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95 lines
3.2 KiB
Plaintext
95 lines
3.2 KiB
Plaintext
.\" This is -*-nroff-*-
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.\" XXX standard disclaimer belongs here....
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.\" $Header: /cvsroot/pgsql/doc/man/Attic/create_aggregate.l,v 1.1.1.1 1996/08/18 22:14:21 scrappy Exp $
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.TH "CREATE AGGREGATE" SQL 11/05/95 Postgres95 Postgres95
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.SH NAME
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create aggregate \(em define a new aggregate
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.SH SYNOPSIS
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.nf
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\fBcreate aggregate\fR agg-name [\fBas\fR]
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\fB(\fP[\fBsfunc1\fR \fB=\fR state-transition-function-1
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,\fP \fBbasetype\fR \fB=\fR data-type
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,\fP \fBstype1\fR \fB=\fR sfunc1-return-type]
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[\fB,\fP \fBsfunc2\fR \fB=\fR state-transition-function-2
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,\fP \fBstype2\fR \fB=\fR sfunc2-return-type]
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[\fB,\fP \fBfinalfunc\fR \fB=\fR final-function]
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[\fB,\fP \fBinitcond1\fR \fB=\fR initial-condition-1]
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[\fB,\fP \fBinitcond2\fR \fB=\fR initial-condition-2]\fB)\fR
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.fi
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.SH DESCRIPTION
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An aggregate function can use up to three functions, two
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.IR "state transition"
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functions, X1 and X2:
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.nf
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X1( internal-state1, next-data_item ) ---> next-internal-state1
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X2( internal-state2 ) ---> next-internal-state2
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.fi
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and a
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.BR "final calculation"
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function, F:
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.nf
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F(internal-state1, internal-state2) ---> aggregate-value
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.fi
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These functions are required to have the following properties:
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.IP
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The arguments to state-transition-function-1 must be
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.BR ( stype1 , basetype ) ,
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and its return value must be stype1.
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.IP
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The argument and return value of state-transition-function-2 must be
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.BR stype2 .
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.IP
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The arguments to the final-calculation-function must be
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.BR ( stype1 , stype2 ) ,
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and its return value must be a POSTGRES base type (not
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necessarily the same as basetype.
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.IP
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The final-calculation-function should be specified if and only if both
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state-transition functions are specified.
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.PP
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Note that it is possible to specify aggregate functions that have
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varying combinations of state and final functions. For example, the
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\*(lqcount\*(rq aggregate requires
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.BR sfunc2
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(an incrementing function) but not
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.BR sfunc1 " or " finalfunc ,
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whereas the \*(lqsum\*(rq aggregate requires
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.BR sfunc1
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(an addition function) but not
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.BR sfunc2 " or " finalfunc
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and the \*(lqaverage\*(rq aggregate requires both of the above state
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functions as well as a
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.BR finalfunc
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(a division function) to produce its answer. In any case, at least
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one state function must be defined, and any
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.BR sfunc2
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must have a corresponding
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.BR initcond2 .
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.PP
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Aggregates also require two initial conditions, one for each
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transition function. These are specified and stored in the database
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as fields of type
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.IR text .
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.SH EXAMPLE
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This
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.IR avg
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aggregate consists of two state transition functions, a addition
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function and a incrementing function. These modify the internal state
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of the aggregate through a running sum and and the number of values
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seen so far. It accepts a new employee salary, increments the count,
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and adds the new salary to produce the next state. The state
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transition functions must be passed correct initialization values.
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The final calculation then divides the sum by the count to produce the
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final answer.
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.nf
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--
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--Create an aggregate for int4 average
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--
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create aggregate avg (sfunc1 = int4add, basetype = int4,
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stype1 = int4, sfunc2 = int4inc, stype2 = int4,
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finalfunc = int4div, initcond1 = "0", initcond2 = "0")
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.fi
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.SH "SEE ALSO"
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create function(l),
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remove aggregate(l).
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