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by the SQL standard. For backwards compatibility, however, continue to accept the syntax without. Minor editorialization in the reference pages for these commands, too.
175 lines
5.0 KiB
Plaintext
175 lines
5.0 KiB
Plaintext
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$PostgreSQL: pgsql/doc/src/sgml/ref/begin.sgml,v 1.33 2004/08/12 21:00:21 tgl Exp $
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PostgreSQL documentation
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-->
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<refentry id="SQL-BEGIN">
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<refmeta>
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<refentrytitle id="SQL-BEGIN-TITLE">BEGIN</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>BEGIN</refname>
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<refpurpose>start a transaction block</refpurpose>
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</refnamediv>
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<indexterm zone="sql-begin">
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<primary>BEGIN</primary>
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</indexterm>
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<refsynopsisdiv>
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<synopsis>
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BEGIN [ WORK | TRANSACTION ] [ <replaceable class="parameter">transaction_mode</replaceable> [, ...] ]
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where <replaceable class="parameter">transaction_mode</replaceable> is one of:
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ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
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READ WRITE | READ ONLY
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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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<command>BEGIN</command> initiates a transaction block, that is,
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all statements after a <command>BEGIN</command> command will be
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executed in a single transaction until an explicit <xref
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linkend="sql-commit" endterm="sql-commit-title"> or <xref
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linkend="sql-rollback" endterm="sql-rollback-title"> is given.
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By default (without <command>BEGIN</command>),
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<productname>PostgreSQL</productname> executes
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transactions in <quote>autocommit</quote> mode, that is, each
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statement is executed in its own transaction and a commit is
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implicitly performed at the end of the statement (if execution was
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successful, otherwise a rollback is done).
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</para>
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<para>
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Statements are executed more quickly in a transaction block, because
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transaction start/commit requires significant CPU and disk
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activity. Execution of multiple statements inside a transaction is
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also useful to ensure consistency when making several related changes:
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other sessions will be unable to see the intermediate states
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wherein not all the related updates have been done.
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</para>
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<para>
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If the isolation level or read/write mode is specified, the new
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transaction has those characteristics, as if
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<xref linkend="sql-set-transaction" endterm="sql-set-transaction-title">
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was executed.
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</para>
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</refsect1>
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<refsect1>
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<title>Parameters</title>
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<variablelist>
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<varlistentry>
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<term><literal>WORK</literal></term>
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<term><literal>TRANSACTION</literal></term>
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<listitem>
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<para>
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Optional key words. They have no effect.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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Refer to <xref linkend="sql-set-transaction"
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endterm="sql-set-transaction-title"> for information on the meaning
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of the other parameters to this statement.
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</para>
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</refsect1>
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<refsect1>
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<title>Notes</title>
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<para>
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<xref linkend="sql-start-transaction"
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endterm="sql-start-transaction-title"> has the same functionality
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as <command>BEGIN</>.
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</para>
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<para>
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Use <xref linkend="SQL-COMMIT" endterm="SQL-COMMIT-TITLE"> or
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<xref linkend="SQL-ROLLBACK" endterm="SQL-ROLLBACK-TITLE">
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to terminate a transaction block.
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</para>
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<para>
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Issuing <command>BEGIN</> when already inside a transaction block will
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provoke a warning message. The state of the transaction is not affected.
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To nest transactions within a transaction block, use savepoints
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(see <xref linkend="sql-savepoint" endterm="sql-savepoint-title">).
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</para>
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<para>
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For reasons of backwards compatibility, the commas between successive
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<replaceable class="parameter">transaction_modes</replaceable> may be
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omitted.
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</para>
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</refsect1>
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<refsect1>
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<title>Examples</title>
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<para>
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To begin a transaction block:
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<programlisting>
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BEGIN;
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</programlisting>
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</para>
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</refsect1>
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<refsect1>
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<title>Compatibility</title>
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<para>
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<command>BEGIN</command> is a <productname>PostgreSQL</productname>
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language extension. It is equivalent to the SQL-standard command
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<xref linkend="sql-start-transaction"
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endterm="sql-start-transaction-title">, which see for additional
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compatibility information.
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</para>
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<para>
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Incidentally, the <literal>BEGIN</literal> key word is used for a
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different purpose in embedded SQL. You are advised to be careful
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about the transaction semantics when porting database applications.
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</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<simplelist type="inline">
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<member><xref linkend="sql-commit" endterm="sql-commit-title"></member>
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<member><xref linkend="sql-rollback" endterm="sql-rollback-title"></member>
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<member><xref linkend="sql-start-transaction" endterm="sql-start-transaction-title"></member>
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<member><xref linkend="sql-savepoint" endterm="sql-savepoint-title"></member>
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</simplelist>
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</refsect1>
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</refentry>
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Local variables:
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End:
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