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Diffstat (limited to 'doc/src/sgml/ref/create_aggregate.sgml')
| -rw-r--r-- | doc/src/sgml/ref/create_aggregate.sgml | 223 |
1 files changed, 92 insertions, 131 deletions
diff --git a/doc/src/sgml/ref/create_aggregate.sgml b/doc/src/sgml/ref/create_aggregate.sgml index 44291458d60..07b45c112aa 100644 --- a/doc/src/sgml/ref/create_aggregate.sgml +++ b/doc/src/sgml/ref/create_aggregate.sgml @@ -1,5 +1,5 @@ <!-- -$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_aggregate.sgml,v 1.9 2000/03/31 14:57:05 tgl Exp $ +$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_aggregate.sgml,v 1.10 2000/07/17 03:04:41 tgl Exp $ Postgres documentation --> @@ -21,20 +21,18 @@ Postgres documentation </refnamediv> <refsynopsisdiv> <refsynopsisdivinfo> - <date>1999-07-20</date> + <date>2000-07-16</date> </refsynopsisdivinfo> <synopsis> -CREATE AGGREGATE <replaceable class="PARAMETER">name</replaceable> ( BASETYPE = <replaceable class="PARAMETER">input_data_type</replaceable> - [ , SFUNC1 = <replaceable class="PARAMETER">sfunc1</replaceable>, STYPE1 = <replaceable class="PARAMETER">state1_type</replaceable> ] - [ , SFUNC2 = <replaceable class="PARAMETER">sfunc2</replaceable>, STYPE2 = <replaceable class="PARAMETER">state2_type</replaceable> ] +CREATE AGGREGATE <replaceable class="PARAMETER">name</replaceable> ( BASETYPE = <replaceable class="PARAMETER">input_data_type</replaceable>, + SFUNC = <replaceable class="PARAMETER">sfunc</replaceable>, STYPE = <replaceable class="PARAMETER">state_type</replaceable> [ , FINALFUNC = <replaceable class="PARAMETER">ffunc</replaceable> ] - [ , INITCOND1 = <replaceable class="PARAMETER">initial_condition1</replaceable> ] - [ , INITCOND2 = <replaceable class="PARAMETER">initial_condition2</replaceable> ] ) + [ , INITCOND = <replaceable class="PARAMETER">initial_condition</replaceable> ] ) </synopsis> <refsect2 id="R2-SQL-CREATEAGGREGATE-1"> <refsect2info> - <date>1998-09-09</date> + <date>2000-07-16</date> </refsect2info> <title> Inputs @@ -55,57 +53,39 @@ CREATE AGGREGATE <replaceable class="PARAMETER">name</replaceable> ( BASETYPE = <listitem> <para> The input data type on which this aggregate function operates. + This can be specified as ANY for an aggregate that does not + examine its input values + (an example is <function>count(*)</function>). </para> </listitem> </varlistentry> <varlistentry> - <term><replaceable class="PARAMETER">sfunc1</replaceable></term> + <term><replaceable class="PARAMETER">sfunc</replaceable></term> <listitem> <para> - A state transition function - to be called for every non-NULL input data value. - This must be a function of two arguments, the first being of - type <replaceable class="PARAMETER">state1_type</replaceable> + The name of the state transition function + to be called for each input data value. + This is normally a function of two arguments, the first being of + type <replaceable class="PARAMETER">state_type</replaceable> and the second of type <replaceable class="PARAMETER">input_data_type</replaceable>. - The function must return a value of - type <replaceable class="PARAMETER">state1_type</replaceable>. - This function takes the current state value 1 and the current - input data item, and returns the next state value 1. - </para> - </listitem> - </varlistentry> - - <varlistentry> - <term><replaceable class="PARAMETER">state1_type</replaceable></term> - <listitem> - <para> - The data type for the first state value of the aggregate. - </para> - </listitem> - </varlistentry> - - <varlistentry> - <term><replaceable class="PARAMETER">sfunc2</replaceable></term> - <listitem> - <para> - A state transition function - to be called for every non-NULL input data value. - This must be a function of one argument of - type <replaceable class="PARAMETER">state2_type</replaceable>, - returning a value of the same type. - This function takes the current state value 2 and - returns the next state value 2. + Alternatively, for an aggregate that does not examine its input + values, the function takes just one argument of + type <replaceable class="PARAMETER">state_type</replaceable>. + In either case the function must return a value of + type <replaceable class="PARAMETER">state_type</replaceable>. + This function takes the current state value and the current + input data item, and returns the next state value. </para> </listitem> </varlistentry> <varlistentry> - <term><replaceable class="PARAMETER">state2_type</replaceable></term> + <term><replaceable class="PARAMETER">state_type</replaceable></term> <listitem> <para> - The data type for the second state value of the aggregate. + The data type for the aggregate's state value. </para> </listitem> </varlistentry> @@ -114,35 +94,28 @@ CREATE AGGREGATE <replaceable class="PARAMETER">name</replaceable> ( BASETYPE = <term><replaceable class="PARAMETER">ffunc</replaceable></term> <listitem> <para> - The final function called to compute the aggregate's result - after all input data has been traversed. - If both state values are used, the final function must - take two arguments of types - <replaceable class="PARAMETER">state1_type</replaceable> - and - <replaceable class="PARAMETER">state2_type</replaceable>. - If only one state value is used, the final function must - take a single argument of that state value's type. + The name of the final function called to compute the aggregate's + result after all input data has been traversed. The function + must take a single argument of type + <replaceable class="PARAMETER">state_type</replaceable>. The output datatype of the aggregate is defined as the return type of this function. + If <replaceable class="PARAMETER">ffunc</replaceable> + is not specified, then the ending state value is used as the + aggregate's result, and the output type is + <replaceable class="PARAMETER">state_type</replaceable>. </para> </listitem> </varlistentry> <varlistentry> - <term><replaceable class="PARAMETER">initial_condition1</replaceable></term> + <term><replaceable class="PARAMETER">initial_condition</replaceable></term> <listitem> <para> - The initial value for state value 1. - </para> - </listitem> - </varlistentry> - - <varlistentry> - <term><replaceable class="PARAMETER">initial_condition2</replaceable></term> - <listitem> - <para> - The initial value for state value 2. + The initial setting for the state value. This must be a literal + constant in the form accepted for the datatype + <replaceable class="PARAMETER">state_type</replaceable>. + If not specified, the state value starts out NULL. </para> </listitem> </varlistentry> @@ -177,7 +150,7 @@ CREATE <refsect1 id="R1-SQL-CREATEAGGREGATE-1"> <refsect1info> - <date>1998-09-09</date> + <date>2000-07-16</date> </refsect1info> <title> Description @@ -199,65 +172,76 @@ CREATE of the same name and input data type as an aggregate. </para> <para> - An aggregate function is made from between one and three ordinary + An aggregate function is made from one or two ordinary functions: - two state transition functions, - <replaceable class="PARAMETER">sfunc1</replaceable> - and <replaceable class="PARAMETER">sfunc2</replaceable>, - and a final calculation function, + a state transition function + <replaceable class="PARAMETER">sfunc</replaceable>, + and an optional final calculation function <replaceable class="PARAMETER">ffunc</replaceable>. These are used as follows: <programlisting> -<replaceable class="PARAMETER">sfunc1</replaceable>( internal-state1, next-data-item ) ---> next-internal-state1 -<replaceable class="PARAMETER">sfunc2</replaceable>( internal-state2 ) ---> next-internal-state2 -<replaceable class="PARAMETER">ffunc</replaceable>(internal-state1, internal-state2) ---> aggregate-value +<replaceable class="PARAMETER">sfunc</replaceable>( internal-state, next-data-item ) ---> next-internal-state +<replaceable class="PARAMETER">ffunc</replaceable>( internal-state ) ---> aggregate-value </programlisting> </para> <para> - <productname>Postgres</productname> creates one or two temporary variables - (of data types <replaceable class="PARAMETER">stype1</replaceable> and/or - <replaceable class="PARAMETER">stype2</replaceable>) to hold the - current internal states of the aggregate. At each input data item, - the state transition function(s) are invoked to calculate new values - for the internal state values. After all the data has been processed, + <productname>Postgres</productname> creates a temporary variable + of data type <replaceable class="PARAMETER">stype</replaceable> + to hold the current internal state of the aggregate. At each input + data item, + the state transition function is invoked to calculate a new + internal state value. After all the data has been processed, the final function is invoked once to calculate the aggregate's output - value. + value. If there is no final function then the ending state value + is returned as-is. </para> + <para> - <replaceable class="PARAMETER">ffunc</replaceable> must be specified if - both transition functions are specified. If only one transition function - is used, then <replaceable class="PARAMETER">ffunc</replaceable> is - optional. The default behavior when - <replaceable class="PARAMETER">ffunc</replaceable> is not provided is - to return the ending value of the internal state value being used - (and, therefore, the aggregate's output type is the same as that - state value's type). - </para> + An aggregate function may provide an initial condition, + that is, an initial value for the internal state value. + This is specified and stored in the database as a field of type + <type>text</type>, but it must be a valid external representation + of a constant of the state value datatype. If it is not supplied + then the state value starts out NULL. + </para> - <para> - An aggregate function may also provide one or two initial conditions, - that is, initial values for the internal state values being used. - These are specified and stored in the database as fields of type - <type>text</type>, but they must be valid external representations - of constants of the state value datatypes. If - <replaceable class="PARAMETER">sfunc1</replaceable> is specified - without an <replaceable class="PARAMETER">initcond1</replaceable> value, - then the system does not call - <replaceable class="PARAMETER">sfunc1</replaceable> - at the first input item; instead, the internal state value 1 is - initialized with the first input value, and - <replaceable class="PARAMETER">sfunc1</replaceable> is called beginning - at the second input item. This is useful for aggregates like MIN and - MAX. Note that an aggregate using this feature will return NULL when - called with no input values. There is no comparable provision for - state value 2; if <replaceable class="PARAMETER">sfunc2</replaceable> is - specified then an <replaceable class="PARAMETER">initcond2</replaceable> is - required. + <para> + If the state transition function is declared "strict" in pg_proc, + then it cannot be called with NULL inputs. With such a transition + function, aggregate execution behaves as follows. 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 the first + non-NULL input value replaces the state value, and the transition + function is invoked beginning with the second non-NULL input value. + This is handy for implementing aggregates like <function>max</function>. + Note that this behavior is only available when + <replaceable class="PARAMETER">state_type</replaceable> + is the same as + <replaceable class="PARAMETER">input_data_type</replaceable>. + When these types are different, you must supply a non-NULL initial + condition or use a non-strict transition function. + </para> + + <para> + If the state transition function is not strict, then it will be called + unconditionally at each input value, and must deal with NULL inputs + and NULL transition values for itself. This allows the aggregate + author to have full control over the aggregate's handling of NULLs. + </para> + + <para> + If the final function is declared "strict", then it will not + be called when the ending state value is NULL; instead a NULL result + will be output automatically. (Of course this is just the normal + behavior of strict functions.) In any case the final function has + the option of returning NULL. For example, the final function for + <function>avg</function> returns NULL when it sees there were zero + input tuples. </para> <refsect2 id="R2-SQL-CREATEAGGREGATE-3"> <refsect2info> - <date>1998-09-09</date> + <date>2000-07-16</date> </refsect2info> <title> Notes @@ -272,29 +256,6 @@ CREATE in any order, not just the order illustrated above. </para> - <para> - It is possible to specify aggregate functions - that have varying combinations of state and final functions. - For example, the <function>count</function> aggregate requires - <replaceable class="PARAMETER">sfunc2</replaceable> - (an incrementing function) but not - <replaceable class="PARAMETER">sfunc1</replaceable> or - <replaceable class="PARAMETER">ffunc</replaceable>, - whereas the <function>sum</function> aggregate requires - <replaceable class="PARAMETER">sfunc1</replaceable> (an addition - function) but not <replaceable class="PARAMETER">sfunc2</replaceable> or - <replaceable class="PARAMETER">ffunc</replaceable>, and the - <function>avg</function> - aggregate requires - both state functions as - well as a <replaceable class="PARAMETER">ffunc</replaceable> (a division - function) to produce its - answer. In any case, at least one state function must be - defined, and any <replaceable class="PARAMETER">sfunc2</replaceable> must - have a corresponding - <replaceable class="PARAMETER">initcond2</replaceable>. - </para> - </refsect2> </refsect1> |