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Implement isolation levels read uncommitted and repeatable read as acting

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like the next higher one.
This commit is contained in:
Peter Eisentraut
2003-11-06 22:08:15 +00:00
parent 144a2ecd57
commit 96889392e9
15 changed files with 156 additions and 69 deletions
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23
doc/src/sgml/mvcc.sgml
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@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/mvcc.sgml,v 2.40 2003/11/04 09:55:38 petere Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/mvcc.sgml,v 2.41 2003/11/06 22:08:14 petere Exp $
-->
<chapter id="mvcc">
@ -206,8 +206,25 @@ $Header: /cvsroot/pgsql/doc/src/sgml/mvcc.sgml,v 2.40 2003/11/04 09:55:38 petere
</table>
<para>
<productname>PostgreSQL</productname>
offers the Read Committed and Serializable isolation levels.
In <productname>PostgreSQL</productname>, you can use all four
possible transaction isolation levels. Internally, there are only
two distinct isolation levels, which correspond to the levels Read
Committed and Serializable. When you select the level Read
Uncommitted you really get Read Committed, and when you select
Repeatable Read you really get Serializable, so the actual
isolation level may be stricter than what you select. This is
permitted by the SQL standard: the four isolation levels only
define which phenomena must not happen, they do not define which
phenomena must happen. The reason that PostgreSQL only provides
two isolation levels is that this is the only sensible way to map
the isolation levels to the multiversion concurrency control
architecture. The behavior of the available isolation levels is
detailed in the following subsections.
</para>
<para>
To set the transaction isolation level of a transaction, use the
command <xref linkend="sql-set-transaction">.
</para>
<sect2 id="xact-read-committed">

29
doc/src/sgml/ref/set_transaction.sgml
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@ -1,4 +1,4 @@
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/ref/set_transaction.sgml,v 1.17 2003/09/11 21:42:20 momjian Exp $ -->
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/ref/set_transaction.sgml,v 1.18 2003/11/06 22:08:14 petere Exp $ -->
<refentry id="SQL-SET-TRANSACTION">
<refmeta>
<refentrytitle id="SQL-SET-TRANSACTION-TITLE">SET TRANSACTION</refentrytitle>
@ -17,9 +17,12 @@
<refsynopsisdiv>
<synopsis>
SET TRANSACTION
[ ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE } ] [ READ WRITE | READ ONLY ]
[ ISOLATION LEVEL { READ UNCOMMITTED | READ COMMITTED | REPEATABLE READ | SERIALIZABLE } ]
[ READ WRITE | READ ONLY ]
SET SESSION CHARACTERISTICS AS TRANSACTION
[ ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE } ] [ READ WRITE | READ ONLY ]
[ ISOLATION LEVEL { READ UNCOMMITTED | READ COMMITTED | REPEATABLE READ | SERIALIZABLE } ]
[ READ WRITE | READ ONLY ]
</synopsis>
</refsynopsisdiv>
@ -76,8 +79,11 @@ SET SESSION CHARACTERISTICS AS TRANSACTION
</varlistentry>
</variablelist>
The transaction isolation level cannot be set after the first query
or data-modification statement (<command>SELECT</command>,
The level <literal>READ UNCOMMITTED</literal> is mapped to
<literal>READ COMMITTED</literal>, the level <literal>REPEATABLE
READ</literal> is mapped to <literal>SERIALIZABLE</literal>, The
transaction isolation level cannot be set after the first query or
data-modification statement (<command>SELECT</command>,
<command>INSERT</command>, <command>DELETE</command>,
<command>UPDATE</command>, <command>FETCH</command>,
<command>COPY</command>) of a transaction has been executed. See
@ -122,13 +128,12 @@ SET default_transaction_isolation = '<replaceable>value</replaceable>'
<para>
Both commands are defined in the <acronym>SQL</acronym> standard.
<literal>SERIALIZABLE</literal> is the default transaction
isolation level in the standard; in <productname>PostgreSQL</productname> the default is
ordinarily <literal>READ COMMITTED</literal>, but you can change it as
described above. <productname>PostgreSQL</productname> does not
provide the isolation levels <literal>READ UNCOMMITTED</literal>
and <literal>REPEATABLE READ</literal>. Because of multiversion
concurrency control, the <literal>SERIALIZABLE</literal> level is
not truly serializable. See <xref linkend="mvcc"> for details.
isolation level in the standard; in
<productname>PostgreSQL</productname> the default is ordinarily
<literal>READ COMMITTED</literal>, but you can change it as
described above. Because of multiversion concurrency control, the
<literal>SERIALIZABLE</literal> level is not truly
serializable. See <xref linkend="mvcc"> for details.
</para>
<para>

6
doc/src/sgml/ref/start_transaction.sgml
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@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/start_transaction.sgml,v 1.7 2003/09/09 18:28:53 tgl Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/ref/start_transaction.sgml,v 1.8 2003/11/06 22:08:14 petere Exp $
PostgreSQL documentation
-->
@ -20,7 +20,9 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
START TRANSACTION [ ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE } ] [ READ WRITE | READ ONLY ]
START TRANSACTION
[ ISOLATION LEVEL { READ UNCOMMITTED | READ COMMITTED | REPEATABLE READ | SERIALIZABLE } ]
[ READ WRITE | READ ONLY ]
</synopsis>
</refsynopsisdiv>

12
doc/src/sgml/runtime.sgml
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@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/runtime.sgml,v 1.216 2003/11/04 09:55:38 petere Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/runtime.sgml,v 1.217 2003/11/06 22:08:14 petere Exp $
-->
<Chapter Id="runtime">
@ -2043,10 +2043,12 @@ SET ENABLE_SEQSCAN TO OFF;
<term><varname>default_transaction_isolation</varname> (<type>string</type>)</term>
<listitem>
<para>
Each SQL transaction has an isolation level, which can be either
<quote>read committed</quote> or <quote>serializable</quote>.
This parameter controls the default isolation level of each new
transaction. The default is <quote>read committed</quote>.
Each SQL transaction has an isolation level, which can be
either <quote>read uncommitted</quote>, <quote>read
committed</quote>, <quote>repeatable read</quote>, or
<quote>serializable</quote>. This parameter controls the
default isolation level of each new transaction. The default
is <quote>read committed</quote>.
</para>
<para>