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312 lines
12 KiB
Plaintext
312 lines
12 KiB
Plaintext
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$PostgreSQL: pgsql/doc/src/sgml/ref/create_aggregate.sgml,v 1.39 2008/11/14 10:22:45 petere Exp $
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PostgreSQL documentation
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-->
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<refentry id="SQL-CREATEAGGREGATE">
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<refmeta>
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<refentrytitle id="sql-createaggregate-title">CREATE AGGREGATE</refentrytitle>
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<manvolnum>7</manvolnum>
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<refmiscinfo>SQL - Language Statements</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>CREATE AGGREGATE</refname>
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<refpurpose>define a new aggregate function</refpurpose>
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</refnamediv>
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<indexterm zone="sql-createaggregate">
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<primary>CREATE AGGREGATE</primary>
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</indexterm>
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<refsynopsisdiv>
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<synopsis>
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CREATE AGGREGATE <replaceable class="PARAMETER">name</replaceable> ( <replaceable class="PARAMETER">input_data_type</replaceable> [ , ... ] ) (
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SFUNC = <replaceable class="PARAMETER">sfunc</replaceable>,
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STYPE = <replaceable class="PARAMETER">state_data_type</replaceable>
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[ , FINALFUNC = <replaceable class="PARAMETER">ffunc</replaceable> ]
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[ , INITCOND = <replaceable class="PARAMETER">initial_condition</replaceable> ]
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[ , SORTOP = <replaceable class="PARAMETER">sort_operator</replaceable> ]
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)
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or the old syntax
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CREATE AGGREGATE <replaceable class="PARAMETER">name</replaceable> (
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BASETYPE = <replaceable class="PARAMETER">base_type</replaceable>,
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SFUNC = <replaceable class="PARAMETER">sfunc</replaceable>,
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STYPE = <replaceable class="PARAMETER">state_data_type</replaceable>
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[ , FINALFUNC = <replaceable class="PARAMETER">ffunc</replaceable> ]
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[ , INITCOND = <replaceable class="PARAMETER">initial_condition</replaceable> ]
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[ , SORTOP = <replaceable class="PARAMETER">sort_operator</replaceable> ]
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)
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</synopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>
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<command>CREATE AGGREGATE</command> defines a new aggregate
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function. Some basic and commonly-used aggregate functions are
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included with the distribution; they are documented in <xref
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linkend="functions-aggregate">. If one defines new types or needs
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an aggregate function not already provided, then <command>CREATE
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AGGREGATE</command> can be used to provide the desired features.
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</para>
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<para>
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If a schema name is given (for example, <literal>CREATE AGGREGATE
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myschema.myagg ...</>) then the aggregate function is created in the
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specified schema. Otherwise it is created in the current schema.
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</para>
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<para>
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An aggregate function is identified by its name and input data type(s).
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Two aggregates in the same schema can have the same name if they operate on
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different input types. The
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name and input data type(s) of an aggregate must also be distinct from
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the name and input data type(s) of every ordinary function in the same
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schema.
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</para>
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<para>
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An aggregate function is made from one or two ordinary
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functions:
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a state transition function
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<replaceable class="PARAMETER">sfunc</replaceable>,
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and an optional final calculation function
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<replaceable class="PARAMETER">ffunc</replaceable>.
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These are used as follows:
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<programlisting>
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<replaceable class="PARAMETER">sfunc</replaceable>( internal-state, next-data-values ) ---> next-internal-state
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<replaceable class="PARAMETER">ffunc</replaceable>( internal-state ) ---> aggregate-value
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</programlisting>
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</para>
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<para>
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<productname>PostgreSQL</productname> creates a temporary variable
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of data type <replaceable class="PARAMETER">stype</replaceable>
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to hold the current internal state of the aggregate. At each input row,
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the aggregate argument value(s) are calculated and
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the state transition function is invoked with the current state value
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and the new argument value(s) to calculate a new
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internal state value. After all the rows have been processed,
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the final function is invoked once to calculate the aggregate's return
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value. If there is no final function then the ending state value
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is returned as-is.
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</para>
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<para>
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An aggregate function can provide an initial condition,
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that is, an initial value for the internal state value.
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This is specified and stored in the database as a value of type
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<type>text</type>, but it must be a valid external representation
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of a constant of the state value data type. If it is not supplied
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then the state value starts out null.
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</para>
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<para>
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If the state transition function is declared <quote>strict</quote>,
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then it cannot be called with null inputs. With such a transition
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function, aggregate execution behaves as follows. Rows with any null input
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values are ignored (the function is not called and the previous state value
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is retained). If the initial state value is null, then at the first row
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with all-nonnull input values, the first argument value replaces the state
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value, and the transition function is invoked at subsequent rows with
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all-nonnull input values.
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This is handy for implementing aggregates like <function>max</function>.
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Note that this behavior is only available when
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<replaceable class="PARAMETER">state_data_type</replaceable>
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is the same as the first
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<replaceable class="PARAMETER">input_data_type</replaceable>.
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When these types are different, you must supply a nonnull initial
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condition or use a nonstrict transition function.
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</para>
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<para>
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If the state transition function is not strict, then it will be called
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unconditionally at each input row, and must deal with null inputs
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and null transition values for itself. This allows the aggregate
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author to have full control over the aggregate's handling of null values.
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</para>
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<para>
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If the final function is declared <quote>strict</quote>, then it will not
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be called when the ending state value is null; instead a null result
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will be returned automatically. (Of course this is just the normal
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behavior of strict functions.) In any case the final function has
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the option of returning a null value. For example, the final function for
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<function>avg</function> returns null when it sees there were zero
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input rows.
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</para>
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<para>
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Aggregates that behave like <function>MIN</> or <function>MAX</> can
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sometimes be optimized by looking into an index instead of scanning every
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input row. If this aggregate can be so optimized, indicate it by
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specifying a <firstterm>sort operator</>. The basic requirement is that
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the aggregate must yield the first element in the sort ordering induced by
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the operator; in other words:
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<programlisting>
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SELECT agg(col) FROM tab;
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</programlisting>
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must be equivalent to:
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<programlisting>
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SELECT col FROM tab ORDER BY col USING sortop LIMIT 1;
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</programlisting>
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Further assumptions are that the aggregate ignores null inputs, and that
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it delivers a null result if and only if there were no non-null inputs.
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Ordinarily, a data type's <literal><</> operator is the proper sort
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operator for <function>MIN</>, and <literal>></> is the proper sort
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operator for <function>MAX</>. Note that the optimization will never
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actually take effect unless the specified operator is the <quote>less
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than</quote> or <quote>greater than</quote> strategy member of a B-tree
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index operator class.
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</para>
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</refsect1>
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<refsect1>
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<title>Parameters</title>
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<variablelist>
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<varlistentry>
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<term><replaceable class="PARAMETER">name</replaceable></term>
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<listitem>
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<para>
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The name (optionally schema-qualified) of the aggregate function
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to create.
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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">input_data_type</replaceable></term>
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<listitem>
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<para>
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An input data type on which this aggregate function operates.
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To create a zero-argument aggregate function, write <literal>*</>
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in place of the list of input data types. (An example of such an
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aggregate is <function>count(*)</function>.)
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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">base_type</replaceable></term>
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<listitem>
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<para>
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In the old syntax for <command>CREATE AGGREGATE</>, the input data type
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is specified by a <literal>basetype</> parameter rather than being
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written next to the aggregate name. Note that this syntax allows
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only one input parameter. To define a zero-argument aggregate function,
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specify the <literal>basetype</> as
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<literal>"ANY"</> (not <literal>*</>).
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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">sfunc</replaceable></term>
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<listitem>
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<para>
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The name of the state transition function to be called for each
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input row. For an <replaceable class="PARAMETER">N</>-argument
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aggregate function, the <replaceable class="PARAMETER">sfunc</>
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must take <replaceable class="PARAMETER">N</>+1 arguments,
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the first being of type <replaceable
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class="PARAMETER">state_data_type</replaceable> and the rest
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matching the declared input data type(s) of the aggregate.
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The function must return a value of type <replaceable
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class="PARAMETER">state_data_type</replaceable>. This function
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takes the current state value and the current input data value(s),
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and returns the next state value.
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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">state_data_type</replaceable></term>
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<listitem>
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<para>
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The data type for the aggregate's state value.
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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">ffunc</replaceable></term>
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<listitem>
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<para>
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The name of the final function called to compute the aggregate's
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result after all input rows have been traversed. The function
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must take a single argument of type <replaceable
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class="PARAMETER">state_data_type</replaceable>. The return
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data type of the aggregate is defined as the return type of this
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function. If <replaceable class="PARAMETER">ffunc</replaceable>
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is not specified, then the ending state value is used as the
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aggregate's result, and the return type is <replaceable
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class="PARAMETER">state_data_type</replaceable>.
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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">initial_condition</replaceable></term>
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<listitem>
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<para>
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The initial setting for the state value. This must be a string
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constant in the form accepted for the data type <replaceable
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class="PARAMETER">state_data_type</replaceable>. If not
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specified, the state value starts out null.
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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">sort_operator</replaceable></term>
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<listitem>
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<para>
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The associated sort operator for a <function>MIN</>- or
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<function>MAX</>-like aggregate.
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This is just an operator name (possibly schema-qualified).
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The operator is assumed to have the same input data types as
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the aggregate (which must be a single-argument aggregate).
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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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The parameters of <command>CREATE AGGREGATE</command> can be
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written in any order, not just the order illustrated above.
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</para>
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</refsect1>
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<refsect1>
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<title>Examples</title>
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<para>
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See <xref linkend="xaggr">.
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</para>
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</refsect1>
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<refsect1>
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<title>Compatibility</title>
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<para>
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<command>CREATE AGGREGATE</command> is a
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<productname>PostgreSQL</productname> language extension. The SQL
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standard does not provide for user-defined aggregate functions.
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</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<simplelist type="inline">
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<member><xref linkend="sql-alteraggregate" endterm="sql-alteraggregate-title"></member>
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<member><xref linkend="sql-dropaggregate" endterm="sql-dropaggregate-title"></member>
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</simplelist>
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</refsect1>
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</refentry>
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