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Revise SELECT reference page for outer joins, subselect in FROM,
ISO-compliant UNION/INTERSECT/EXCEPT. Revise discussion of rule rewriter to reflect new subselect-in-FROM implementation of views. Miscellaneous other cleanups.
This commit is contained in:
Tom Lane
parent
c4e3b0c3aa
commit
1045304a3b
@ -1,5 +1,5 @@
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<!--
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$Header: /cvsroot/pgsql/doc/src/sgml/ref/select.sgml,v 1.34 2000/12/08 20:11:11 momjian Exp $
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$Header: /cvsroot/pgsql/doc/src/sgml/ref/select.sgml,v 1.35 2000/12/12 05:07:59 tgl Exp $
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Postgres documentation
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-->
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@ -19,25 +19,36 @@ Postgres documentation
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</refpurpose></refnamediv>
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<refsynopsisdiv>
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<refsynopsisdivinfo>
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<date>1999-07-20</date>
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<date>2000-12-11</date>
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</refsynopsisdivinfo>
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<synopsis>
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SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replaceable> [, ...] ) ] ]
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<replaceable class="PARAMETER">expression</replaceable> [ AS <replaceable class="PARAMETER">name</replaceable> ] [, ...]
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* | <replaceable class="PARAMETER">expression</replaceable> [ AS <replaceable class="PARAMETER">output_name</replaceable> ] [, ...]
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[ INTO [ TEMPORARY | TEMP ] [ TABLE ] <replaceable class="PARAMETER">new_table</replaceable> ]
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[ FROM [ ONLY ]<replaceable class="PARAMETER">table</replaceable> [ <replaceable class="PARAMETER">alias</replaceable> ] [, ...] ]
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[ FROM <replaceable class="PARAMETER">from_item</replaceable> [, ...] ]
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[ WHERE <replaceable class="PARAMETER">condition</replaceable> ]
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[ GROUP BY <replaceable class="PARAMETER">column</replaceable> [, ...] ]
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[ GROUP BY <replaceable class="PARAMETER">expression</replaceable> [, ...] ]
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[ HAVING <replaceable class="PARAMETER">condition</replaceable> [, ...] ]
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[ { UNION [ ALL ] | INTERSECT | EXCEPT } <replaceable class="PARAMETER">select</replaceable> ]
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[ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC | USING <replaceable class="PARAMETER">operator</replaceable> ] [, ...] ]
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[ { UNION | INTERSECT | EXCEPT [ ALL ] } <replaceable class="PARAMETER">select</replaceable> ]
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[ ORDER BY <replaceable class="PARAMETER">expression</replaceable> [ ASC | DESC | USING <replaceable class="PARAMETER">operator</replaceable> ] [, ...] ]
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[ FOR UPDATE [ OF <replaceable class="PARAMETER">class_name</replaceable> [, ...] ] ]
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[ LIMIT { <replaceable class="PARAMETER">count</replaceable> | ALL } [ { OFFSET | , } <replaceable class="PARAMETER">start</replaceable> ]]
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where <replaceable class="PARAMETER">from_item</replaceable> can be:
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[ ONLY ] <replaceable class="PARAMETER">table_name</replaceable> [ * ]
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[ [ AS ] <replaceable class="PARAMETER">alias</replaceable> [ ( <replaceable class="PARAMETER">column_alias_list</replaceable> ) ] ]
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|
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( <replaceable class="PARAMETER">select</replaceable> )
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[ AS ] <replaceable class="PARAMETER">alias</replaceable> [ ( <replaceable class="PARAMETER">column_alias_list</replaceable> ) ]
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|
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<replaceable class="PARAMETER">from_item</replaceable> [ NATURAL ] <replaceable class="PARAMETER">join_type</replaceable> <replaceable class="PARAMETER">from_item</replaceable>
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[ ON <replaceable class="PARAMETER">join_condition</replaceable> | USING ( <replaceable class="PARAMETER">join_column_list</replaceable> ) ]
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</synopsis>
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<refsect2 id="R2-SQL-SELECT-1">
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<refsect2info>
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<date>2000-03-15</date>
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<date>2000-12-11</date>
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</refsect2info>
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<title>
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Inputs
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@ -55,14 +66,14 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replac
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</varlistentry>
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<varlistentry>
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<term><replaceable class="PARAMETER">name</replaceable></term>
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<term><replaceable class="PARAMETER">output_name</replaceable></term>
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<listitem>
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<para>
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Specifies another name for a column or an expression using
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Specifies another name for an output column using
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the AS clause. This name is primarily used to label the column
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for display. It can also be used to refer to the column's value in
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ORDER BY and GROUP BY clauses. But the
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<replaceable class="PARAMETER">name</replaceable>
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<replaceable class="PARAMETER">output_name</replaceable>
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cannot be used in the WHERE or HAVING clauses; write out the
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expression instead.
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</para>
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@ -75,7 +86,7 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replac
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<listitem>
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<para>
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If TEMPORARY or TEMP is specified,
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the table is created unique to this session, and is
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the output table is created unique to this session, and is
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automatically dropped on session exit.
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</para>
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</listitem>
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@ -87,7 +98,7 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replac
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<para>
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If the INTO TABLE clause is specified, the result of the
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query will be stored in a new table with the indicated
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name.
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name, rather than being returned to the client.
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The target table (<replaceable class="PARAMETER">new_table</replaceable>) will
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be created automatically and must not exist before this command.
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Refer to <command>SELECT INTO</command> for more information.
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@ -95,7 +106,7 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replac
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<note>
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<para>
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The <command>CREATE TABLE AS</command> statement will also
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create a new table from a select query.
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create a new table from a SELECT query.
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</para>
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</note>
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</para>
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@ -103,22 +114,10 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replac
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</varlistentry>
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<varlistentry>
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<term><replaceable class="PARAMETER">table</replaceable></term>
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<term><replaceable class="PARAMETER">from_item</replaceable></term>
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<listitem>
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<para>
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The name of an existing table referenced by the FROM clause.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="PARAMETER">alias</replaceable></term>
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<listitem>
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<para>
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An alternate name for the preceding
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<replaceable class="PARAMETER">table</replaceable>.
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It is used for brevity or to eliminate ambiguity for joins
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within a single table.
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A table reference, sub-SELECT, or JOIN clause. See below for details.
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</para>
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</listitem>
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</varlistentry>
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@ -128,16 +127,7 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replac
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<listitem>
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<para>
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A boolean expression giving a result of true or false.
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See the WHERE clause.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="PARAMETER">column</replaceable></term>
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<listitem>
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<para>
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The name of a table's column.
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See the WHERE and HAVING clause descriptions below.
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</para>
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</listitem>
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</varlistentry>
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@ -146,14 +136,102 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replac
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<term><replaceable class="PARAMETER">select</replaceable></term>
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<listitem>
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<para>
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A select statement with all features except the ORDER BY and
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LIMIT clauses.
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A select statement with all features except the ORDER BY, FOR UPDATE,
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and LIMIT clauses (even those can be used when the select is
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parenthesized).
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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<para>
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FROM items can contain:
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<variablelist>
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<varlistentry>
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<term><replaceable class="PARAMETER">table_name</replaceable></term>
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<listitem>
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<para>
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The name of an existing table or view. If ONLY is specified, only that
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table is scanned. If ONLY is not specified, the table and all its
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descendant tables (if any) are scanned. * can be appended to the
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table name to indicate that descendant tables are to be scanned,
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but as of <Productname>Postgres</Productname> 7.1 this is the default
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behavior. (In releases before 7.1, ONLY was the default behavior.)
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="PARAMETER">alias</replaceable></term>
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<listitem>
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<para>
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A substitute name for the preceding
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<replaceable class="PARAMETER">table_name</replaceable>.
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An alias is used for brevity or to eliminate ambiguity for self-joins
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(where the same table is scanned multiple times). If an alias is
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written, a column alias list can also be written to provide
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substitute names for one or more columns of the table.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="PARAMETER">select</replaceable></term>
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<listitem>
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<para>
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A sub-SELECT can appear in the FROM clause. This acts as though
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its output were created as a temporary table for the duration of
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this single SELECT command. Note that the sub-SELECT must be
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surrounded by parentheses, and an alias <emphasis>must</emphasis>
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be provided for it.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="PARAMETER">join_type</replaceable></term>
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<listitem>
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<para>
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One of
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<command>[ INNER ] JOIN</command>,
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<command>LEFT [ OUTER ] JOIN</command>,
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<command>RIGHT [ OUTER ] JOIN</command>,
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<command>FULL [ OUTER ] JOIN</command>, or
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<command>CROSS JOIN</command>.
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For INNER and OUTER join types, exactly one of NATURAL,
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ON <replaceable class="PARAMETER">join_condition</replaceable>, or
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USING ( <replaceable class="PARAMETER">join_column_list</replaceable> )
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must appear. For CROSS JOIN, none of these items may appear.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="PARAMETER">join_condition</replaceable></term>
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<listitem>
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<para>
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A qualification condition. This is similar to the WHERE condition
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except that it only applies to the two from_items being joined in
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this JOIN clause.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="PARAMETER">join_column_list</replaceable></term>
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<listitem>
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<para>
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A USING column list ( a, b, ... ) is shorthand for the ON condition
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left_table.a = right_table.a AND left_table.b = right_table.b ...
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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</refsect2>
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<refsect2 id="R2-SQL-SELECT-2">
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@ -192,7 +270,7 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replac
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<refsect1 id="R1-SQL-SELECT-1">
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<refsect1info>
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<date>2000-03-15</date>
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<date>2000-12-11</date>
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</refsect1info>
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<title>
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Description
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@ -203,12 +281,15 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replac
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if WHERE is omitted, all rows are candidates.
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(See <xref linkend="sql-where" endterm="sql-where-title">.)
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</para>
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<para>
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<command>ONLY</command> will eliminate rows from subclasses of the table.
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This was previously the default result, and getting subclasses was
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obtained by appending <command>*</command> to the table name.
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The old behaviour is available via the command
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<command>SET SQL_Inheritance TO OFF;</command>
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Actually, the returned rows are not directly the rows produced by the
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FROM/WHERE/GROUP BY/HAVING clauses; rather, the output rows are formed
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by computing the SELECT output expressions for each selected row.
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<command>*</command> can be written in the output list as a shorthand
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for all the columns of the selected rows. Also, one can write
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<replaceable class="PARAMETER">table_name</replaceable><command>.*</command>
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as a shorthand for the columns coming from just that table.
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</para>
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<para>
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@ -257,19 +338,28 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replac
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</para>
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<para>
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The UNION operator allows the result to be the collection of rows
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SELECT queries can be combined using UNION, INTERSECT, and EXCEPT
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operators. Use parentheses if necessary to determine the ordering
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of these operators.
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</para>
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<para>
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The UNION operator computes the collection of rows
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returned by the queries involved.
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Duplicate rows are eliminated unless ALL is specified.
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(See <xref linkend="sql-union" endterm="sql-union-title">.)
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</para>
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<para>
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The INTERSECT operator gives you the rows that are common to both queries.
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The INTERSECT operator computes the rows that are common to both queries.
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Duplicate rows are eliminated unless ALL is specified.
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(See <xref linkend="sql-intersect" endterm="sql-intersect-title">.)
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</para>
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<para>
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The EXCEPT operator gives you the rows returned by the first query but
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The EXCEPT operator computes the rows returned by the first query but
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not the second query.
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Duplicate rows are eliminated unless ALL is specified.
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(See <xref linkend="sql-except" endterm="sql-except-title">.)
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</para>
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@ -289,6 +379,95 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replac
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(See the <command>GRANT</command>/<command>REVOKE</command> statements).
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</para>
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<refsect2 id="SQL-FROM">
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<refsect2info>
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<date>2000-12-11</date>
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</refsect2info>
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<title id="sql-from-title">
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FROM Clause
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</title>
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<para>
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The FROM clause specifies one or more source tables for the SELECT.
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If multiple sources are specified, the result is conceptually the
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Cartesian product of all the rows in all the sources --- but usually
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qualification conditions are added to restrict the returned rows to
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a small subset of the Cartesian product.
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</para>
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<para>
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When a FROM item is a simple table name, it implicitly includes rows
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from subclasses (inheritance children) of the table.
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<command>ONLY</command> will
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suppress rows from subclasses of the table. Before
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<Productname>Postgres</Productname> 7.1,
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this was the default result, and adding subclasses was done
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by appending <command>*</command> to the table name.
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This old behaviour is available via the command
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<command>SET SQL_Inheritance TO OFF;</command>
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</para>
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<para>
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A FROM item can also be a parenthesized sub-SELECT (note that an
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alias clause is required for a sub-SELECT!). This is an extremely
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handy feature since it's the only way to get multiple levels of
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grouping, aggregation, or sorting in a single query.
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</para>
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<para>
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Finally, a FROM item can be a JOIN clause, which combines two simpler
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FROM items. (Use parentheses if necessary to determine the order
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of nesting.)
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</para>
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<para>
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A CROSS JOIN or INNER JOIN is a simple Cartesian product,
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the same as you get from listing the two items at the top level of FROM.
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CROSS JOIN is equivalent to INNER JOIN ON (TRUE), that is, no rows are
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removed by qualification. These join types are just a notational
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convenience, since they do nothing you couldn't do with plain FROM and
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WHERE.
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</para>
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<para>
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LEFT OUTER JOIN returns all rows in the qualified Cartesian product
|
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(ie, all combined rows that pass its ON condition), plus one copy of each
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row in the left-hand table for which there was no right-hand row that
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passed the ON condition. This left-hand row is extended to the full
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width of the joined table by inserting NULLs for the right-hand columns.
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Note that only the JOIN's own ON or USING condition is considered while
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deciding which rows have matches. Outer ON or WHERE conditions are
|
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applied afterwards.
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</para>
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<para>
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Conversely, RIGHT OUTER JOIN returns all the joined rows, plus one row
|
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for each unmatched right-hand row (extended with nulls on the left).
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This is just a notational
|
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convenience, since you could convert it to a LEFT OUTER JOIN by switching
|
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the left and right inputs.
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</para>
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<para>
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FULL OUTER JOIN returns all the joined rows, plus one row for each
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unmatched left-hand row (extended with nulls on the right), plus one row
|
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for each unmatched right-hand row (extended with nulls on the left).
|
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</para>
|
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|
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<para>
|
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For all the JOIN types except CROSS JOIN, you must write exactly one of
|
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ON <replaceable class="PARAMETER">join_condition</replaceable>,
|
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USING ( <replaceable class="PARAMETER">join_column_list</replaceable> ),
|
||||
or NATURAL. ON is the most general case: you can write any qualification
|
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expression involving the two tables to be joined.
|
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A USING column list ( a, b, ... ) is shorthand for the ON condition
|
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left_table.a = right_table.a AND left_table.b = right_table.b ...
|
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Also, USING implies that only one of each pair of equivalent columns will
|
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be included in the JOIN output, not both. NATURAL is shorthand for
|
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a USING list that mentions all similarly-named columns in the tables.
|
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</para>
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</refsect2>
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<refsect2 id="SQL-WHERE">
|
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<refsect2info>
|
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<date>2000-03-15</date>
|
||||
@ -340,7 +519,7 @@ WHERE <replaceable class="PARAMETER">boolean_expr</replaceable>
|
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GROUP BY specifies a grouped table derived by the application
|
||||
of this clause:
|
||||
<synopsis>
|
||||
GROUP BY <replaceable class="PARAMETER">column</replaceable> [, ...]
|
||||
GROUP BY <replaceable class="PARAMETER">expression</replaceable> [, ...]
|
||||
</synopsis>
|
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</para>
|
||||
|
||||
@ -357,9 +536,10 @@ GROUP BY <replaceable class="PARAMETER">column</replaceable> [, ...]
|
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</para>
|
||||
|
||||
<para>
|
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An item in GROUP BY can also be the name or ordinal number of an output
|
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column (SELECT expression), or it can be an arbitrary expression formed
|
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from input-column values. In case of ambiguity, a GROUP BY name will
|
||||
A GROUP BY item can be an input column name, or the name or ordinal
|
||||
number of an output column (SELECT expression), or it can be an arbitrary
|
||||
expression formed from input-column values. In case of ambiguity, a GROUP
|
||||
BY name will
|
||||
be interpreted as an input-column name rather than an output column name.
|
||||
</para>
|
||||
</refsect2>
|
||||
@ -375,17 +555,17 @@ GROUP BY <replaceable class="PARAMETER">column</replaceable> [, ...]
|
||||
The optional HAVING condition has the general form:
|
||||
|
||||
<synopsis>
|
||||
HAVING <replaceable class="PARAMETER">cond_expr</replaceable>
|
||||
HAVING <replaceable class="PARAMETER">boolean_expr</replaceable>
|
||||
</synopsis>
|
||||
|
||||
where <replaceable class="PARAMETER">cond_expr</replaceable> is the same
|
||||
where <replaceable class="PARAMETER">boolean_expr</replaceable> is the same
|
||||
as specified for the WHERE clause.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
HAVING specifies a grouped table derived by the elimination
|
||||
of group rows that do not satisfy the
|
||||
<replaceable class="PARAMETER">cond_expr</replaceable>.
|
||||
<replaceable class="PARAMETER">boolean_expr</replaceable>.
|
||||
HAVING is different from WHERE:
|
||||
WHERE filters individual rows before application of GROUP BY,
|
||||
while HAVING filters group rows created by GROUP BY.
|
||||
@ -393,7 +573,7 @@ HAVING <replaceable class="PARAMETER">cond_expr</replaceable>
|
||||
|
||||
<para>
|
||||
Each column referenced in
|
||||
<replaceable class="PARAMETER">cond_expr</replaceable> shall unambiguously
|
||||
<replaceable class="PARAMETER">boolean_expr</replaceable> shall unambiguously
|
||||
reference a grouping column, unless the reference appears within an
|
||||
aggregate function.
|
||||
</para>
|
||||
@ -408,15 +588,17 @@ HAVING <replaceable class="PARAMETER">cond_expr</replaceable>
|
||||
</title>
|
||||
<para>
|
||||
<synopsis>
|
||||
ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC ] [, ...]
|
||||
ORDER BY <replaceable class="PARAMETER">expression</replaceable> [ ASC | DESC | USING <replaceable class="PARAMETER">operator</replaceable> ] [, ...]
|
||||
</synopsis></para>
|
||||
|
||||
<para>
|
||||
<replaceable class="PARAMETER">column</replaceable> can be either a
|
||||
result column name or an ordinal number.
|
||||
An ORDER BY item can be the name or ordinal
|
||||
number of an output column (SELECT expression), or it can be an arbitrary
|
||||
expression formed from input-column values. In case of ambiguity, an
|
||||
ORDER BY name will be interpreted as an output-column name.
|
||||
</para>
|
||||
<para>
|
||||
The ordinal numbers refers to the ordinal (left-to-right) position
|
||||
The ordinal number refers to the ordinal (left-to-right) position
|
||||
of the result column. This feature makes it possible to define an ordering
|
||||
on the basis of a column that does not have a proper name.
|
||||
This is never absolutely necessary because it is always possible
|
||||
@ -447,13 +629,13 @@ SELECT name FROM distributors ORDER BY code;
|
||||
or ASC (ascending) after each column name in the ORDER BY clause.
|
||||
If not specified, ASC is assumed by default. Alternatively, a
|
||||
specific ordering operator name may be specified. ASC is equivalent
|
||||
to USING '<' and DESC is equivalent to USING '>'.
|
||||
to USING < and DESC is equivalent to USING >.
|
||||
</para>
|
||||
</refsect2>
|
||||
|
||||
<refsect2 id="SQL-UNION">
|
||||
<refsect2info>
|
||||
<date>1998-09-24</date>
|
||||
<date>2000-12-11</date>
|
||||
</refsect2info>
|
||||
<title id="sql-union-title">
|
||||
UNION Clause
|
||||
@ -461,16 +643,21 @@ SELECT name FROM distributors ORDER BY code;
|
||||
<para>
|
||||
<synopsis>
|
||||
<replaceable class="PARAMETER">table_query</replaceable> UNION [ ALL ] <replaceable class="PARAMETER">table_query</replaceable>
|
||||
[ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC ] [, ...] ]
|
||||
[ ORDER BY <replaceable class="PARAMETER">expression</replaceable> [ ASC | DESC | USING <replaceable class="PARAMETER">operator</replaceable> ] [, ...] ]
|
||||
[ LIMIT { <replaceable class="PARAMETER">count</replaceable> | ALL } [ { OFFSET | , } <replaceable class="PARAMETER">start</replaceable> ]]
|
||||
</synopsis>
|
||||
|
||||
where
|
||||
<replaceable class="PARAMETER">table_query</replaceable>
|
||||
specifies any select expression without an ORDER BY or LIMIT clause.
|
||||
specifies any select expression without an ORDER BY, FOR UPDATE,
|
||||
or LIMIT clause. (ORDER BY and LIMIT can be attached to a sub-expression
|
||||
if it is enclosed in parentheses. Without parentheses, these clauses
|
||||
will be taken to apply to the result of the UNION, not to its right-hand
|
||||
input expression.)
|
||||
</para>
|
||||
|
||||
<para>
|
||||
The UNION operator allows the result to be the collection of rows
|
||||
The UNION operator computes the collection (set union) of the rows
|
||||
returned by the queries involved.
|
||||
The two SELECTs that represent the direct operands of the UNION must
|
||||
produce the same number of columns, and corresponding columns must be
|
||||
@ -478,79 +665,98 @@ SELECT name FROM distributors ORDER BY code;
|
||||
</para>
|
||||
|
||||
<para>
|
||||
By default, the result of UNION does not contain any duplicate rows
|
||||
unless the ALL clause is specified.
|
||||
The result of UNION does not contain any duplicate rows
|
||||
unless the ALL option is specified. ALL prevents elimination of
|
||||
duplicates.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
Multiple UNION operators in the same SELECT statement are
|
||||
evaluated left to right.
|
||||
Note that the ALL keyword is not global in nature, being
|
||||
applied only for the current pair of table results.
|
||||
evaluated left to right, unless otherwise indicated by parentheses.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
Currently, FOR UPDATE may not be specified either for a UNION result
|
||||
or for the inputs of a UNION.
|
||||
</para>
|
||||
|
||||
</refsect2>
|
||||
|
||||
<refsect2 id="SQL-INTERSECT">
|
||||
<refsect2info>
|
||||
<date>1998-09-24</date>
|
||||
<date>2000-12-11</date>
|
||||
</refsect2info>
|
||||
<title id="sql-intersect-title">
|
||||
INTERSECT Clause
|
||||
</title>
|
||||
<para>
|
||||
<synopsis>
|
||||
<replaceable class="PARAMETER">table_query</replaceable> INTERSECT <replaceable class="PARAMETER">table_query</replaceable>
|
||||
[ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC ] [, ...] ]
|
||||
<replaceable class="PARAMETER">table_query</replaceable> INTERSECT [ ALL ] <replaceable class="PARAMETER">table_query</replaceable>
|
||||
[ ORDER BY <replaceable class="PARAMETER">expression</replaceable> [ ASC | DESC | USING <replaceable class="PARAMETER">operator</replaceable> ] [, ...] ]
|
||||
[ LIMIT { <replaceable class="PARAMETER">count</replaceable> | ALL } [ { OFFSET | , } <replaceable class="PARAMETER">start</replaceable> ]]
|
||||
</synopsis>
|
||||
|
||||
where
|
||||
<replaceable class="PARAMETER">table_query</replaceable>
|
||||
specifies any select expression without an ORDER BY or LIMIT clause.
|
||||
specifies any select expression without an ORDER BY, FOR UPDATE,
|
||||
or LIMIT clause.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
The INTERSECT operator gives you the rows that are common to both queries.
|
||||
The two SELECTs that represent the direct operands of the INTERSECT must
|
||||
produce the same number of columns, and corresponding columns must be
|
||||
of compatible data types.
|
||||
INTERSECT is similar to UNION, except that it produces only rows that
|
||||
appear in both query outputs, rather than rows that appear in either.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
The result of INTERSECT does not contain any duplicate rows
|
||||
unless the ALL option is specified. With ALL, a row that has
|
||||
m duplicates in L and n duplicates in R will appear min(m,n) times.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
Multiple INTERSECT operators in the same SELECT statement are
|
||||
evaluated left to right, unless parentheses dictate otherwise.
|
||||
INTERSECT binds more tightly than UNION --- that is,
|
||||
A UNION B INTERSECT C will be read as
|
||||
A UNION (B INTERSECT C) unless otherwise specified by parentheses.
|
||||
</para>
|
||||
</refsect2>
|
||||
|
||||
<refsect2 id="SQL-EXCEPT">
|
||||
<refsect2info>
|
||||
<date>1998-09-24</date>
|
||||
<date>2000-12-11</date>
|
||||
</refsect2info>
|
||||
<title id="sql-except-title">
|
||||
EXCEPT Clause
|
||||
</title>
|
||||
<para>
|
||||
<synopsis>
|
||||
<replaceable class="PARAMETER">table_query</replaceable> EXCEPT <replaceable class="PARAMETER">table_query</replaceable>
|
||||
[ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC ] [, ...] ]
|
||||
<replaceable class="PARAMETER">table_query</replaceable> EXCEPT [ ALL ] <replaceable class="PARAMETER">table_query</replaceable>
|
||||
[ ORDER BY <replaceable class="PARAMETER">expression</replaceable> [ ASC | DESC | USING <replaceable class="PARAMETER">operator</replaceable> ] [, ...] ]
|
||||
[ LIMIT { <replaceable class="PARAMETER">count</replaceable> | ALL } [ { OFFSET | , } <replaceable class="PARAMETER">start</replaceable> ]]
|
||||
</synopsis>
|
||||
|
||||
where
|
||||
<replaceable class="PARAMETER">table_query</replaceable>
|
||||
specifies any select expression without an ORDER BY or LIMIT clause.
|
||||
specifies any select expression without an ORDER BY, FOR UPDATE,
|
||||
or LIMIT clause.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
The EXCEPT operator gives you the rows returned by the first query but
|
||||
not the second query.
|
||||
The two SELECTs that represent the direct operands of the EXCEPT must
|
||||
produce the same number of columns, and corresponding columns must be
|
||||
of compatible data types.
|
||||
EXCEPT is similar to UNION, except that it produces only rows that
|
||||
appear in the left query's output but not in the right query's output.
|
||||
</para>
|
||||
|
||||
|
||||
<para>
|
||||
The result of EXCEPT does not contain any duplicate rows
|
||||
unless the ALL option is specified. With ALL, a row that has
|
||||
m duplicates in L and n duplicates in R will appear max(m-n,0) times.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
Multiple EXCEPT operators in the same SELECT statement are
|
||||
evaluated left to right, unless parentheses dictate otherwise.
|
||||
EXCEPT binds at the same level as UNION.
|
||||
</para>
|
||||
</refsect2>
|
||||
|
||||
@ -758,7 +964,22 @@ SELECT actors.name
|
||||
<para>
|
||||
<productname>Postgres</productname> allows one to omit
|
||||
the <command>FROM</command> clause from a query. This feature
|
||||
was retained from the original PostQuel query language:
|
||||
was retained from the original PostQuel query language. It has
|
||||
a straightforward use to compute the results of simple constant
|
||||
expressions:
|
||||
|
||||
<programlisting>
|
||||
SELECT 2+2;
|
||||
|
||||
?column?
|
||||
----------
|
||||
4
|
||||
</programlisting>
|
||||
|
||||
Some other DBMSes cannot do this except by introducing a dummy one-row
|
||||
table to do the select from. A less obvious use is to abbreviate a
|
||||
normal select from one or more tables:
|
||||
|
||||
<programlisting>
|
||||
SELECT distributors.* WHERE name = 'Westward';
|
||||
|
||||
@ -766,6 +987,26 @@ SELECT distributors.* WHERE name = 'Westward';
|
||||
-----+----------
|
||||
108 | Westward
|
||||
</programlisting>
|
||||
|
||||
This works because an implicit FROM item is added for each table that is
|
||||
referenced in the query but not mentioned in FROM. While this is a convenient
|
||||
shorthand, it's easy to misuse. For example, the query
|
||||
<programlisting>
|
||||
SELECT distributors.* FROM distributors d;
|
||||
</programlisting>
|
||||
is probably a mistake; most likely the user meant
|
||||
<programlisting>
|
||||
SELECT d.* FROM distributors d;
|
||||
</programlisting>
|
||||
rather than the unconstrained join
|
||||
<programlisting>
|
||||
SELECT distributors.* FROM distributors d, distributors distributors;
|
||||
</programlisting>
|
||||
that he will actually get. To help detect this sort of mistake,
|
||||
<Productname>Postgres</Productname> 7.1
|
||||
and later will warn if the implicit-FROM feature is used in a query that also
|
||||
contains an explicit FROM clause.
|
||||
|
||||
</para>
|
||||
</refsect2>
|
||||
|
||||
@ -791,9 +1032,9 @@ SELECT distributors.* WHERE name = 'Westward';
|
||||
is just noise and can be
|
||||
omitted without affecting the meaning.
|
||||
The <productname>Postgres</productname> parser requires this keyword when
|
||||
renaming columns because the type extensibility features lead to
|
||||
renaming output columns because the type extensibility features lead to
|
||||
parsing ambiguities
|
||||
in this context.</para>
|
||||
in this context. "AS" is optional in FROM items, however.</para>
|
||||
|
||||
<para>
|
||||
The DISTINCT ON phrase is not part of <acronym>SQL92</acronym>.
|
||||
@ -818,11 +1059,11 @@ SELECT distributors.* WHERE name = 'Westward';
|
||||
<date>1998-09-24</date>
|
||||
</refsect3info>
|
||||
<title>
|
||||
UNION Clause
|
||||
UNION/INTERSECT/EXCEPT Clause
|
||||
</title>
|
||||
<para>
|
||||
The <acronym>SQL92</acronym> syntax for UNION allows an
|
||||
additional CORRESPONDING BY clause:
|
||||
The <acronym>SQL92</acronym> syntax for UNION/INTERSECT/EXCEPT allows an
|
||||
additional CORRESPONDING BY option:
|
||||
<synopsis>
|
||||
<replaceable class="PARAMETER">table_query</replaceable> UNION [ALL]
|
||||
[CORRESPONDING [BY (<replaceable class="PARAMETER">column</replaceable> [,...])]]
|
||||
|
||||
@ -1,5 +1,5 @@
|
||||
<!--
|
||||
$Header: /cvsroot/pgsql/doc/src/sgml/ref/select_into.sgml,v 1.6 2000/07/21 18:51:24 momjian Exp $
|
||||
$Header: /cvsroot/pgsql/doc/src/sgml/ref/select_into.sgml,v 1.7 2000/12/12 05:07:59 tgl Exp $
|
||||
Postgres documentation
|
||||
-->
|
||||
|
||||
@ -19,20 +19,31 @@ Postgres documentation
|
||||
</refpurpose></refnamediv>
|
||||
<refsynopsisdiv>
|
||||
<refsynopsisdivinfo>
|
||||
<date>1999-07-20</date>
|
||||
<date>2000-12-11</date>
|
||||
</refsynopsisdivinfo>
|
||||
<synopsis>
|
||||
SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replaceable> [, ...] ) ] ]
|
||||
<replaceable class="PARAMETER">expression</replaceable> [ AS <replaceable class="PARAMETER">name</replaceable> ] [, ...]
|
||||
[ INTO [ TEMPORARY | TEMP ] [ TABLE ] <replaceable class="PARAMETER">new_table</replaceable> ]
|
||||
[ FROM <replaceable class="PARAMETER">table</replaceable> [ <replaceable class="PARAMETER">alias</replaceable> ] [, ...] ]
|
||||
* | <replaceable class="PARAMETER">expression</replaceable> [ AS <replaceable class="PARAMETER">output_name</replaceable> ] [, ...]
|
||||
INTO [ TEMPORARY | TEMP ] [ TABLE ] <replaceable class="PARAMETER">new_table</replaceable>
|
||||
[ FROM <replaceable class="PARAMETER">from_item</replaceable> [, ...] ]
|
||||
[ WHERE <replaceable class="PARAMETER">condition</replaceable> ]
|
||||
[ GROUP BY <replaceable class="PARAMETER">column</replaceable> [, ...] ]
|
||||
[ GROUP BY <replaceable class="PARAMETER">expression</replaceable> [, ...] ]
|
||||
[ HAVING <replaceable class="PARAMETER">condition</replaceable> [, ...] ]
|
||||
[ { UNION [ ALL ] | INTERSECT | EXCEPT } <replaceable class="PARAMETER">select</replaceable> ]
|
||||
[ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC | USING <replaceable class="PARAMETER">operator</replaceable> ] [, ...] ]
|
||||
[ { UNION | INTERSECT | EXCEPT [ ALL ] } <replaceable class="PARAMETER">select</replaceable> ]
|
||||
[ ORDER BY <replaceable class="PARAMETER">expression</replaceable> [ ASC | DESC | USING <replaceable class="PARAMETER">operator</replaceable> ] [, ...] ]
|
||||
[ FOR UPDATE [ OF <replaceable class="PARAMETER">class_name</replaceable> [, ...] ] ]
|
||||
[ LIMIT { <replaceable class="PARAMETER">count</replaceable> | ALL } [ { OFFSET | , } <replaceable class="PARAMETER">start</replaceable> ] ]
|
||||
[ LIMIT { <replaceable class="PARAMETER">count</replaceable> | ALL } [ { OFFSET | , } <replaceable class="PARAMETER">start</replaceable> ]]
|
||||
|
||||
where <replaceable class="PARAMETER">from_item</replaceable> can be:
|
||||
|
||||
[ ONLY ] <replaceable class="PARAMETER">table_name</replaceable> [ * ]
|
||||
[ [ AS ] <replaceable class="PARAMETER">alias</replaceable> [ ( <replaceable class="PARAMETER">column_alias_list</replaceable> ) ] ]
|
||||
|
|
||||
( <replaceable class="PARAMETER">select</replaceable> )
|
||||
[ AS ] <replaceable class="PARAMETER">alias</replaceable> [ ( <replaceable class="PARAMETER">column_alias_list</replaceable> ) ]
|
||||
|
|
||||
<replaceable class="PARAMETER">from_item</replaceable> [ NATURAL ] <replaceable class="PARAMETER">join_type</replaceable> <replaceable class="PARAMETER">from_item</replaceable>
|
||||
[ ON <replaceable class="PARAMETER">join_condition</replaceable> | USING ( <replaceable class="PARAMETER">join_column_list</replaceable> ) ]
|
||||
</synopsis>
|
||||
|
||||
<refsect2 id="R2-SQL-SELECTINTO-1">
|
||||
|
||||
File diff suppressed because it is too large
Load Diff
@ -1,5 +1,5 @@
|
||||
<!--
|
||||
$Header: /cvsroot/pgsql/doc/src/sgml/sql.sgml,v 1.14 2000/12/09 22:59:25 momjian Exp $
|
||||
$Header: /cvsroot/pgsql/doc/src/sgml/sql.sgml,v 1.15 2000/12/12 05:07:58 tgl Exp $
|
||||
-->
|
||||
|
||||
<chapter id="sql">
|
||||
@ -854,17 +854,17 @@ A < B + 3.
|
||||
used to retrieve data. The syntax is:
|
||||
|
||||
<synopsis>
|
||||
SELECT [ALL|DISTINCT]
|
||||
{ * | <replaceable class="parameter">expr_1</replaceable> [AS <replaceable class="parameter">c_alias_1</replaceable>] [, ...
|
||||
[, <replaceable class="parameter">expr_k</replaceable> [AS <replaceable class="parameter">c_alias_k</replaceable>]]]}
|
||||
FROM <replaceable class="parameter">table_name_1</replaceable> [<replaceable class="parameter">t_alias_1</replaceable>]
|
||||
[, ... [, <replaceable class="parameter">table_name_n</replaceable> [<replaceable class="parameter">t_alias_n</replaceable>]]]
|
||||
[WHERE <replaceable class="parameter">condition</replaceable>]
|
||||
[GROUP BY <replaceable class="parameter">name_of_attr_i</replaceable>
|
||||
[,... [, <replaceable class="parameter">name_of_attr_j</replaceable>]] [HAVING <replaceable class="parameter">condition</replaceable>]]
|
||||
[{UNION [ALL] | INTERSECT | EXCEPT} SELECT ...]
|
||||
[ORDER BY <replaceable class="parameter">name_of_attr_i</replaceable> [ASC|DESC]
|
||||
[, ... [, <replaceable class="parameter">name_of_attr_j</replaceable> [ASC|DESC]]]];
|
||||
SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replaceable> [, ...] ) ] ]
|
||||
* | <replaceable class="PARAMETER">expression</replaceable> [ AS <replaceable class="PARAMETER">output_name</replaceable> ] [, ...]
|
||||
[ INTO [ TEMPORARY | TEMP ] [ TABLE ] <replaceable class="PARAMETER">new_table</replaceable> ]
|
||||
[ FROM <replaceable class="PARAMETER">from_item</replaceable> [, ...] ]
|
||||
[ WHERE <replaceable class="PARAMETER">condition</replaceable> ]
|
||||
[ GROUP BY <replaceable class="PARAMETER">expression</replaceable> [, ...] ]
|
||||
[ HAVING <replaceable class="PARAMETER">condition</replaceable> [, ...] ]
|
||||
[ { UNION | INTERSECT | EXCEPT [ ALL ] } <replaceable class="PARAMETER">select</replaceable> ]
|
||||
[ ORDER BY <replaceable class="PARAMETER">expression</replaceable> [ ASC | DESC | USING <replaceable class="PARAMETER">operator</replaceable> ] [, ...] ]
|
||||
[ FOR UPDATE [ OF <replaceable class="PARAMETER">class_name</replaceable> [, ...] ] ]
|
||||
[ LIMIT { <replaceable class="PARAMETER">count</replaceable> | ALL } [ { OFFSET | , } <replaceable class="PARAMETER">start</replaceable> ]]
|
||||
</synopsis>
|
||||
</para>
|
||||
|
||||
@ -1037,11 +1037,13 @@ SELECT S.SNAME, P.PNAME
|
||||
<para>
|
||||
<acronym>SQL</acronym> provides aggregate operators
|
||||
(e.g. AVG, COUNT, SUM, MIN, MAX) that
|
||||
take the name of an attribute as an argument. The value of the
|
||||
aggregate operator is calculated over all values of the specified
|
||||
attribute (column) of the whole table. If groups are specified in the
|
||||
query the calculation is done only over the values of a group (see next
|
||||
section).
|
||||
take an expression as argument. The expression is evaluated at
|
||||
each row that satisfies the WHERE clause, and the aggregate operator
|
||||
is calculated over this set of input values. Normally, an aggregate
|
||||
delivers a single result for a whole SELECT statement. But if
|
||||
grouping is specified in the query, then a separate calculation is done
|
||||
over the rows of each group, and an aggregate result is delivered per
|
||||
group (see next section).
|
||||
|
||||
<example>
|
||||
<title id="aggregates-example">Aggregates</title>
|
||||
@ -1094,11 +1096,11 @@ SELECT COUNT(PNO)
|
||||
<para>
|
||||
<acronym>SQL</acronym> allows one to partition the tuples of a table
|
||||
into groups. Then the
|
||||
aggregate operators described above can be applied to the groups
|
||||
(i.e. the value of the aggregate operator is no longer calculated over
|
||||
aggregate operators described above can be applied to the groups ---
|
||||
i.e. the value of the aggregate operator is no longer calculated over
|
||||
all the values of the specified column but over all values of a
|
||||
group. Thus the aggregate operator is evaluated individually for every
|
||||
group.)
|
||||
group.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
@ -1179,19 +1181,26 @@ SELECT S.SNO, S.SNAME, COUNT(SE.PNO)
|
||||
|
||||
<para>
|
||||
In our example we got four groups and now we can apply the aggregate
|
||||
operator COUNT to every group leading to the total result of the query
|
||||
operator COUNT to every group leading to the final result of the query
|
||||
given above.
|
||||
</para>
|
||||
</example>
|
||||
</para>
|
||||
|
||||
<para>
|
||||
Note that for the result of a query using GROUP BY and aggregate
|
||||
operators to make sense the attributes grouped by must also appear in
|
||||
the target list. All further attributes not appearing in the GROUP
|
||||
BY clause can only be selected by using an aggregate function. On
|
||||
the other hand you can not use aggregate functions on attributes
|
||||
appearing in the GROUP BY clause.
|
||||
Note that for a query using GROUP BY and aggregate
|
||||
operators to make sense the target list can only refer directly to
|
||||
the attributes being grouped by. Other attributes may only be used
|
||||
inside the argument of an aggregate function. Otherwise there would
|
||||
not be a unique value to associate with the other attributes.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
Also observe that it makes no sense to ask for an aggregate of an
|
||||
aggregate, eg, AVG(MAX(sno)), because a SELECT only does one pass
|
||||
of grouping and aggregation. You can get a result of this kind by
|
||||
using a temporary table or a sub-SELECT in the FROM clause to
|
||||
do the first level of aggregation.
|
||||
</para>
|
||||
</sect3>
|
||||
|
||||
@ -1201,11 +1210,14 @@ SELECT S.SNO, S.SNAME, COUNT(SE.PNO)
|
||||
<para>
|
||||
The HAVING clause works much like the WHERE clause and is used to
|
||||
consider only those groups satisfying the qualification given in the
|
||||
HAVING clause. The expressions allowed in the HAVING clause must
|
||||
involve aggregate functions. Every expression using only plain
|
||||
attributes belongs to the WHERE clause. On the other hand every
|
||||
expression involving an aggregate function must be put to the HAVING
|
||||
clause.
|
||||
HAVING clause. Essentially, WHERE filters out unwanted input rows
|
||||
before grouping and aggregation are done, whereas HAVING filters out
|
||||
unwanted group rows post-GROUP. Therefore, WHERE cannot refer to the
|
||||
results of aggregate functions. On the other hand, there's no point
|
||||
in writing a HAVING condition that doesn't involve an aggregate
|
||||
function! If your condition doesn't involve aggregates, you might
|
||||
as well write it in WHERE, and thereby avoid the computation of
|
||||
aggregates for groups that you're just going to throw away anyway.
|
||||
|
||||
<example>
|
||||
<title id="having-example">Having</title>
|
||||
@ -1280,7 +1292,9 @@ SELECT *
|
||||
SELECT. For every tuple of the outer SELECT the inner SELECT has to be
|
||||
evaluated. After every evaluation we know the price of the tuple named
|
||||
'Screw' and we can check if the price of the actual tuple is
|
||||
greater.
|
||||
greater. (Actually, in this example the inner query need only be
|
||||
evaluated once, since it does not depend on the state of the outer
|
||||
query.)
|
||||
</para>
|
||||
|
||||
<para>
|
||||
@ -1299,9 +1313,43 @@ SELECT *
|
||||
<para>
|
||||
In our example the result will be empty because every supplier sells
|
||||
at least one part. Note that we use S.SNO from the outer SELECT within
|
||||
the WHERE clause of the inner SELECT. As described above the subquery
|
||||
is evaluated for every tuple from the outer query i.e. the value for
|
||||
S.SNO is always taken from the actual tuple of the outer SELECT.
|
||||
the WHERE clause of the inner SELECT. Here the subquery must be
|
||||
evaluated for every tuple from the outer query, i.e. the value for
|
||||
S.SNO is always taken from the current tuple of the outer SELECT.
|
||||
</para>
|
||||
</example>
|
||||
</para>
|
||||
</sect3>
|
||||
|
||||
<sect3>
|
||||
<title>Subqueries in FROM</title>
|
||||
|
||||
<para>
|
||||
A somewhat different way of using subqueries is to put them in the
|
||||
FROM clause. This is a useful feature because a subquery of this
|
||||
kind can output multiple columns and rows, whereas a subquery used
|
||||
in an expression must deliver just a single result. It also lets
|
||||
us get more than one round of grouping/aggregation without resorting
|
||||
to a temporary table.
|
||||
|
||||
<example>
|
||||
<title id="subselect-in-from-example">Subselect in FROM</title>
|
||||
|
||||
<para>
|
||||
If we want to know the highest average part price among all our
|
||||
suppliers, we can't write MAX(AVG(PRICE)), but we can write:
|
||||
|
||||
<programlisting>
|
||||
SELECT MAX(subtable.avgprice)
|
||||
FROM (SELECT AVG(P.PRICE) AS avgprice
|
||||
FROM SUPPLIER S, PART P, SELLS SE
|
||||
WHERE S.SNO = SE.SNO AND
|
||||
P.PNO = SE.PNO
|
||||
GROUP BY S.SNO) subtable;
|
||||
</programlisting>
|
||||
|
||||
The subquery returns one row per supplier (because of its GROUP BY)
|
||||
and then we aggregate over those rows in the outer query.
|
||||
</para>
|
||||
</example>
|
||||
</para>
|
||||
@ -1311,7 +1359,7 @@ SELECT *
|
||||
<title>Union, Intersect, Except</title>
|
||||
|
||||
<para>
|
||||
These operations calculate the union, intersect and set theoretic
|
||||
These operations calculate the union, intersection and set theoretic
|
||||
difference of the tuples derived by two subqueries.
|
||||
|
||||
<example>
|
||||
@ -1341,7 +1389,7 @@ gives the result:
|
||||
</para>
|
||||
|
||||
<para>
|
||||
Here an example for INTERSECT:
|
||||
Here is an example for INTERSECT:
|
||||
|
||||
<programlisting>
|
||||
SELECT S.SNO, S.SNAME, S.CITY
|
||||
@ -1361,7 +1409,7 @@ SELECT S.SNO, S.SNAME, S.CITY
|
||||
2 | Jones | Paris
|
||||
</programlisting>
|
||||
|
||||
The only tuple returned by both parts of the query is the one having $SNO=2$.
|
||||
The only tuple returned by both parts of the query is the one having SNO=2.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
@ -1469,14 +1517,11 @@ CREATE TABLE SELLS
|
||||
<listitem>
|
||||
<para>
|
||||
DECIMAL (<replaceable class="parameter">p</replaceable>[,<replaceable class="parameter">q</replaceable>]):
|
||||
signed packed decimal number of
|
||||
signed packed decimal number of up to
|
||||
<replaceable class="parameter">p</replaceable>
|
||||
digits precision with assumed
|
||||
digits, with
|
||||
<replaceable class="parameter">q</replaceable>
|
||||
of them right to the decimal point.
|
||||
|
||||
(15 ≥ <replaceable class="parameter">p</replaceable> ≥ <replaceable class="parameter">q</replaceable> ≥ 0).
|
||||
|
||||
digits to the right of the decimal point.
|
||||
If <replaceable class="parameter">q</replaceable>
|
||||
is omitted it is assumed to be 0.
|
||||
</para>
|
||||
@ -1547,8 +1592,8 @@ CREATE INDEX I ON SUPPLIER (SNAME);
|
||||
<para>
|
||||
The created index is maintained automatically, i.e. whenever a new tuple
|
||||
is inserted into the relation SUPPLIER the index I is adapted. Note
|
||||
that the only changes a user can percept when an index is present
|
||||
are an increased speed.
|
||||
that the only changes a user can perceive when an index is present
|
||||
are increased speed for SELECT and decreases in speed of updates.
|
||||
</para>
|
||||
</example>
|
||||
</para>
|
||||
|
||||
Loading…
Reference in New Issue
Block a user