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bb4eefe7bf
DocBook XML is superficially compatible with DocBook SGML but has a slightly stricter DTD that we have been violating in a few cases. Although XSLT doesn't care whether the document is valid, the style sheets don't necessarily process invalid documents correctly, so we need to work toward fixing this. This first commit moves the indexterms in refentry elements to an allowed position. It has no impact on the output.
273 lines
10 KiB
Plaintext
273 lines
10 KiB
Plaintext
<!-- doc/src/sgml/ref/set_transaction.sgml -->
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<refentry id="SQL-SET-TRANSACTION">
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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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<indexterm>
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<primary>transaction isolation level</primary>
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<secondary>setting</secondary>
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</indexterm>
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<indexterm>
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<primary>read-only transaction</primary>
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<secondary>setting</secondary>
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</indexterm>
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<indexterm>
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<primary>deferrable transaction</primary>
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<secondary>setting</secondary>
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</indexterm>
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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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<refsynopsisdiv>
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<synopsis>
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SET TRANSACTION <replaceable class="parameter">transaction_mode</replaceable> [, ...]
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SET TRANSACTION SNAPSHOT <replaceable class="parameter">snapshot_id</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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[ NOT ] DEFERRABLE
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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, the transaction access mode (read/write or
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read-only), and the deferrable mode.
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In addition, a snapshot can be selected, though only for the current
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transaction, not as a session default.
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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>REPEATABLE READ</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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<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. If a pattern of reads and writes among concurrent
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serializable transactions would create a situation which could not
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have occurred for any serial (one-at-a-time) execution of those
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transactions, one of them will be rolled back with a
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<literal>serialization_failure</literal> error.
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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 one additional level, <literal>READ
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UNCOMMITTED</literal>.
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In <productname>PostgreSQL</productname> <literal>READ
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UNCOMMITTED</literal> is treated as <literal>READ COMMITTED</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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<para>
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The <literal>DEFERRABLE</literal> transaction property has no effect
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unless the transaction is also <literal>SERIALIZABLE</literal> and
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<literal>READ ONLY</literal>. When all three of these properties are
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selected for a
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transaction, the transaction may block when first acquiring its snapshot,
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after which it is able to run without the normal overhead of a
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<literal>SERIALIZABLE</literal> transaction and without any risk of
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contributing to or being canceled by a serialization failure. This mode
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is well suited for long-running reports or backups.
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</para>
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<para>
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The <literal>SET TRANSACTION SNAPSHOT</literal> command allows a new
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transaction to run with the same <firstterm>snapshot</> as an existing
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transaction. The pre-existing transaction must have exported its snapshot
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with the <literal>pg_export_snapshot</literal> function (see <xref
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linkend="functions-snapshot-synchronization">). That function returns a
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snapshot identifier, which must be given to <literal>SET TRANSACTION
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SNAPSHOT</literal> to specify which snapshot is to be imported. The
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identifier must be written as a string literal in this command, for example
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<literal>'000003A1-1'</>.
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<literal>SET TRANSACTION SNAPSHOT</literal> can only be executed at the
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start of a transaction, before 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 the transaction. Furthermore, the transaction
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must already be set to <literal>SERIALIZABLE</literal> or
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<literal>REPEATABLE READ</literal> isolation level (otherwise, the snapshot
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would be discarded immediately, since <literal>READ COMMITTED</> mode takes
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a new snapshot for each command). If the importing transaction uses
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<literal>SERIALIZABLE</literal> isolation level, then the transaction that
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exported the snapshot must also use that isolation level. Also, a
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non-read-only serializable transaction cannot import a snapshot from a
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read-only transaction.
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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 emits a warning and otherwise has no effect.
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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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But that option is not available for <command>SET TRANSACTION
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SNAPSHOT</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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<xref linkend="guc-default-transaction-read-only">, and
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<xref linkend="guc-default-transaction-deferrable">.
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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>
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<title>Examples</title>
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<para>
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To begin a new transaction with the same snapshot as an already
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existing transaction, first export the snapshot from the existing
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transaction. That will return the snapshot identifier, for example:
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<programlisting>
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BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
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SELECT pg_export_snapshot();
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pg_export_snapshot
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--------------------
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000003A1-1
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(1 row)
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</programlisting>
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Then give the snapshot identifier in a <command>SET TRANSACTION
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SNAPSHOT</command> command at the beginning of the newly opened
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transaction:
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<programlisting>
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BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
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SET TRANSACTION SNAPSHOT '000003A1-1';
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</programlisting></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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These commands are defined in the <acronym>SQL</acronym> standard,
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except for the <literal>DEFERRABLE</literal> transaction mode
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and the <command>SET TRANSACTION SNAPSHOT</> form, which are
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<productname>PostgreSQL</productname> extensions.
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</para>
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<para>
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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.
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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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