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Revise aggregate functions per earlier discussions in pghackers.
There's now only one transition value and transition function. NULL handling in aggregates is a lot cleaner. Also, use Numeric accumulators instead of integer accumulators for sum/avg on integer datatypes --- this avoids overflow at the cost of being a little slower. Implement VARIANCE() and STDDEV() aggregates in the standard backend. Also, enable new LIKE selectivity estimators by default. Unrelated change, but as long as I had to force initdb anyway...
This commit is contained in:
Tom Lane
@@ -1,5 +1,5 @@
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<!--
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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_aggregate.sgml,v 1.9 2000/03/31 14:57:05 tgl Exp $
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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_aggregate.sgml,v 1.10 2000/07/17 03:04:41 tgl Exp $
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Postgres documentation
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-->
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@@ -21,20 +21,18 @@ Postgres documentation
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</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>2000-07-16</date>
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</refsynopsisdivinfo>
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<synopsis>
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CREATE AGGREGATE <replaceable class="PARAMETER">name</replaceable> ( BASETYPE = <replaceable class="PARAMETER">input_data_type</replaceable>
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[ , SFUNC1 = <replaceable class="PARAMETER">sfunc1</replaceable>, STYPE1 = <replaceable class="PARAMETER">state1_type</replaceable> ]
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[ , SFUNC2 = <replaceable class="PARAMETER">sfunc2</replaceable>, STYPE2 = <replaceable class="PARAMETER">state2_type</replaceable> ]
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CREATE AGGREGATE <replaceable class="PARAMETER">name</replaceable> ( BASETYPE = <replaceable class="PARAMETER">input_data_type</replaceable>,
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SFUNC = <replaceable class="PARAMETER">sfunc</replaceable>, STYPE = <replaceable class="PARAMETER">state_type</replaceable>
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[ , FINALFUNC = <replaceable class="PARAMETER">ffunc</replaceable> ]
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[ , INITCOND1 = <replaceable class="PARAMETER">initial_condition1</replaceable> ]
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[ , INITCOND2 = <replaceable class="PARAMETER">initial_condition2</replaceable> ] )
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[ , INITCOND = <replaceable class="PARAMETER">initial_condition</replaceable> ] )
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</synopsis>
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<refsect2 id="R2-SQL-CREATEAGGREGATE-1">
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<refsect2info>
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<date>1998-09-09</date>
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<date>2000-07-16</date>
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</refsect2info>
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<title>
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Inputs
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@@ -55,57 +53,39 @@ CREATE AGGREGATE <replaceable class="PARAMETER">name</replaceable> ( BASETYPE =
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<listitem>
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<para>
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The input data type on which this aggregate function operates.
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This can be specified as ANY for an aggregate that does not
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examine its input values
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(an example 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">sfunc1</replaceable></term>
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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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A state transition function
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to be called for every non-NULL input data value.
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This must be a function of two arguments, the first being of
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type <replaceable class="PARAMETER">state1_type</replaceable>
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The name of the state transition function
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to be called for each input data value.
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This is normally a function of two arguments, the first being of
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type <replaceable class="PARAMETER">state_type</replaceable>
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and the second of
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type <replaceable class="PARAMETER">input_data_type</replaceable>.
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The function must return a value of
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type <replaceable class="PARAMETER">state1_type</replaceable>.
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This function takes the current state value 1 and the current
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input data item, and returns the next state value 1.
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Alternatively, for an aggregate that does not examine its input
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values, the function takes just one argument of
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type <replaceable class="PARAMETER">state_type</replaceable>.
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In either case the function must return a value of
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type <replaceable class="PARAMETER">state_type</replaceable>.
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This function takes the current state value and the current
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input data item, 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">state1_type</replaceable></term>
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<term><replaceable class="PARAMETER">state_type</replaceable></term>
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<listitem>
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<para>
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The data type for the first state value of the aggregate.
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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">sfunc2</replaceable></term>
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<listitem>
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<para>
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A state transition function
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to be called for every non-NULL input data value.
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This must be a function of one argument of
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type <replaceable class="PARAMETER">state2_type</replaceable>,
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returning a value of the same type.
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This function takes the current state value 2 and
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returns the next state value 2.
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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">state2_type</replaceable></term>
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<listitem>
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<para>
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The data type for the second state value of the aggregate.
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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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@@ -114,35 +94,28 @@ CREATE AGGREGATE <replaceable class="PARAMETER">name</replaceable> ( BASETYPE =
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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 final function called to compute the aggregate's result
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after all input data has been traversed.
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If both state values are used, the final function must
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take two arguments of types
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<replaceable class="PARAMETER">state1_type</replaceable>
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and
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<replaceable class="PARAMETER">state2_type</replaceable>.
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If only one state value is used, the final function must
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take a single argument of that state value's type.
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The name of the final function called to compute the aggregate's
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result after all input data has been traversed. The function
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must take a single argument of type
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<replaceable class="PARAMETER">state_type</replaceable>.
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The output datatype of the aggregate is defined as the return
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type of this function.
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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 output type is
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<replaceable class="PARAMETER">state_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_condition1</replaceable></term>
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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 value for state value 1.
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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_condition2</replaceable></term>
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<listitem>
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<para>
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The initial value for state value 2.
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The initial setting for the state value. This must be a literal
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constant in the form accepted for the datatype
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<replaceable class="PARAMETER">state_type</replaceable>.
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If not 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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@@ -177,7 +150,7 @@ CREATE
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<refsect1 id="R1-SQL-CREATEAGGREGATE-1">
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<refsect1info>
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<date>1998-09-09</date>
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<date>2000-07-16</date>
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</refsect1info>
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<title>
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Description
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@@ -199,65 +172,76 @@ CREATE
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of the same name and input data type as an aggregate.
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</para>
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<para>
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An aggregate function is made from between one and three ordinary
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An aggregate function is made from one or two ordinary
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functions:
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two state transition functions,
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<replaceable class="PARAMETER">sfunc1</replaceable>
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and <replaceable class="PARAMETER">sfunc2</replaceable>,
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and a final calculation function,
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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">sfunc1</replaceable>( internal-state1, next-data-item ) ---> next-internal-state1
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<replaceable class="PARAMETER">sfunc2</replaceable>( internal-state2 ) ---> next-internal-state2
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<replaceable class="PARAMETER">ffunc</replaceable>(internal-state1, internal-state2) ---> aggregate-value
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<replaceable class="PARAMETER">sfunc</replaceable>( internal-state, next-data-item ) ---> 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>Postgres</productname> creates one or two temporary variables
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(of data types <replaceable class="PARAMETER">stype1</replaceable> and/or
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<replaceable class="PARAMETER">stype2</replaceable>) to hold the
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current internal states of the aggregate. At each input data item,
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the state transition function(s) are invoked to calculate new values
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for the internal state values. After all the data has been processed,
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<productname>Postgres</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
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data item,
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the state transition function is invoked to calculate a new
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internal state value. After all the data has been processed,
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the final function is invoked once to calculate the aggregate's output
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value.
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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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<replaceable class="PARAMETER">ffunc</replaceable> must be specified if
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both transition functions are specified. If only one transition function
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is used, then <replaceable class="PARAMETER">ffunc</replaceable> is
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optional. The default behavior when
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<replaceable class="PARAMETER">ffunc</replaceable> is not provided is
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to return the ending value of the internal state value being used
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(and, therefore, the aggregate's output type is the same as that
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state value's type).
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</para>
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<para>
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An aggregate function may also provide one or two initial conditions,
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that is, initial values for the internal state values being used.
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These are specified and stored in the database as fields of type
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<type>text</type>, but they must be valid external representations
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of constants of the state value datatypes. If
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<replaceable class="PARAMETER">sfunc1</replaceable> is specified
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without an <replaceable class="PARAMETER">initcond1</replaceable> value,
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then the system does not call
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<replaceable class="PARAMETER">sfunc1</replaceable>
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at the first input item; instead, the internal state value 1 is
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initialized with the first input value, and
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<replaceable class="PARAMETER">sfunc1</replaceable> is called beginning
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at the second input item. This is useful for aggregates like MIN and
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MAX. Note that an aggregate using this feature will return NULL when
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called with no input values. There is no comparable provision for
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state value 2; if <replaceable class="PARAMETER">sfunc2</replaceable> is
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specified then an <replaceable class="PARAMETER">initcond2</replaceable> is
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required.
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<para>
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An aggregate function may 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 field 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 datatype. 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 "strict" in pg_proc,
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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. NULL input values
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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 the first
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non-NULL input value replaces the state value, and the transition
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function is invoked beginning with the second non-NULL input value.
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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_type</replaceable>
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is the same as
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<replaceable class="PARAMETER">input_data_type</replaceable>.
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When these types are different, you must supply a non-NULL initial
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condition or use a non-strict 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 value, 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 NULLs.
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</para>
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<para>
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If the final function is declared "strict", 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 output 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 NULL. 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 tuples.
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</para>
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<refsect2 id="R2-SQL-CREATEAGGREGATE-3">
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<refsect2info>
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<date>1998-09-09</date>
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<date>2000-07-16</date>
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</refsect2info>
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<title>
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Notes
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@@ -272,29 +256,6 @@ CREATE
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in any order, not just the order illustrated above.
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</para>
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<para>
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It is possible to specify aggregate functions
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that have varying combinations of state and final functions.
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For example, the <function>count</function> aggregate requires
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<replaceable class="PARAMETER">sfunc2</replaceable>
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(an incrementing function) but not
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<replaceable class="PARAMETER">sfunc1</replaceable> or
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<replaceable class="PARAMETER">ffunc</replaceable>,
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whereas the <function>sum</function> aggregate requires
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<replaceable class="PARAMETER">sfunc1</replaceable> (an addition
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function) but not <replaceable class="PARAMETER">sfunc2</replaceable> or
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<replaceable class="PARAMETER">ffunc</replaceable>, and the
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<function>avg</function>
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aggregate requires
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both state functions as
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well as a <replaceable class="PARAMETER">ffunc</replaceable> (a division
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function) to produce its
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answer. In any case, at least one state function must be
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defined, and any <replaceable class="PARAMETER">sfunc2</replaceable> must
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have a corresponding
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<replaceable class="PARAMETER">initcond2</replaceable>.
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</para>
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</refsect2>
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</refsect1>
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Reference in New Issue
Block a user