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Chapter on multi-version concurrency control from Vadim.

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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 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 data queried.
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>
Accordingly, the four isolation levels are defined to be:
<segmentedlist>
<segtitle>
Isolation Level
</segtitle>
<segtitle>
Dirty Read
</segtitle>
<segtitle>
Non-Repeatable Read
</segtitle>
<segtitle>
Phantom Read
</segtitle>
<seglistitem>
<seg>
Read uncommitted
</seg>
<seg>
Possible
</seg>
<seg>
Possible
</seg>
<seg>
Possible
</seg>
</seglistitem>
<seglistitem>
<seg>
Read committed
</seg>
<seg>
Not possible
</seg>
<seg>
Possible
</seg>
<seg>
Possible
</seg>
</seglistitem>
<seglistitem>
<seg>
Repeatable read
</seg>
<seg>
Not possible
</seg>
<seg>
Not possible
</seg>
<seg>
Possible
</seg>
</seglistitem>
<seglistitem>
<seg>
Serializable
</seg>
<seg>
Not possible
</seg>
<seg>
Not possible
</seg>
<seg>
Not possible
</seg>
</seglistitem>
</segmentedlist>
<productname>Postgres</productname>
offers the read committed and serializable isolation levels.
</para>
</sect1>
<sect1>
<title>Read Committed Isolation Level</title>
<para>
This 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 SELECT or INSERT (with a query)
statements will not be affected by concurrent transactions.
</para>
</sect1>
<sect1>
<title>Serializable Isolation Level</title>
<para>
This level provides the highest transaction isolation. When a
transaction is on the <firstterm>serializable</firstterm> 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
<command>UPDATE</command>/<command>DELETE</command>/<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>
Note that only AccessExclusiveLock blocks <command>SELECT</command> (without FOR
UPDATE) statement.
</para>
</note>
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</sect2>
<sect2>
<title>Row-level locks</title>
<para>
These locks are acquired by means of modification of internal
fields of row being updated/deleted/marked for update.
<productname>Postgres</productname>
doesn't remember any information about modified rows in memory and
so hasn't limit for locked rows 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, it is not the case for all 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 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/updation by concurrent
transactions before the current transaction 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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