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168 lines
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168 lines
6.1 KiB
Plaintext
<!-- $PostgreSQL: pgsql/doc/src/sgml/ref/set_transaction.sgml,v 1.25 2006/09/16 00:30:20 momjian 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 transaction</refpurpose>
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</refnamediv>
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<indexterm zone="sql-set-transaction">
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<primary>SET TRANSACTION</primary>
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</indexterm>
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<refsynopsisdiv>
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<synopsis>
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SET TRANSACTION <replaceable class="parameter">transaction_mode</replaceable> [, ...]
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SET SESSION CHARACTERISTICS AS 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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The <command>SET TRANSACTION</command> command sets the
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characteristics of the current transaction. It has no effect on any
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subsequent transactions. <command>SET SESSION
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CHARACTERISTICS</command> sets the default transaction
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characteristics for subsequent transactions of a session. These
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defaults can be overridden by <command>SET TRANSACTION</command>
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for an individual transaction.
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</para>
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<para>
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The available transaction characteristics are the transaction
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isolation level and the transaction access mode (read/write or
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read-only).
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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><literal>READ COMMITTED</literal></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><literal>SERIALIZABLE</literal></term>
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<listitem>
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<para>
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All statements of the current transaction can only see rows committed
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before the first query or data-modification statement was executed in
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this transaction.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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The SQL standard defines two additional levels, <literal>READ
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UNCOMMITTED</literal> and <literal>REPEATABLE READ</literal>.
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In <productname>PostgreSQL</productname> <literal>READ
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UNCOMMITTED</literal> is treated as
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<literal>READ COMMITTED</literal>, while <literal>REPEATABLE
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READ</literal> is treated as <literal>SERIALIZABLE</literal>.
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</para>
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<para>
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The transaction isolation level cannot be changed after the first query or
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data-modification statement (<command>SELECT</command>,
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<command>INSERT</command>, <command>DELETE</command>,
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<command>UPDATE</command>, <command>FETCH</command>, or
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<command>COPY</command>) of a transaction has been executed. See
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<xref linkend="mvcc"> for more information about transaction
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isolation and concurrency control.
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</para>
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<para>
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The transaction access mode determines whether the transaction is
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read/write or read-only. Read/write is the default. When a
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transaction is read-only, the following SQL commands are
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disallowed: <literal>INSERT</literal>, <literal>UPDATE</literal>,
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<literal>DELETE</literal>, and <literal>COPY FROM</literal> if the
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table they would write to is not a temporary table; all
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<literal>CREATE</literal>, <literal>ALTER</literal>, and
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<literal>DROP</literal> commands; <literal>COMMENT</literal>,
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<literal>GRANT</literal>, <literal>REVOKE</literal>,
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<literal>TRUNCATE</literal>; and <literal>EXPLAIN ANALYZE</literal>
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and <literal>EXECUTE</literal> if the command they would execute is
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among those listed. This is a high-level notion of read-only that
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does not prevent all writes to disk.
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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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If <command>SET TRANSACTION</command> is executed without a prior
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<command>START TRANSACTION</command> or <command>BEGIN</command>,
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it will appear to have no effect, since the transaction will immediately
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end.
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</para>
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<para>
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It is possible to dispense with <command>SET TRANSACTION</command>
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by instead specifying the desired <replaceable
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class="parameter">transaction_modes</replaceable> in
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<command>BEGIN</command> or <command>START TRANSACTION</command>.
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</para>
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<para>
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The session default transaction modes can also be set by setting the
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configuration parameters <xref linkend="guc-default-transaction-isolation">
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and <xref linkend="guc-default-transaction-read-only">.
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(In fact <command>SET SESSION CHARACTERISTICS</command> is just a
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verbose equivalent for setting these variables with <command>SET</>.)
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This means the defaults can be set in the configuration file, via
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<command>ALTER DATABASE</>, etc. Consult <xref linkend="runtime-config">
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for more information.
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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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<para>
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Both commands are defined in the <acronym>SQL</acronym> standard.
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<literal>SERIALIZABLE</literal> is the default transaction
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isolation level in the standard. In
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<productname>PostgreSQL</productname> the default is ordinarily
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<literal>READ COMMITTED</literal>, but you can change it as
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mentioned above. Because of lack of predicate locking, the
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<literal>SERIALIZABLE</literal> level is not truly
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serializable. See <xref linkend="mvcc"> for details.
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</para>
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<para>
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In the SQL standard, there is one other transaction characteristic
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that can be set with these commands: the size of the diagnostics
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area. This concept is specific to embedded SQL, and therefore is
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not implemented in the <productname>PostgreSQL</productname> server.
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</para>
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<para>
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The SQL standard requires commas between successive <replaceable
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class="parameter">transaction_modes</replaceable>, but for historical
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reasons <productname>PostgreSQL</productname> allows the commas to be
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omitted.
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</para>
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</refsect1>
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</refentry>
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