regardless of
|
|
transaction isolation level, data read by one transaction can be
|
|
overwritten by another concurrent transaction. In other words,
|
|
if a row is returned by <command>SELECT</command> it doesn't mean that
|
|
the row is still current at the instant it is returned (i.e., sometime
|
|
after the current query began). The row might have been modified or
|
|
deleted by an already-committed transaction that committed after this one
|
|
started.
|
|
Even if the row is still valid <quote>now</quote>, it could be changed or
|
|
deleted
|
|
before the current transaction does a commit or rollback.
|
|
</para>
|
|
|
|
<para>
|
|
Another way to think about it is that each
|
|
transaction sees a snapshot of the database contents, and concurrently
|
|
executing transactions may very well see different snapshots. So the
|
|
whole concept of <quote>now</quote> is somewhat ill-defined anyway.
|
|
This is not normally
|
|
a big problem if the client applications are isolated from each other,
|
|
but if the clients can communicate via channels outside the database
|
|
then serious confusion may ensue.
|
|
</para>
|
|
|
|
<para>
|
|
To ensure the current validity of a row and protect it against
|
|
concurrent updates one must use <command>SELECT FOR UPDATE</command>,
|
|
<command>SELECT FOR SHARE</command>, or an appropriate <command>LOCK
|
|
TABLE</command> statement. (<command>SELECT FOR UPDATE</command>
|
|
or <command>SELECT FOR SHARE</command> locks just the
|
|
returned rows against concurrent updates, while <command>LOCK
|
|
TABLE</command> locks the whole table.) This should be taken into
|
|
account when porting applications to
|
|
<productname>PostgreSQL</productname> from other environments.
|
|
(Before version 6.5 <productname>PostgreSQL</productname> used
|
|
read locks, and so this above consideration is also relevant when
|
|
upgrading from <productname>PostgreSQL</productname> versions
|
|
prior to 6.5.)
|
|
</para>
|
|
|
|
<para>
|
|
Global validity checks require extra thought under <acronym>MVCC</acronym>.
|
|
For example, a banking application might wish to check that the sum of
|
|
all credits in one table equals the sum of debits in another table,
|
|
when both tables are being actively updated. Comparing the results of two
|
|
successive <literal>SELECT sum(...)</literal> commands will not work reliably under
|
|
Read Committed mode, since the second query will likely include the results
|
|
of transactions not counted by the first. Doing the two sums in a
|
|
single serializable transaction will give an accurate picture of the
|
|
effects of transactions that committed before the serializable transaction
|
|
started — but one might legitimately wonder whether the answer is still
|
|
relevant by the time it is delivered. If the serializable transaction
|
|
itself applied some changes before trying to make the consistency check,
|
|
the usefulness of the check becomes even more debatable, since now it
|
|
includes some but not all post-transaction-start changes. In such cases
|
|
a careful person might wish to lock all tables needed for the check,
|
|
in order to get an indisputable picture of current reality. A
|
|
<literal>SHARE</> mode (or higher) lock guarantees that there are no
|
|
uncommitted changes in the locked table, other than those of the current
|
|
transaction.
|
|
</para>
|
|
|
|
<para>
|
|
Note also that if one is
|
|
relying on explicit locking to prevent concurrent changes, one should use
|
|
Read Committed mode, or in Serializable mode be careful to obtain the
|
|
lock(s) before performing queries. A lock obtained by a
|
|
serializable transaction guarantees that no other transactions modifying
|
|
the table are still running, but if the snapshot seen by the
|
|
transaction predates obtaining the lock, it may predate some now-committed
|
|
changes in the table. A serializable transaction's snapshot is actually
|
|
frozen at the start of its first query or data-modification command
|
|
(<literal>SELECT</>, <literal>INSERT</>,
|
|
<literal>UPDATE</>, or <literal>DELETE</>), so
|
|
it's possible to obtain locks explicitly before the snapshot is
|
|
frozen.
|
|
</para>
|
|
</sect1>
|
|
|
|
<sect1 id="locking-indexes">
|
|
<title>Locking and Indexes</title>
|
|
|
|
<indexterm zone="locking-indexes">
|
|
<primary>index</primary>
|
|
<secondary>locks</secondary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
Though <productname>PostgreSQL</productname>
|
|
provides nonblocking read/write access to table
|
|
data, nonblocking read/write access is not currently offered for every
|
|
index access method implemented
|
|
in <productname>PostgreSQL</productname>.
|
|
The various index types are handled as follows:
|
|
|
|
<variablelist>
|
|
<varlistentry>
|
|
<term>
|
|
B-tree and <acronym>GiST</acronym> indexes
|
|
</term>
|
|
<listitem>
|
|
<para>
|
|
Short-term share/exclusive page-level locks are used for
|
|
read/write access. Locks are released immediately after each
|
|
index row is fetched or inserted. These index types provide
|
|
the highest concurrency without deadlock conditions.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term>
|
|
Hash indexes
|
|
</term>
|
|
<listitem>
|
|
<para>
|
|
Share/exclusive hash-bucket-level locks are used for read/write
|
|
access. Locks are released after the whole bucket is processed.
|
|
Bucket-level locks provide better concurrency than index-level
|
|
ones, but deadlock is possible since the locks are held longer
|
|
than one index operation.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
</para>
|
|
|
|
<para>
|
|
Currently, B-tree indexes offer the best performance for concurrent
|
|
applications; since they also have more features than hash
|
|
indexes, they are the recommended index type for concurrent
|
|
applications that need to index scalar data. When dealing with
|
|
non-scalar data, B-trees are not useful, and GiST indexes should
|
|
be used instead.
|
|
</para>
|
|
</sect1>
|
|
</chapter>
|
|
|
|
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|
|
Local variables:
|
|
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|
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|
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|
|
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|
|
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|
|
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|
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|
|
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|
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|
|
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|