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<!-- $Header: /cvsroot/pgsql/doc/src/sgml/ref/set_transaction.sgml,v 1.2 2000/07/14 15:27:14 thomas Exp $ -->
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<refentry id="SQL-SET-TRANSACTION">
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<refmeta>
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<refentrytitle id="SQL-SET-TRANSACTION-TITLE">SET TRANSACTION</refentrytitle>
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<refmiscinfo>SQL - Language Statements</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>SET TRANSACTION</refname>
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<refpurpose>Set the characteristics of the current SQL-transaction</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<refsynopsisdivinfo>
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<date>2000-06-01</date>
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</refsynopsisdivinfo>
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<synopsis>
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SET TRANSACTION ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE }
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</synopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>
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The <command>SET TRANSACTION</command> command sets the
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characteristics for the current SQL-transaction. It has no effect
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on any subsequent transactions. This command cannot be used after
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the first DML statement (<command>SELECT</command>,
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<command>INSERT</command>, <command>DELETE</command>,
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<command>UPDATE</command>, <command>FETCH</command>,
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<command>COPY</command>) of a transaction has been executed.
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</para>
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<para>
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The isolation level of a transaction determines what data the
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transaction can see when other transactions are running concurrently.
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<variablelist>
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<varlistentry>
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<term>READ COMMITTED</term>
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<listitem>
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<para>
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A statement can only see rows committed before it began. This
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is the default.
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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>SERIALIZABLE</term>
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<listitem>
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<para>
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The current transaction can only see rows committed before
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first DML statement was executed in this transaction.
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</para>
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<tip>
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<para>
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Intuitively, serializable means that two concurrent
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transactions will leave the database in the same state as if
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the two has been executed strictly after one another in either
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order.
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</para>
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</tip>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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</refsect1>
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<refsect1 id="R1-SQL-SET-TRANSACTION-3">
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<title>Compatibility</title>
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<refsect2 id="R2-SQL-SET-TRANSACTION-4">
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<title>SQL92, SQL99</title>
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<para>
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SERIALIZABLE is the default level in <acronym>SQL</acronym>.
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Postgres does not provide the isolation levels <option>READ
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UNCOMMITTED</option> and <option>REPEATABLE READ</option>. Because
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of multi-version concurrency control, the serializable level is not
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truly serializable. See the <citetitle>User's Guide</citetitle> for
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details.
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</para>
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<para>
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In <acronym>SQL</acronym> there are two other transaction
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characteristics that can be set with this command: whether the
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transaction is read-only and the size of the diagnostics area.
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Neither of these concepts are supported in Postgres.
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</para>
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</refsect2>
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</refsect1>
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</refentry>
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2000-07-14 17:27:14 +02:00
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