diff options
| -rw-r--r-- | doc/src/sgml/ref/begin.sgml | 47 | ||||
| -rw-r--r-- | doc/src/sgml/ref/start_transaction.sgml | 23 |
2 files changed, 45 insertions, 25 deletions
diff --git a/doc/src/sgml/ref/begin.sgml b/doc/src/sgml/ref/begin.sgml index 850bfd77e68..11ca82e6d49 100644 --- a/doc/src/sgml/ref/begin.sgml +++ b/doc/src/sgml/ref/begin.sgml @@ -1,5 +1,5 @@ <!-- -$Header: /cvsroot/pgsql/doc/src/sgml/ref/begin.sgml,v 1.18 2002/08/04 04:31:44 momjian Exp $ +$Header: /cvsroot/pgsql/doc/src/sgml/ref/begin.sgml,v 1.19 2002/08/30 22:45:25 tgl Exp $ PostgreSQL documentation --> @@ -103,33 +103,35 @@ WARNING: BEGIN: already a transaction in progress <command>BEGIN</command> initiates a user transaction in chained mode, i.e., all user statements after <command>BEGIN</command> command will be executed in a single transaction until an explicit - <xref linkend="sql-commit" endterm="sql-commit-title">, - <xref linkend="sql-rollback" endterm="sql-rollback-title">, - or execution abort. Statements in chained mode are executed much faster, + <xref linkend="sql-commit" endterm="sql-commit-title"> or + <xref linkend="sql-rollback" endterm="sql-rollback-title">. + Statements are executed more quickly in chained mode, because transaction start/commit requires significant CPU and disk activity. Execution of multiple statements inside a transaction - is also required for consistency when changing several - related tables. + is also useful to ensure consistency when changing several + related tables: other clients will be unable to see the intermediate + states wherein not all the related updates have been done. </para> <para> The default transaction isolation level in <productname>PostgreSQL</productname> - is READ COMMITTED, where queries inside the transaction see only changes - committed before query execution. So, you have to use + is READ COMMITTED, wherein each query inside the transaction sees changes + committed before that query begins execution. So, you have to use <command>SET TRANSACTION ISOLATION LEVEL SERIALIZABLE</command> - just after <command>BEGIN</command> if you need more rigorous transaction isolation. + just after <command>BEGIN</command> if you need more rigorous transaction + isolation. (Alternatively, you can change the default transaction + isolation level; see the <citetitle>PostgreSQL Administrator's + Guide</citetitle> for details.) In SERIALIZABLE mode queries will see only changes committed before the entire transaction began (actually, before execution of the first DML statement - in a serializable transaction). + in the transaction). </para> <para> - If the transaction is committed, <productname>PostgreSQL</productname> - will ensure either that all updates are done or else that none of - them are done. Transactions have the standard <acronym>ACID</acronym> - (atomic, consistent, isolatable, and durable) property. + Transactions have the standard <acronym>ACID</acronym> + (atomic, consistent, isolatable, and durable) properties. </para> <refsect2 id="R2-SQL-BEGIN-3"> @@ -140,9 +142,9 @@ WARNING: BEGIN: already a transaction in progress Notes </title> <para> - Refer to <xref linkend="sql-lock" endterm="sql-lock-title"> - for further information - about locking tables inside a transaction. + <xref linkend="sql-start-transaction" + endterm="sql-start-transaction-title"> has the same functionality + as <command>BEGIN</>. </para> <para> @@ -151,6 +153,17 @@ WARNING: BEGIN: already a transaction in progress <xref linkend="SQL-ROLLBACK" endterm="SQL-ROLLBACK-TITLE"> to terminate a transaction. </para> + + <para> + Refer to <xref linkend="sql-lock" endterm="sql-lock-title"> + for further information + about locking tables inside a transaction. + </para> + + <para> + If you turn <literal>autocommit</> mode off, then <command>BEGIN</> + is not required: any SQL command automatically starts a transaction. + </para> </refsect2> </refsect1> diff --git a/doc/src/sgml/ref/start_transaction.sgml b/doc/src/sgml/ref/start_transaction.sgml index 2e657c610bb..fb8dd131932 100644 --- a/doc/src/sgml/ref/start_transaction.sgml +++ b/doc/src/sgml/ref/start_transaction.sgml @@ -1,4 +1,8 @@ -<!-- $Header: /cvsroot/pgsql/doc/src/sgml/ref/start_transaction.sgml,v 1.2 2002/08/06 05:32:16 momjian Exp $ --> +<!-- +$Header: /cvsroot/pgsql/doc/src/sgml/ref/start_transaction.sgml,v 1.3 2002/08/30 22:45:25 tgl Exp $ +PostgreSQL documentation +--> + <refentry id="SQL-START-TRANSACTION"> <docinfo> <date>2002-07-26</date> @@ -76,7 +80,7 @@ WARNING: BEGIN: already a transaction in progress This command begins a new transaction. If the isolation level is specified, the new transaction has that isolation level. In all other respects, the behavior of this command is identical to the - <command>BEGIN</command> command. + <xref linkend="sql-begin" endterm="sql-begin-title"> command. </para> </refsect1> @@ -87,8 +91,8 @@ WARNING: BEGIN: already a transaction in progress <para> The isolation level of a transaction can also be set with the <xref linkend="sql-set-transaction" endterm="sql-set-transaction-title"> - command. If no isolation level is specified, the level defaults to - <option>READ COMMITTED</option>. + command. If no isolation level is specified, the default isolation + level is used. </para> </refsect1> @@ -99,11 +103,14 @@ WARNING: BEGIN: already a transaction in progress <title>SQL99</title> <para> - <option>SERIALIZABLE</option> is the default level in - <acronym>SQL</acronym>. <productname>PostgreSQL</productname> + <option>SERIALIZABLE</option> is the default isolation level in + <acronym>SQL99</acronym>, but it is not the usual default in + <productname>PostgreSQL</productname>: the factory default setting + is READ COMMITTED. + <productname>PostgreSQL</productname> does not provide the isolation levels <option>READ UNCOMMITTED</option> - and <option>REPEATABLE READ</option>. Because of multiversion - concurrency control, the <option>SERIALIZABLE</option> level is + and <option>REPEATABLE READ</option>. Because of lack of predicate + locking, the <option>SERIALIZABLE</option> level is not truly serializable. See the <citetitle>User's Guide</citetitle> for details. </para> |