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 <chapter id="mvcc">
  <title>Multi-Version Concurrency Control</title>

  <abstract>
   <para>
    Multi-Version Concurrency Control
    (MVCC)
    is an advanced technique for improving database performance in a
    multi-user environment. 
    <ulink url="mailto:vadim@krs.ru">Vadim Mikheev</ulink> provided
    the implementation for <productname>Postgres</productname>.
   </para>
  </abstract>

  <sect1>
   <title>Introduction</title>

   <para>
    Unlike most other database systems which use locks for concurrency control,
    <productname>Postgres</productname>
    maintains data consistency by using a multiversion model. 
    This means that while querying a database each transaction sees
    a snapshot of data (a <firstterm>database version</firstterm>)
    as it was some
    time ago, regardless of the current state of the underlying data.
    This protects the transaction from viewing inconsistent data that
    could be caused by (other) concurrent transaction updates on the same
    data rows, providing <firstterm>transaction isolation</firstterm>
    for each database session.
   </para>

   <para>
    The main difference between multiversion and lock models is that
    in MVCC locks acquired for querying (reading) data don't conflict
    with locks acquired for writing data and so reading never blocks
    writing and writing never blocks reading.
   </para>
  </sect1>

  <sect1>
   <title>Transaction Isolation</title>

   <para>
    The <acronym>ANSI</acronym>/<acronym>ISO</acronym> <acronym>SQL</acronym>
    standard defines four levels of transaction
    isolation in terms of three phenomena that must be prevented 
    between concurrent transactions.
    These undesirable phenomena are:

    <variablelist>
     <varlistentry>
      <term>
       dirty reads
      </term>
     <listitem>
      <para>
	A transaction reads data written by concurrent uncommitted transaction.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
       non-repeatable reads
      </term>
     <listitem>
      <para>
	A transaction re-reads data it has previously read and finds that data
	has been modified by another committed transaction.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
       phantom read
      </term>
     <listitem>
      <para>
	A transaction re-executes a query returning a set of rows that satisfy a
	search condition and finds that additional rows satisfying the condition
	has been inserted by another committed transaction.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    The four isolation levels and the corresponding behaviors are described below.

    <table tocentry="1">
     <title><productname>Postgres</productname> Isolation Levels</title>
     <titleabbrev>Isolation Levels</titleabbrev>
     <tgroup cols="4">
      <thead>
       <row>
	<entry>
	</entry>
	<entry>
	 Dirty Read
	</entry>
	<entry>
	 Non-Repeatable Read
	</entry>
	<entry>
	 Phantom Read
	</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry>
	 Read uncommitted
	</entry>
	<entry>
	 Possible
	</entry>
	<entry>
	 Possible
	</entry>
	<entry>
	 Possible
	</entry>
       </row>

       <row>
	<entry>
	 Read committed
	</entry>
	<entry>
	 Not possible
	</entry>
	<entry>
	 Possible
	</entry>
	<entry>
	 Possible
	</entry>
       </row>

       <row>
	<entry>
	 Repeatable read
	</entry>
	<entry>
	 Not possible
	</entry>
	<entry>
	 Not possible
	</entry>
	<entry>
	 Possible
	</entry>
       </row>

       <row>
	<entry>
	 Serializable
	</entry>
	<entry>
	 Not possible
	</entry>
	<entry>
	 Not possible
	</entry>
	<entry>
	 Not possible
	</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <productname>Postgres</productname>
    offers the read committed and serializable isolation levels.
   </para>
  </sect1>

  <sect1>
   <title>Read Committed Isolation Level</title>

   <para>
    <firstterm>Read Committed</firstterm>
    is the default isolation level in <productname>Postgres</productname>. 
    When a transaction runs on this isolation level, a query sees only
    data committed before the query began and never sees either dirty data or
    concurrent transaction changes committed during query execution.
   </para>

   <para>
    If a row returned by a query while executing an
    <command>UPDATE</command> statement
    (or <command>DELETE</command>
    or <command>SELECT FOR UPDATE</command>)
    is being updated by a
    concurrent uncommitted transaction then the second transaction
    that tries to update this row will wait for the other transaction to
    commit or rollback. In the case of rollback, the waiting transaction
    can proceed to change the row. In the case of commit (and if the
    row still exists; i.e. was not deleted by the other transaction), the
    query will be re-executed for this row to check that new row
    version satisfies query search condition. If the new row version
    satisfies the query search condition then row will be
    updated (or deleted or marked for update).
   </para>

   <para>
    Note that the results of execution of <command>SELECT</command>
    or <command>INSERT</command> (with a query) 
    statements will not be affected by concurrent transactions.
   </para>
  </sect1>

  <sect1>
   <title>Serializable Isolation Level</title>

   <para>
    <firstterm>Serializable</firstterm> provides the highest transaction isolation.
    When a transaction is on the serializable level,
    a query sees only data
    committed before the transaction began and never see either dirty data
    or concurrent transaction changes committed during transaction
    execution. So, this level emulates serial transaction execution,
    as if transactions would be executed one after another, serially,
    rather than concurrently.
   </para>

   <para>
    If a row returned by query while executing a
    <command>UPDATE</command>
    (or <command>DELETE</command> or <command>SELECT FOR UPDATE</command>)
    statement is being updated by
    a concurrent uncommitted transaction then the second transaction
    that tries to update this row will wait for the other transaction to
    commit or rollback. In the case of rollback, the waiting transaction
    can proceed to change the row. In the case of a concurrent
    transaction commit, a serializable transaction will be rolled back
    with the message

    <programlisting>
ERROR:  Can't serialize access due to concurrent update
    </programlisting>

    because a serializable transaction cannot modify rows changed by
    other transactions after the serializable transaction began.
   </para>

   <note>
    <para>
     Note that results of execution of <command>SELECT</command>
     or <command>INSERT</command> (with a query) 
     will not be affected by concurrent transactions.
    </para>
   </note>
  </sect1>

  <sect1>
   <title>Locking and Tables</title>

   <para>
    <productname>Postgres</productname>
    provides various lock modes to control concurrent
    access to data in tables. Some of these lock modes are acquired by
    <productname>Postgres</productname>
    automatically before statement execution, while others are
    provided to be used by applications. All lock modes (except for
    AccessShareLock) acquired in a transaction are held for the duration
    of the transaction.
   </para>

   <para>
    In addition to locks, short-term share/exclusive latches are used
    to control read/write access to table pages in shared buffer pool.
    Latches are released immediately after a tuple is fetched or updated.
   </para>

   <sect2>
    <title>Table-level locks</title>

    <para>
     <variablelist>
      <varlistentry>
       <term>
	AccessShareLock
       </term>
       <listitem>
	<para>
	 An internal lock mode acquiring automatically over tables
	 being queried. <productname>Postgres</productname>
	 releases these locks after statement is
	 done.
	</para>

	<para>
	 Conflicts with AccessExclusiveLock only.
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>
	RowShareLock
       </term>
       <listitem>
	<para>
	 Acquired by <command>SELECT FOR UPDATE</command>
	 and <command>LOCK TABLE</command>
	 for <option>IN ROW SHARE MODE</option> statements.
	</para>

	<para>
	 Conflicts with ExclusiveLock and AccessExclusiveLock modes.
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>
	RowExclusiveLock
       </term>
       <listitem>
	<para>
	 Acquired by <command>UPDATE</command>, <command>DELETE</command>,
	 <command>INSERT</command> and <command>LOCK TABLE</command>
	 for <option>IN ROW EXCLUSIVE MODE</option> statements.
	</para>

	<para>
	 Conflicts with ShareLock, ShareRowExclusiveLock, ExclusiveLock and
	 AccessExclusiveLock modes.
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>
	ShareLock
       </term>
       <listitem>
	<para>
	 Acquired by <command>CREATE INDEX</command>
	 and <command>LOCK TABLE</command> table
	 for <option>IN SHARE MODE</option>
	 statements.
	</para>

	<para>
	 Conflicts with RowExclusiveLock, ShareRowExclusiveLock,
	 ExclusiveLock and AccessExclusiveLock modes.
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>
	ShareRowExclusiveLock
       </term>
       <listitem>
	<para>
	 Acquired by <command>LOCK TABLE</command> for
	 <option>IN SHARE ROW EXCLUSIVE MODE</option> statements.
	</para>

	<para>
	 Conflicts with RowExclusiveLock, ShareLock, ShareRowExclusiveLock,
	 ExclusiveLock and AccessExclusiveLock modes.
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>
	ExclusiveLock
       </term>
       <listitem>
	<para>
	 Acquired by <command>LOCK TABLE</command> table 
	 for <option>IN EXCLUSIVE MODE</option> statements.
	</para>

	<para>
	 Conflicts with RowShareLock, RowExclusiveLock, ShareLock,
	 ShareRowExclusiveLock, ExclusiveLock and AccessExclusiveLock
	 modes.
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>
	AccessExclusiveLock
       </term>
       <listitem>
	<para>
	 Acquired by <command>ALTER TABLE</command>,
	 <command>DROP TABLE</command>,
	 <command>VACUUM</command> and <command>LOCK TABLE</command>
	 statements.
	</para>

	<para>
	 Conflicts with RowShareLock, RowExclusiveLock, ShareLock,
	 ShareRowExclusiveLock, ExclusiveLock and AccessExclusiveLock
	 modes.

	 <note>
	  <para>
	   Only AccessExclusiveLock blocks <command>SELECT</command> (without
	   <option>FOR UPDATE</option>) statement.
	  </para>
	 </note>
	</para>
       </listitem>
      </varlistentry>
     </variablelist>
    </para>
   </sect2>

   <sect2>
    <title>Row-level locks</title>

    <para>
     These locks are acquired when internal
     fields of a row are being updated (or deleted or marked for update).
     <productname>Postgres</productname>
     doesn't remember any information about modified rows in memory and
     so has no limit to the number of rows locked without lock escalation.
    </para>

    <para>
     However, take into account that <command>SELECT FOR UPDATE</command> will modify
     selected rows to mark them and so will results in disk writes.
    </para>

    <para>
     Row-level locks don't affect data querying. They are used to block
     writers to <emphasis>the same row</emphasis> only.
    </para>
   </sect2>
  </sect1>

  <sect1>
   <title>Locking and Indices</title>

   <para>
    Though <productname>Postgres</productname>
    provides unblocking read/write access to table
    data, unblocked read/write access is not provided for every
    index access methods implemented
    in <productname>Postgres</productname>.
   </para>

   <para>
    The various index types are handled as follows:

    <variablelist>
     <varlistentry>
      <term>
       GiST and R-Tree indices
      </term>
      <listitem>
       <para>
	Share/exclusive index-level locks are used for read/write access.
	Locks are released after statement is done.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
       Hash indices
      </term>
      <listitem>
       <para>
	Share/exclusive page-level locks are used for read/write access.
	Locks are released after page is processed.
       </para>

       <para>
	Page-level locks produces better concurrency than index-level ones
	but are subject to deadlocks.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
       Btree
      </term>
      <listitem>
       <para>
	Short-term share/exclusive page-level latches are used for
	read/write access. Latches are released immediately after the index
	tuple is inserted/fetched.
       </para>

       <para>
	Btree indices provide the highest concurrency without deadlock
	conditions.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </sect1>

  <sect1>
   <title>Data consistency checks at the application level</title>

   <para>
    Because readers in <productname>Postgres</productname>
    don't lock data, regardless of
    transaction isolation level, data read by one transaction can be
    overwritten by another. In the other words, if a row is returned
    by <command>SELECT</command> it doesn't mean that this row really
    exists at the time it is returned (i.e. sometime after the
    statement or transaction began) nor
    that the row is protected from deletion or update by concurrent
    transactions before the current transaction does a commit or rollback. 
   </para>

   <para>
    To ensure the actual existance of a row and protect it against
    concurrent updates one must use <command>SELECT FOR UPDATE</command> or
    an appropriate <command>LOCK TABLE</command> statement.
    This should be taken into account when porting applications using
    serializable mode to <productname>Postgres</productname> from other environments.

    <note>
     <para>
      Before version 6.5 <productname>Postgres</productname>
      used read-locks and so the
      above consideration is also the case
      when upgrading to 6.5 (or higher) from previous
      <productname>Postgres</productname> versions.
     </para>
    </note>
   </para>
  </sect1>
 </chapter>

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