Baoding-FenShuaJiang
/
Green_GoodERP18_FSJ_Standard


			
				
					
						
						
							
							<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>CREATE AGGREGATE</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="sql-create-access-method.html" title="CREATE ACCESS METHOD" /><link rel="next" href="sql-createcast.html" title="CREATE CAST" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">CREATE AGGREGATE</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-create-access-method.html" title="CREATE ACCESS METHOD">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><th width="60%" align="center">SQL Commands</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-createcast.html" title="CREATE CAST">Next</a></td></tr></table><hr></hr></div><div class="refentry" id="SQL-CREATEAGGREGATE"><div class="titlepage"></div><a id="id-1.9.3.57.1" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">CREATE AGGREGATE</span></h2><p>CREATE AGGREGATE — define a new aggregate function</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><pre class="synopsis">
CREATE [ OR REPLACE ] AGGREGATE <em class="replaceable"><code>name</code></em> ( [ <em class="replaceable"><code>argmode</code></em> ] [ <em class="replaceable"><code>argname</code></em> ] <em class="replaceable"><code>arg_data_type</code></em> [ , ... ] ) (
    SFUNC = <em class="replaceable"><code>sfunc</code></em>,
    STYPE = <em class="replaceable"><code>state_data_type</code></em>
    [ , SSPACE = <em class="replaceable"><code>state_data_size</code></em> ]
    [ , FINALFUNC = <em class="replaceable"><code>ffunc</code></em> ]
    [ , FINALFUNC_EXTRA ]
    [ , FINALFUNC_MODIFY = { READ_ONLY | SHAREABLE | READ_WRITE } ]
    [ , COMBINEFUNC = <em class="replaceable"><code>combinefunc</code></em> ]
    [ , SERIALFUNC = <em class="replaceable"><code>serialfunc</code></em> ]
    [ , DESERIALFUNC = <em class="replaceable"><code>deserialfunc</code></em> ]
    [ , INITCOND = <em class="replaceable"><code>initial_condition</code></em> ]
    [ , MSFUNC = <em class="replaceable"><code>msfunc</code></em> ]
    [ , MINVFUNC = <em class="replaceable"><code>minvfunc</code></em> ]
    [ , MSTYPE = <em class="replaceable"><code>mstate_data_type</code></em> ]
    [ , MSSPACE = <em class="replaceable"><code>mstate_data_size</code></em> ]
    [ , MFINALFUNC = <em class="replaceable"><code>mffunc</code></em> ]
    [ , MFINALFUNC_EXTRA ]
    [ , MFINALFUNC_MODIFY = { READ_ONLY | SHAREABLE | READ_WRITE } ]
    [ , MINITCOND = <em class="replaceable"><code>minitial_condition</code></em> ]
    [ , SORTOP = <em class="replaceable"><code>sort_operator</code></em> ]
    [ , PARALLEL = { SAFE | RESTRICTED | UNSAFE } ]
)

CREATE [ OR REPLACE ] AGGREGATE <em class="replaceable"><code>name</code></em> ( [ [ <em class="replaceable"><code>argmode</code></em> ] [ <em class="replaceable"><code>argname</code></em> ] <em class="replaceable"><code>arg_data_type</code></em> [ , ... ] ]
                        ORDER BY [ <em class="replaceable"><code>argmode</code></em> ] [ <em class="replaceable"><code>argname</code></em> ] <em class="replaceable"><code>arg_data_type</code></em> [ , ... ] ) (
    SFUNC = <em class="replaceable"><code>sfunc</code></em>,
    STYPE = <em class="replaceable"><code>state_data_type</code></em>
    [ , SSPACE = <em class="replaceable"><code>state_data_size</code></em> ]
    [ , FINALFUNC = <em class="replaceable"><code>ffunc</code></em> ]
    [ , FINALFUNC_EXTRA ]
    [ , FINALFUNC_MODIFY = { READ_ONLY | SHAREABLE | READ_WRITE } ]
    [ , INITCOND = <em class="replaceable"><code>initial_condition</code></em> ]
    [ , PARALLEL = { SAFE | RESTRICTED | UNSAFE } ]
    [ , HYPOTHETICAL ]
)

<span class="phrase">or the old syntax</span>

CREATE [ OR REPLACE ] AGGREGATE <em class="replaceable"><code>name</code></em> (
    BASETYPE = <em class="replaceable"><code>base_type</code></em>,
    SFUNC = <em class="replaceable"><code>sfunc</code></em>,
    STYPE = <em class="replaceable"><code>state_data_type</code></em>
    [ , SSPACE = <em class="replaceable"><code>state_data_size</code></em> ]
    [ , FINALFUNC = <em class="replaceable"><code>ffunc</code></em> ]
    [ , FINALFUNC_EXTRA ]
    [ , FINALFUNC_MODIFY = { READ_ONLY | SHAREABLE | READ_WRITE } ]
    [ , COMBINEFUNC = <em class="replaceable"><code>combinefunc</code></em> ]
    [ , SERIALFUNC = <em class="replaceable"><code>serialfunc</code></em> ]
    [ , DESERIALFUNC = <em class="replaceable"><code>deserialfunc</code></em> ]
    [ , INITCOND = <em class="replaceable"><code>initial_condition</code></em> ]
    [ , MSFUNC = <em class="replaceable"><code>msfunc</code></em> ]
    [ , MINVFUNC = <em class="replaceable"><code>minvfunc</code></em> ]
    [ , MSTYPE = <em class="replaceable"><code>mstate_data_type</code></em> ]
    [ , MSSPACE = <em class="replaceable"><code>mstate_data_size</code></em> ]
    [ , MFINALFUNC = <em class="replaceable"><code>mffunc</code></em> ]
    [ , MFINALFUNC_EXTRA ]
    [ , MFINALFUNC_MODIFY = { READ_ONLY | SHAREABLE | READ_WRITE } ]
    [ , MINITCOND = <em class="replaceable"><code>minitial_condition</code></em> ]
    [ , SORTOP = <em class="replaceable"><code>sort_operator</code></em> ]
)
</pre></div><div class="refsect1" id="id-1.9.3.57.5"><h2>Description</h2><p>
   <code class="command">CREATE AGGREGATE</code> defines a new aggregate function.
   <code class="command">CREATE OR REPLACE AGGREGATE</code> will either define a new
   aggregate function or replace an existing definition. Some basic and
   commonly-used aggregate functions are included with the distribution; they
   are documented in <a class="xref" href="functions-aggregate.html" title="9.20. Aggregate Functions">Section 9.20</a>. If one defines new
   types or needs an aggregate function not already provided, then
   <code class="command">CREATE AGGREGATE</code> can be used to provide the desired
   features.
  </p><p>
   When replacing an existing definition, the argument types, result type,
   and number of direct arguments may not be changed. Also, the new definition
   must be of the same kind (ordinary aggregate, ordered-set aggregate, or
   hypothetical-set aggregate) as the old one.
  </p><p>
   If a schema name is given (for example, <code class="literal">CREATE AGGREGATE
   myschema.myagg ...</code>) then the aggregate function is created in the
   specified schema.  Otherwise it is created in the current schema.
  </p><p>
   An aggregate function is identified by its name and input data type(s).
   Two aggregates in the same schema can have the same name if they operate on
   different input types.  The
   name and input data type(s) of an aggregate must also be distinct from
   the name and input data type(s) of every ordinary function in the same
   schema.
   This behavior is identical to overloading of ordinary function names
   (see <a class="xref" href="sql-createfunction.html" title="CREATE FUNCTION"><span class="refentrytitle">CREATE FUNCTION</span></a>).
  </p><p>
   A simple aggregate function is made from one or two ordinary
   functions:
   a state transition function
   <em class="replaceable"><code>sfunc</code></em>,
   and an optional final calculation function
   <em class="replaceable"><code>ffunc</code></em>.
   These are used as follows:
</p><pre class="programlisting">
<em class="replaceable"><code>sfunc</code></em>( internal-state, next-data-values ) ---&gt; next-internal-state
<em class="replaceable"><code>ffunc</code></em>( internal-state ) ---&gt; aggregate-value
</pre><p>
  </p><p>
   <span class="productname">PostgreSQL</span> creates a temporary variable
   of data type <em class="replaceable"><code>stype</code></em>
   to hold the current internal state of the aggregate.  At each input row,
   the aggregate argument value(s) are calculated and
   the state transition function is invoked with the current state value
   and the new argument value(s) to calculate a new
   internal state value.  After all the rows have been processed,
   the final function is invoked once to calculate the aggregate's return
   value.  If there is no final function then the ending state value
   is returned as-is.
  </p><p>
   An aggregate function can provide an initial condition,
   that is, an initial value for the internal state value.
   This is specified and stored in the database as a value of type
   <code class="type">text</code>, but it must be a valid external representation
   of a constant of the state value data type.  If it is not supplied
   then the state value starts out null.
  </p><p>
   If the state transition function is declared <span class="quote">“<span class="quote">strict</span>”</span>,
   then it cannot be called with null inputs.  With such a transition
   function, aggregate execution behaves as follows.  Rows with any null input
   values are ignored (the function is not called and the previous state value
   is retained).  If the initial state value is null, then at the first row
   with all-nonnull input values, the first argument value replaces the state
   value, and the transition function is invoked at each subsequent row with
   all-nonnull input values.
   This is handy for implementing aggregates like <code class="function">max</code>.
   Note that this behavior is only available when
   <em class="replaceable"><code>state_data_type</code></em>
   is the same as the first
   <em class="replaceable"><code>arg_data_type</code></em>.
   When these types are different, you must supply a nonnull initial
   condition or use a nonstrict transition function.
  </p><p>
   If the state transition function is not strict, then it will be called
   unconditionally at each input row, and must deal with null inputs
   and null state values for itself.  This allows the aggregate
   author to have full control over the aggregate's handling of null values.
  </p><p>
   If the final function is declared <span class="quote">“<span class="quote">strict</span>”</span>, then it will not
   be called when the ending state value is null; instead a null result
   will be returned automatically.  (Of course this is just the normal
   behavior of strict functions.)  In any case the final function has
   the option of returning a null value.  For example, the final function for
   <code class="function">avg</code> returns null when it sees there were zero
   input rows.
  </p><p>
   Sometimes it is useful to declare the final function as taking not just
   the state value, but extra parameters corresponding to the aggregate's
   input values.  The main reason for doing this is if the final function
   is polymorphic and the state value's data type would be inadequate to
   pin down the result type.  These extra parameters are always passed as
   NULL (and so the final function must not be strict when
   the <code class="literal">FINALFUNC_EXTRA</code> option is used), but nonetheless they
   are valid parameters.  The final function could for example make use
   of <code class="function">get_fn_expr_argtype</code> to identify the actual argument type
   in the current call.
  </p><p>
   An aggregate can optionally support <em class="firstterm">moving-aggregate mode</em>,
   as described in <a class="xref" href="xaggr.html#XAGGR-MOVING-AGGREGATES" title="37.12.1. Moving-Aggregate Mode">Section 37.12.1</a>.  This requires
   specifying the <code class="literal">MSFUNC</code>, <code class="literal">MINVFUNC</code>,
   and <code class="literal">MSTYPE</code> parameters, and optionally
   the <code class="literal">MSSPACE</code>, <code class="literal">MFINALFUNC</code>,
   <code class="literal">MFINALFUNC_EXTRA</code>, <code class="literal">MFINALFUNC_MODIFY</code>,
   and <code class="literal">MINITCOND</code> parameters.  Except for <code class="literal">MINVFUNC</code>,
   these parameters work like the corresponding simple-aggregate parameters
   without <code class="literal">M</code>; they define a separate implementation of the
   aggregate that includes an inverse transition function.
  </p><p>
   The syntax with <code class="literal">ORDER BY</code> in the parameter list creates
   a special type of aggregate called an <em class="firstterm">ordered-set
   aggregate</em>; or if <code class="literal">HYPOTHETICAL</code> is specified, then
   a <em class="firstterm">hypothetical-set aggregate</em> is created.  These
   aggregates operate over groups of sorted values in order-dependent ways,
   so that specification of an input sort order is an essential part of a
   call.  Also, they can have <em class="firstterm">direct</em> arguments, which are
   arguments that are evaluated only once per aggregation rather than once
   per input row.  Hypothetical-set aggregates are a subclass of ordered-set
   aggregates in which some of the direct arguments are required to match,
   in number and data types, the aggregated argument columns.  This allows
   the values of those direct arguments to be added to the collection of
   aggregate-input rows as an additional <span class="quote">“<span class="quote">hypothetical</span>”</span> row.
  </p><p>
   An aggregate can optionally support <em class="firstterm">partial aggregation</em>,
   as described in <a class="xref" href="xaggr.html#XAGGR-PARTIAL-AGGREGATES" title="37.12.4. Partial Aggregation">Section 37.12.4</a>.
   This requires specifying the <code class="literal">COMBINEFUNC</code> parameter.
   If the <em class="replaceable"><code>state_data_type</code></em>
   is <code class="type">internal</code>, it's usually also appropriate to provide the
   <code class="literal">SERIALFUNC</code> and <code class="literal">DESERIALFUNC</code> parameters so that
   parallel aggregation is possible.  Note that the aggregate must also be
   marked <code class="literal">PARALLEL SAFE</code> to enable parallel aggregation.
  </p><p>
   Aggregates that behave like <code class="function">MIN</code> or <code class="function">MAX</code> can
   sometimes be optimized by looking into an index instead of scanning every
   input row.  If this aggregate can be so optimized, indicate it by
   specifying a <em class="firstterm">sort operator</em>.  The basic requirement is that
   the aggregate must yield the first element in the sort ordering induced by
   the operator; in other words:
</p><pre class="programlisting">
SELECT agg(col) FROM tab;
</pre><p>
   must be equivalent to:
</p><pre class="programlisting">
SELECT col FROM tab ORDER BY col USING sortop LIMIT 1;
</pre><p>
   Further assumptions are that the aggregate ignores null inputs, and that
   it delivers a null result if and only if there were no non-null inputs.
   Ordinarily, a data type's <code class="literal">&lt;</code> operator is the proper sort
   operator for <code class="function">MIN</code>, and <code class="literal">&gt;</code> is the proper sort
   operator for <code class="function">MAX</code>.  Note that the optimization will never
   actually take effect unless the specified operator is the <span class="quote">“<span class="quote">less
   than</span>”</span> or <span class="quote">“<span class="quote">greater than</span>”</span> strategy member of a B-tree
   index operator class.
  </p><p>
   To be able to create an aggregate function, you must
   have <code class="literal">USAGE</code> privilege on the argument types, the state
   type(s), and the return type, as well as <code class="literal">EXECUTE</code>
   privilege on the supporting functions.
  </p></div><div class="refsect1" id="id-1.9.3.57.6"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="replaceable"><code>name</code></em></span></dt><dd><p>
      The name (optionally schema-qualified) of the aggregate function
      to create.
     </p></dd><dt><span class="term"><em class="replaceable"><code>argmode</code></em></span></dt><dd><p>
      The mode of an argument: <code class="literal">IN</code> or <code class="literal">VARIADIC</code>.
      (Aggregate functions do not support <code class="literal">OUT</code> arguments.)
      If omitted, the default is <code class="literal">IN</code>.  Only the last argument
      can be marked <code class="literal">VARIADIC</code>.
     </p></dd><dt><span class="term"><em class="replaceable"><code>argname</code></em></span></dt><dd><p>
      The name of an argument.  This is currently only useful for
      documentation purposes.  If omitted, the argument has no name.
     </p></dd><dt><span class="term"><em class="replaceable"><code>arg_data_type</code></em></span></dt><dd><p>
      An input data type on which this aggregate function operates.
      To create a zero-argument aggregate function, write <code class="literal">*</code>
      in place of the list of argument specifications.  (An example of such an
      aggregate is <code class="function">count(*)</code>.)
     </p></dd><dt><span class="term"><em class="replaceable"><code>base_type</code></em></span></dt><dd><p>
      In the old syntax for <code class="command">CREATE AGGREGATE</code>, the input data type
      is specified by a <code class="literal">basetype</code> parameter rather than being
      written next to the aggregate name.  Note that this syntax allows
      only one input parameter.  To define a zero-argument aggregate function
      with this syntax, specify the <code class="literal">basetype</code> as
      <code class="literal">"ANY"</code> (not <code class="literal">*</code>).
      Ordered-set aggregates cannot be defined with the old syntax.
     </p></dd><dt><span class="term"><em class="replaceable"><code>sfunc</code></em></span></dt><dd><p>
      The name of the state transition function to be called for each
      input row.  For a normal <em class="replaceable"><code>N</code></em>-argument
      aggregate function, the <em class="replaceable"><code>sfunc</code></em>
      must take <em class="replaceable"><code>N</code></em>+1 arguments,
      the first being of type <em class="replaceable"><code>state_data_type</code></em> and the rest
      matching the declared input data type(s) of the aggregate.
      The function must return a value of type <em class="replaceable"><code>state_data_type</code></em>.  This function
      takes the current state value and the current input data value(s),
      and returns the next state value.
     </p><p>
      For ordered-set (including hypothetical-set) aggregates, the state
      transition function receives only the current state value and the
      aggregated arguments, not the direct arguments.  Otherwise it is the
      same.
     </p></dd><dt><span class="term"><em class="replaceable"><code>state_data_type</code></em></span></dt><dd><p>
      The data type for the aggregate's state value.
     </p></dd><dt><span class="term"><em class="replaceable"><code>state_data_size</code></em></span></dt><dd><p>
      The approximate average size (in bytes) of the aggregate's state value.
      If this parameter is omitted or is zero, a default estimate is used
      based on the <em class="replaceable"><code>state_data_type</code></em>.
      The planner uses this value to estimate the memory required for a
      grouped aggregate query.  The planner will consider using hash
      aggregation for such a query only if the hash table is estimated to fit
      in <a class="xref" href="runtime-config-resource.html#GUC-WORK-MEM">work_mem</a>; therefore, large values of this
      parameter discourage use of hash aggregation.
     </p></dd><dt><span class="term"><em class="replaceable"><code>ffunc</code></em></span></dt><dd><p>
      The name of the final function called to compute the aggregate's
      result after all input rows have been traversed.
      For a normal aggregate, this function
      must take a single argument of type <em class="replaceable"><code>state_data_type</code></em>.  The return
      data type of the aggregate is defined as the return type of this
      function.  If <em class="replaceable"><code>ffunc</code></em>
      is not specified, then the ending state value is used as the
      aggregate's result, and the return type is <em class="replaceable"><code>state_data_type</code></em>.
     </p><p>
      For ordered-set (including hypothetical-set) aggregates, the
      final function receives not only the final state value,
      but also the values of all the direct arguments.
     </p><p>
      If <code class="literal">FINALFUNC_EXTRA</code> is specified, then in addition to the
      final state value and any direct arguments, the final function
      receives extra NULL values corresponding to the aggregate's regular
      (aggregated) arguments.  This is mainly useful to allow correct
      resolution of the aggregate result type when a polymorphic aggregate
      is being defined.
     </p></dd><dt><span class="term"><code class="literal">FINALFUNC_MODIFY</code> = { <code class="literal">READ_ONLY</code> | <code class="literal">SHAREABLE</code> | <code class="literal">READ_WRITE</code> }</span></dt><dd><p>
      This option specifies whether the final function is a pure function
      that does not modify its arguments.  <code class="literal">READ_ONLY</code> indicates
      it does not; the other two values indicate that it may change the
      transition state value.  See <a class="xref" href="sql-createaggregate.html#SQL-CREATEAGGREGATE-NOTES" title="Notes">Notes</a> below for more detail.  The
      default is <code class="literal">READ_ONLY</code>, except for ordered-set aggregates,
      for which the default is <code class="literal">READ_WRITE</code>.
     </p></dd><dt><span class="term"><em class="replaceable"><code>combinefunc</code></em></span></dt><dd><p>
      The <em class="replaceable"><code>combinefunc</code></em> function
      may optionally be specified to allow the aggregate function to support
      partial aggregation.  If provided,
      the <em class="replaceable"><code>combinefunc</code></em> must
      combine two <em class="replaceable"><code>state_data_type</code></em>
      values, each containing the result of aggregation over some subset of
      the input values, to produce a
      new <em class="replaceable"><code>state_data_type</code></em> that
      represents the result of aggregating over both sets of inputs.  This
      function can be thought of as
      an <em class="replaceable"><code>sfunc</code></em>, where instead of
      acting upon an individual input row and adding it to the running
      aggregate state, it adds another aggregate state to the running state.
     </p><p>
      The <em class="replaceable"><code>combinefunc</code></em> must be
      declared as taking two arguments of
      the <em class="replaceable"><code>state_data_type</code></em> and
      returning a value of
      the <em class="replaceable"><code>state_data_type</code></em>.
      Optionally this function may be <span class="quote">“<span class="quote">strict</span>”</span>. In this case the
      function will not be called when either of the input states are null;
      the other state will be taken as the correct result.
     </p><p>
      For aggregate functions
      whose <em class="replaceable"><code>state_data_type</code></em>
      is <code class="type">internal</code>,
      the <em class="replaceable"><code>combinefunc</code></em> must not
      be strict. In this case
      the <em class="replaceable"><code>combinefunc</code></em> must
      ensure that null states are handled correctly and that the state being
      returned is properly stored in the aggregate memory context.
     </p></dd><dt><span class="term"><em class="replaceable"><code>serialfunc</code></em></span></dt><dd><p>
      An aggregate function
      whose <em class="replaceable"><code>state_data_type</code></em>
      is <code class="type">internal</code> can participate in parallel aggregation only if it
      has a <em class="replaceable"><code>serialfunc</code></em> function,
      which must serialize the aggregate state into a <code class="type">bytea</code> value for
      transmission to another process.  This function must take a single
      argument of type <code class="type">internal</code> and return type <code class="type">bytea</code>.  A
      corresponding <em class="replaceable"><code>deserialfunc</code></em>
      is also required.
     </p></dd><dt><span class="term"><em class="replaceable"><code>deserialfunc</code></em></span></dt><dd><p>
      Deserialize a previously serialized aggregate state back into
      <em class="replaceable"><code>state_data_type</code></em>. This
      function must take two arguments of types <code class="type">bytea</code>
      and <code class="type">internal</code>, and produce a result of type <code class="type">internal</code>.
      (Note: the second, <code class="type">internal</code> argument is unused, but is required
      for type safety reasons.)
     </p></dd><dt><span class="term"><em class="replaceable"><code>initial_condition</code></em></span></dt><dd><p>
      The initial setting for the state value.  This must be a string
      constant in the form accepted for the data type <em class="replaceable"><code>state_data_type</code></em>.  If not
      specified, the state value starts out null.
     </p></dd><dt><span class="term"><em class="replaceable"><code>msfunc</code></em></span></dt><dd><p>
      The name of the forward state transition function to be called for each
      input row in moving-aggregate mode.  This is exactly like the regular
      transition function, except that its first argument and result are of
      type <em class="replaceable"><code>mstate_data_type</code></em>, which might be different
      from <em class="replaceable"><code>state_data_type</code></em>.
     </p></dd><dt><span class="term"><em class="replaceable"><code>minvfunc</code></em></span></dt><dd><p>
      The name of the inverse state transition function to be used in
      moving-aggregate mode.  This function has the same argument and
      result types as <em class="replaceable"><code>msfunc</code></em>, but it is used to remove
      a value from the current aggregate state, rather than add a value to
      it.  The inverse transition function must have the same strictness
      attribute as the forward state transition function.
     </p></dd><dt><span class="term"><em class="replaceable"><code>mstate_data_type</code></em></span></dt><dd><p>
      The data type for the aggregate's state value, when using
      moving-aggregate mode.
     </p></dd><dt><span class="term"><em class="replaceable"><code>mstate_data_size</code></em></span></dt><dd><p>
      The approximate average size (in bytes) of the aggregate's state
      value, when using moving-aggregate mode.  This works the same as
      <em class="replaceable"><code>state_data_size</code></em>.
     </p></dd><dt><span class="term"><em class="replaceable"><code>mffunc</code></em></span></dt><dd><p>
      The name of the final function called to compute the aggregate's
      result after all input rows have been traversed, when using
      moving-aggregate mode.  This works the same as <em class="replaceable"><code>ffunc</code></em>,
      except that its first argument's type
      is <em class="replaceable"><code>mstate_data_type</code></em> and extra dummy arguments are
      specified by writing <code class="literal">MFINALFUNC_EXTRA</code>.
      The aggregate result type determined by <em class="replaceable"><code>mffunc</code></em>
      or <em class="replaceable"><code>mstate_data_type</code></em> must match that determined by the
      aggregate's regular implementation.
     </p></dd><dt><span class="term"><code class="literal">MFINALFUNC_MODIFY</code> = { <code class="literal">READ_ONLY</code> | <code class="literal">SHAREABLE</code> | <code class="literal">READ_WRITE</code> }</span></dt><dd><p>
      This option is like <code class="literal">FINALFUNC_MODIFY</code>, but it describes
      the behavior of the moving-aggregate final function.
     </p></dd><dt><span class="term"><em class="replaceable"><code>minitial_condition</code></em></span></dt><dd><p>
      The initial setting for the state value, when using moving-aggregate
      mode.  This works the same as <em class="replaceable"><code>initial_condition</code></em>.
     </p></dd><dt><span class="term"><em class="replaceable"><code>sort_operator</code></em></span></dt><dd><p>
      The associated sort operator for a <code class="function">MIN</code>- or
      <code class="function">MAX</code>-like aggregate.
      This is just an operator name (possibly schema-qualified).
      The operator is assumed to have the same input data types as
      the aggregate (which must be a single-argument normal aggregate).
     </p></dd><dt><span class="term"><code class="literal">PARALLEL =</code> { <code class="literal">SAFE</code> | <code class="literal">RESTRICTED</code> | <code class="literal">UNSAFE</code> }</span></dt><dd><p>
      The meanings of <code class="literal">PARALLEL SAFE</code>, <code class="literal">PARALLEL
      RESTRICTED</code>, and <code class="literal">PARALLEL UNSAFE</code> are the same as
      in <a class="xref" href="sql-createfunction.html" title="CREATE FUNCTION"><span class="refentrytitle">CREATE FUNCTION</span></a>.  An aggregate will not be
      considered for parallelization if it is marked <code class="literal">PARALLEL
      UNSAFE</code> (which is the default!) or <code class="literal">PARALLEL RESTRICTED</code>.
      Note that the parallel-safety markings of the aggregate's support
      functions are not consulted by the planner, only the marking of the
      aggregate itself.
     </p></dd><dt><span class="term"><code class="literal">HYPOTHETICAL</code></span></dt><dd><p>
      For ordered-set aggregates only, this flag specifies that the aggregate
      arguments are to be processed according to the requirements for
      hypothetical-set aggregates: that is, the last few direct arguments must
      match the data types of the aggregated (<code class="literal">WITHIN GROUP</code>)
      arguments.  The <code class="literal">HYPOTHETICAL</code> flag has no effect on
      run-time behavior, only on parse-time resolution of the data types and
      collations of the aggregate's arguments.
     </p></dd></dl></div><p>
   The parameters of <code class="command">CREATE AGGREGATE</code> can be
   written in any order, not just the order illustrated above.
  </p></div><div class="refsect1" id="SQL-CREATEAGGREGATE-NOTES"><h2>Notes</h2><p>
    In parameters that specify support function names, you can write
    a schema name if needed, for example <code class="literal">SFUNC = public.sum</code>.
    Do not write argument types there, however — the argument types
    of the support functions are determined from other parameters.
   </p><p>
    Ordinarily, PostgreSQL functions are expected to be true functions that
    do not modify their input values.  However, an aggregate transition
    function, <span class="emphasis"><em>when used in the context of an aggregate</em></span>,
    is allowed to cheat and modify its transition-state argument in place.
    This can provide substantial performance benefits compared to making
    a fresh copy of the transition state each time.
   </p><p>
    Likewise, while an aggregate final function is normally expected not to
    modify its input values, sometimes it is impractical to avoid modifying
    the transition-state argument.  Such behavior must be declared using
    the <code class="literal">FINALFUNC_MODIFY</code> parameter.
    The <code class="literal">READ_WRITE</code>
    value indicates that the final function modifies the transition state in
    unspecified ways.  This value prevents use of the aggregate as a window
    function, and it also prevents merging of transition states for aggregate
    calls that share the same input values and transition functions.
    The <code class="literal">SHAREABLE</code> value indicates that the transition function
    cannot be applied after the final function, but multiple final-function
    calls can be performed on the ending transition state value.  This value
    prevents use of the aggregate as a window function, but it allows merging
    of transition states.  (That is, the optimization of interest here is not
    applying the same final function repeatedly, but applying different final
    functions to the same ending transition state value.  This is allowed as
    long as none of the final functions are marked <code class="literal">READ_WRITE</code>.)
   </p><p>
    If an aggregate supports moving-aggregate mode, it will improve
    calculation efficiency when the aggregate is used as a window function
    for a window with moving frame start (that is, a frame start mode other
    than <code class="literal">UNBOUNDED PRECEDING</code>).  Conceptually, the forward
    transition function adds input values to the aggregate's state when
    they enter the window frame from the bottom, and the inverse transition
    function removes them again when they leave the frame at the top.  So,
    when values are removed, they are always removed in the same order they
    were added.  Whenever the inverse transition function is invoked, it will
    thus receive the earliest added but not yet removed argument value(s).
    The inverse transition function can assume that at least one row will
    remain in the current state after it removes the oldest row.  (When this
    would not be the case, the window function mechanism simply starts a
    fresh aggregation, rather than using the inverse transition function.)
   </p><p>
    The forward transition function for moving-aggregate mode is not
    allowed to return NULL as the new state value. If the inverse
    transition function returns NULL, this is taken as an indication that
    the inverse function cannot reverse the state calculation for this
    particular input, and so the aggregate calculation will be redone from
    scratch for the current frame starting position.  This convention
    allows moving-aggregate mode to be used in situations where there are
    some infrequent cases that are impractical to reverse out of the
    running state value.
   </p><p>
    If no moving-aggregate implementation is supplied,
    the aggregate can still be used with moving frames,
    but <span class="productname">PostgreSQL</span> will recompute the whole
    aggregation whenever the start of the frame moves.
    Note that whether or not the aggregate supports moving-aggregate
    mode, <span class="productname">PostgreSQL</span> can handle a moving frame
    end without recalculation; this is done by continuing to add new values
    to the aggregate's state.  This is why use of an aggregate as a window
    function requires that the final function be read-only: it must
    not damage the aggregate's state value, so that the aggregation can be
    continued even after an aggregate result value has been obtained for
    one set of frame boundaries.
   </p><p>
    The syntax for ordered-set aggregates allows <code class="literal">VARIADIC</code>
    to be specified for both the last direct parameter and the last
    aggregated (<code class="literal">WITHIN GROUP</code>) parameter.  However, the
    current implementation restricts use of <code class="literal">VARIADIC</code>
    in two ways.  First, ordered-set aggregates can only use
    <code class="literal">VARIADIC "any"</code>, not other variadic array types.
    Second, if the last direct parameter is <code class="literal">VARIADIC "any"</code>,
    then there can be only one aggregated parameter and it must also
    be <code class="literal">VARIADIC "any"</code>.  (In the representation used in the
    system catalogs, these two parameters are merged into a single
    <code class="literal">VARIADIC "any"</code> item, since <code class="structname">pg_proc</code> cannot
    represent functions with more than one <code class="literal">VARIADIC</code> parameter.)
    If the aggregate is a hypothetical-set aggregate, the direct arguments
    that match the <code class="literal">VARIADIC "any"</code> parameter are the hypothetical
    ones; any preceding parameters represent additional direct arguments
    that are not constrained to match the aggregated arguments.
   </p><p>
    Currently, ordered-set aggregates do not need to support
    moving-aggregate mode, since they cannot be used as window functions.
   </p><p>
    Partial (including parallel) aggregation is currently not supported for
    ordered-set aggregates.  Also, it will never be used for aggregate calls
    that include <code class="literal">DISTINCT</code> or <code class="literal">ORDER BY</code> clauses, since
    those semantics cannot be supported during partial aggregation.
  </p></div><div class="refsect1" id="id-1.9.3.57.8"><h2>Examples</h2><p>
   See <a class="xref" href="xaggr.html" title="37.12. User-Defined Aggregates">Section 37.12</a>.
  </p></div><div class="refsect1" id="id-1.9.3.57.9"><h2>Compatibility</h2><p>
   <code class="command">CREATE AGGREGATE</code> is a
   <span class="productname">PostgreSQL</span> language extension.  The SQL
   standard does not provide for user-defined aggregate functions.
  </p></div><div class="refsect1" id="id-1.9.3.57.10"><h2>See Also</h2><span class="simplelist"><a class="xref" href="sql-alteraggregate.html" title="ALTER AGGREGATE"><span class="refentrytitle">ALTER AGGREGATE</span></a>, <a class="xref" href="sql-dropaggregate.html" title="DROP AGGREGATE"><span class="refentrytitle">DROP AGGREGATE</span></a></span></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sql-create-access-method.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-createcast.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">CREATE ACCESS METHOD </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> CREATE CAST</td></tr></table></div></body></html>