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More about chained mode and isolation levels.

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This commit is contained in:
Vadim B. Mikheev 1999-06-11 05:19:53 +00:00
parent 3b9ef4d073
commit 962c66d83f
1 changed files with 35 additions and 16 deletions
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doc/src/sgml/ref/begin.sgml
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@ -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>