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Correct erroneous description of MVCC Read Committed semantics.

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This commit is contained in:
Tom Lane 2001-03-28 20:46:34 +00:00
parent f155cc82ec
commit 0cadec788e
1 changed files with 17 additions and 10 deletions
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doc/src/sgml/mvcc.sgml
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@ -1,5 +1,5 @@
<!-- <!--
$Header: /cvsroot/pgsql/doc/src/sgml/mvcc.sgml,v 2.11 2000/12/22 21:51:58 petere Exp $ $Header: /cvsroot/pgsql/doc/src/sgml/mvcc.sgml,v 2.12 2001/03/28 20:46:34 tgl Exp $
--> -->
<chapter id="mvcc"> <chapter id="mvcc">
@ -95,7 +95,7 @@ $Header: /cvsroot/pgsql/doc/src/sgml/mvcc.sgml,v 2.11 2000/12/22 21:51:58 petere
The four isolation levels and the corresponding behaviors are described below. The four isolation levels and the corresponding behaviors are described below.
<table tocentry="1"> <table tocentry="1">
<title><productname>Postgres</productname> Isolation Levels</title> <title><acronym>ANSI</acronym>/<acronym>ISO</acronym> <acronym>SQL</acronym> Isolation Levels</title>
<titleabbrev>Isolation Levels</titleabbrev> <titleabbrev>Isolation Levels</titleabbrev>
<tgroup cols="4"> <tgroup cols="4">
<thead> <thead>
@ -192,10 +192,13 @@ $Header: /cvsroot/pgsql/doc/src/sgml/mvcc.sgml,v 2.11 2000/12/22 21:51:58 petere
is the default isolation level in <productname>Postgres</productname>. is the default isolation level in <productname>Postgres</productname>.
When a transaction runs on this isolation level, When a transaction runs on this isolation level,
a <command>SELECT</command> query sees only data committed before the a <command>SELECT</command> query sees only data committed before the
transaction began and never sees either dirty data or concurrent query began and never sees either uncommitted data or changes committed
transaction changes committed during transaction execution. (However, the during query execution by concurrent transactions. (However, the
<command>SELECT</command> does see the effects of previous updates <command>SELECT</command> does see the effects of previous updates
executed within this same transaction.) executed within this same transaction, even though they are not yet
committed.) Notice that two successive <command>SELECT</command>s can see different data,
even though they are within a single transaction, when other transactions
commit changes during execution of the first <command>SELECT</command>.
</para> </para>
<para> <para>
@ -241,11 +244,15 @@ $Header: /cvsroot/pgsql/doc/src/sgml/mvcc.sgml,v 2.11 2000/12/22 21:51:58 petere
<para> <para>
When a transaction is on the serializable level, When a transaction is on the serializable level,
a <command>SELECT</command> query sees only data committed before the a <command>SELECT</command> query sees only data committed before the
transaction began and never sees either dirty data or concurrent transaction began and never sees either uncommitted data or changes
transaction changes committed during transaction execution. (However, the committed
during transaction execution by concurrent transactions. (However, the
<command>SELECT</command> does see the effects of previous updates <command>SELECT</command> does see the effects of previous updates
executed within this same transaction.) This is the same behavior as executed within this same transaction, even though they are not yet
for Read Committed level. committed.) This is different from Read Committed in that the
<command>SELECT</command>
sees a snapshot as of the start of the transaction, not as of the start
of the current query within the transaction.
</para> </para>
<para> <para>
@ -286,7 +293,7 @@ ERROR: Can't serialize access due to concurrent update
updates make it impossible to sustain the illusion of serial execution, updates make it impossible to sustain the illusion of serial execution,
and the cost of redoing complex transactions may be significant. So and the cost of redoing complex transactions may be significant. So
this level is recommended only when update queries contain logic this level is recommended only when update queries contain logic
sufficiently complex that it may give wrong answers in Read Committed sufficiently complex that they may give wrong answers in Read Committed
level. level.
</para> </para>
</sect1> </sect1>