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The endterm attribute is mainly useful when the toolchain does not support automatic link target text generation for a particular situation. In the past, this was required by the man page tools for all reference page links, but that is no longer the case, and it now actually gets in the way of proper automatic link text generation. The only remaining use cases are currently xrefs to refsects.
169 lines
6.2 KiB
Plaintext
169 lines
6.2 KiB
Plaintext
<!-- $PostgreSQL: pgsql/doc/src/sgml/ref/set_transaction.sgml,v 1.28 2010/04/03 07:23:02 petere Exp $ -->
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<refentry id="SQL-SET-TRANSACTION">
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<refmeta>
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<refentrytitle>SET TRANSACTION</refentrytitle>
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<manvolnum>7</manvolnum>
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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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<phrase>where <replaceable class="parameter">transaction_mode</replaceable> is one of:</phrase>
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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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