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<!-- $Header: /cvsroot/pgsql/doc/src/sgml/ref/set_transaction.sgml,v 1.9 2002/08/04 04:31:44 momjian Exp $ --> |
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<!-- $Header: /cvsroot/pgsql/doc/src/sgml/ref/set_transaction.sgml,v 1.10 2003/01/10 22:03:27 petere Exp $ --> |
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<refentry id="SQL-SET-TRANSACTION"> |
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<docinfo> |
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<date>2000-11-24</date> |
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@ -16,9 +16,10 @@ |
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<refsynopsisdiv> |
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<synopsis> |
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SET TRANSACTION ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE } |
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SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL |
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{ READ COMMITTED | SERIALIZABLE } |
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SET TRANSACTION |
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[ ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE } ] [ READ WRITE | READ ONLY ] |
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SET SESSION CHARACTERISTICS AS TRANSACTION |
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[ ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE } ] [ READ WRITE | READ ONLY ] |
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</synopsis> |
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</refsynopsisdiv> |
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@ -26,17 +27,19 @@ SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL |
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<title>Description</title> |
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<para> |
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This command sets the transaction isolation level. The |
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<command>SET TRANSACTION</command> command sets the characteristics |
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for the current SQL-transaction. It has no effect on any subsequent |
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transactions. This command cannot be used after the first query or data-modification |
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statement (<command>SELECT</command>, <command>INSERT</command>, |
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<command>DELETE</command>, <command>UPDATE</command>, |
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<command>FETCH</command>, <command>COPY</command>) of a transaction |
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has been executed. <command>SET SESSION CHARACTERISTICS</command> |
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sets the default transaction isolation level for each transaction |
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for a session. <command>SET TRANSACTION</command> can override it |
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for an individual transaction. |
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The <command>SET TRANSACTION</command> command sets the transaction |
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characteristics of the current SQL-transaction. It has no effect on |
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any subsequent transactions. <command>SET SESSION |
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CHARACTERISTICS</command> sets the default transaction |
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characteristics for each transaction of a session. <command>SET |
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TRANSACTION</command> can override it for an individual |
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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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@ -45,7 +48,7 @@ SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL |
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<variablelist> |
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<varlistentry> |
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<term>READ COMMITTED</term> |
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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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@ -55,7 +58,7 @@ SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL |
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</varlistentry> |
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<varlistentry> |
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<term>SERIALIZABLE</term> |
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<term><literal>SERIALIZABLE</literal></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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@ -72,6 +75,28 @@ SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL |
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</listitem> |
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</varlistentry> |
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</variablelist> |
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The transaction isolation level cannot be set after the first query |
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or 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>, |
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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 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 TO</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 writes to disk. |
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</para> |
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</refsect1> |
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@ -97,7 +122,7 @@ SET default_transaction_isolation = '<replaceable>value</replaceable>' |
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<title>SQL92, SQL99</title> |
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<para> |
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<option>SERIALIZABLE</option> is the default level in |
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<option>SERIALIZABLE</option> is the default transaction isolation level in |
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<acronym>SQL</acronym>. <productname>PostgreSQL</productname> does |
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not provide the isolation levels <option>READ UNCOMMITTED</option> |
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and <option>REPEATABLE READ</option>. Because of multiversion |
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@ -107,11 +132,10 @@ SET default_transaction_isolation = '<replaceable>value</replaceable>' |
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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 these commands: 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 |
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<productname>PostgreSQL</productname>. |
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In <acronym>SQL</acronym> there is one other transaction |
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characteristic that can be set with these commands: the size of |
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the diagnostics area. This concept is not supported in |
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<productname>PostgreSQL</productname>. |
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</para> |
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</refsect2> |
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</refsect1> |
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Peter Eisentraut
23 years ago