1
0
mirror of https://github.com/postgres/postgres.git synced 2025-08-25 20:23:07 +03:00
Files
postgres/doc/src/sgml/ref/begin.sgml
Peter Eisentraut 9081bddbd7 Improve <xref> vs. <command> formatting in the documentation
SQL commands are generally marked up as <command>, except when a link
to a reference page is used using <xref>.  But the latter doesn't
create monospace markup, so this looks strange especially when a
paragraph contains a mix of links and non-links.

We considered putting <command> in the <refentrytitle> on the target
side, but that creates some formatting side effects elsewhere.
Generally, it seems safer to solve this on the link source side.

We can't put the <xref> inside the <command>; the DTD doesn't allow
this.  DocBook 5 would allow the <command> to have the linkend
attribute itself, but we are not there yet.

So to solve this for now, convert the <xref>s to <link> plus
<command>.  This gives the correct look and also gives some more
flexibility what we can put into the link text (e.g., subcommands or
other clauses).  In the future, these could then be converted to
DocBook 5 style.

I haven't converted absolutely all xrefs to SQL command reference
pages, only those where we care about the appearance of the link text
or where it was otherwise appropriate to make the appearance match a
bit better.  Also in some cases, the links where repetitive, so in
those cases the links where just removed and replaced by a plain
<command>.  In cases where we just want the link and don't
specifically care about the generated link text (typically phrased
"for further information see <xref ...>") the xref is kept.

Reported-by: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Discussion: https://www.postgresql.org/message-id/flat/87o8pco34z.fsf@wibble.ilmari.org
2020-10-03 16:40:02 +02:00

162 lines
4.7 KiB
Plaintext

<!--
doc/src/sgml/ref/begin.sgml
PostgreSQL documentation
-->
<refentry id="sql-begin">
<indexterm zone="sql-begin">
<primary>BEGIN</primary>
</indexterm>
<refmeta>
<refentrytitle>BEGIN</refentrytitle>
<manvolnum>7</manvolnum>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>BEGIN</refname>
<refpurpose>start a transaction block</refpurpose>
</refnamediv>
<refsynopsisdiv>
<synopsis>
BEGIN [ WORK | TRANSACTION ] [ <replaceable class="parameter">transaction_mode</replaceable> [, ...] ]
<phrase>where <replaceable class="parameter">transaction_mode</replaceable> is one of:</phrase>
ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
READ WRITE | READ ONLY
[ NOT ] DEFERRABLE
</synopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
<command>BEGIN</command> initiates a transaction block, that is,
all statements after a <command>BEGIN</command> command will be
executed in a single transaction until an explicit <link
linkend="sql-commit"><command>COMMIT</command></link> or <link
linkend="sql-rollback"><command>ROLLBACK</command></link> is given.
By default (without <command>BEGIN</command>),
<productname>PostgreSQL</productname> executes
transactions in <quote>autocommit</quote> mode, that is, each
statement is executed in its own transaction and a commit is
implicitly performed at the end of the statement (if execution was
successful, otherwise a rollback is done).
</para>
<para>
Statements are executed more quickly in a transaction block, because
transaction start/commit requires significant CPU and disk
activity. Execution of multiple statements inside a transaction is
also useful to ensure consistency when making several related changes:
other sessions will be unable to see the intermediate states
wherein not all the related updates have been done.
</para>
<para>
If the isolation level, read/write mode, or deferrable mode is specified, the new
transaction has those characteristics, as if
<link linkend="sql-set-transaction"><command>SET TRANSACTION</command></link>
was executed.
</para>
</refsect1>
<refsect1>
<title>Parameters</title>
<variablelist>
<varlistentry>
<term><literal>WORK</literal></term>
<term><literal>TRANSACTION</literal></term>
<listitem>
<para>
Optional key words. They have no effect.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
Refer to <xref linkend="sql-set-transaction"/> for information on the meaning
of the other parameters to this statement.
</para>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
<link linkend="sql-start-transaction"><command>START TRANSACTION</command></link> has the same functionality
as <command>BEGIN</command>.
</para>
<para>
Use <link linkend="sql-commit"><command>COMMIT</command></link> or
<link linkend="sql-rollback"><command>ROLLBACK</command></link>
to terminate a transaction block.
</para>
<para>
Issuing <command>BEGIN</command> when already inside a transaction block will
provoke a warning message. The state of the transaction is not affected.
To nest transactions within a transaction block, use savepoints
(see <xref linkend="sql-savepoint"/>).
</para>
<para>
For reasons of backwards compatibility, the commas between successive
<replaceable class="parameter">transaction_modes</replaceable> can be
omitted.
</para>
</refsect1>
<refsect1>
<title>Examples</title>
<para>
To begin a transaction block:
<programlisting>
BEGIN;
</programlisting></para>
</refsect1>
<refsect1>
<title>Compatibility</title>
<para>
<command>BEGIN</command> is a <productname>PostgreSQL</productname>
language extension. It is equivalent to the SQL-standard command
<link linkend="sql-start-transaction"><command>START TRANSACTION</command></link>, whose reference page
contains additional compatibility information.
</para>
<para>
The <literal>DEFERRABLE</literal>
<replaceable class="parameter">transaction_mode</replaceable>
is a <productname>PostgreSQL</productname> language extension.
</para>
<para>
Incidentally, the <literal>BEGIN</literal> key word is used for a
different purpose in embedded SQL. You are advised to be careful
about the transaction semantics when porting database applications.
</para>
</refsect1>
<refsect1>
<title>See Also</title>
<simplelist type="inline">
<member><xref linkend="sql-commit"/></member>
<member><xref linkend="sql-rollback"/></member>
<member><xref linkend="sql-start-transaction"/></member>
<member><xref linkend="sql-savepoint"/></member>
</simplelist>
</refsect1>
</refentry>