diff options
| -rw-r--r-- | doc/src/sgml/ref/begin.sgml | 51 |
1 files changed, 35 insertions, 16 deletions
diff --git a/doc/src/sgml/ref/begin.sgml b/doc/src/sgml/ref/begin.sgml index cd9842a87de..62e5b88d0a7 100644 --- a/doc/src/sgml/ref/begin.sgml +++ b/doc/src/sgml/ref/begin.sgml @@ -10,7 +10,7 @@ BEGIN WORK </REFNAME> <REFPURPOSE> - Begins a transaction + Begins a transaction in chained mode </REFPURPOSE> </refnamediv> @@ -78,15 +78,31 @@ BEGIN [ WORK | TRANSACTION ] Description </TITLE> <para> - <command>BEGIN</command> initiates a user transaction - which <productname>Postgres</productname> will - guarantee is serializable with respect to all concurrently - executing transactions. <productname>Postgres</productname> uses two-phase - locking - to perform this task. If the transaction is committed, - <productname>Postgres</productname> will ensure either that all updates are - done or else - that none of + By default, <productname>Postgres</productname> executes transactions + in unchained mode (also known as autocommit feature in other DBMSes). + In other words, each user statement is executed in its own transaction + and commit is implicit (if execution was successfull). + <command>BEGIN</command> initiates a user transaction in chained mode, + i.e. all user statements after <command>BEGIN</command> command will + be executed in single transaction untill explicit COMMIT, ROLLBACK + or execution abort. Statements in chained mode are executed much faster, + because of transaction start/commit requires significant CPU and disk + activity. This mode is also required for consistency when changing + one of related tables. + </para> + <para> + Default transaction isolation level in <productname>Postgres</productname> + is READ COMMITTED one, when queries inside transaction see only changes + committed before query execution. So, you have to use + <command>SET TRANSACTION ISOLATION LEVEL SERIALIZABLE</command> + command just after BEGIN if you need in better transaction isolation. + In SERIALIZABLE mode queries will see only changes committed before entire + transaction began (actually, before execution of first DML statement + in serializable transaction). + </para> + <para> + If the transaction is committed, <productname>Postgres</productname> + will ensure either that all updates are done or else that none of them are done. Transactions have the standard ACID (atomic, consistent, isolatable, and durable) property. </para> @@ -105,12 +121,12 @@ BEGIN [ WORK | TRANSACTION ] <PARA> Refer to the <command>LOCK</command> statement for further information - about locking tables inside a transaction. + about locking tables inside a transaction. </PARA> <PARA> Use <command>COMMIT</command> or <command>ROLLBACK</command> - to terminate a transaction. + to terminate a transaction. </PARA> </REFSECT2> </refsect1> @@ -133,7 +149,7 @@ BEGIN WORK; </TITLE> <PARA> <command>BEGIN</command> - is a <productname>Postgres</productname> language extension. + is a <productname>Postgres</productname> language extension. </para> <REFSECT2 ID="R2-SQL-BEGINWORK-4"> <REFSECT2INFO> @@ -144,9 +160,12 @@ BEGIN WORK; </TITLE> <PARA> There is no explicit BEGIN WORK command in <acronym>SQL92</acronym>; - transaction initiation - is always implicit and it terminates either with a COMMIT or with - a ROLLBACK statement. + transaction initiation is always implicit and it terminates either + with a COMMIT or with a ROLLBACK statement. + </PARA> + <PARA> + <acronym>SQL92</acronym> also requires SERIALIZABLE to be default + transaction isolation level. </PARA> </refsect2> </refsect1> |