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Adjust the API for aggregate function calls so that a C-coded function

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can tell whether it is being used as an aggregate or not.  This allows
such a function to avoid re-pallocing a pass-by-reference transition
value; normally it would be unsafe for a function to scribble on an input,
but in the aggregate case it's safe to reuse the old transition value.
Make int8inc() do this.  This gets a useful improvement in the speed of
COUNT(*), at least on narrow tables (it seems to be swamped by I/O when
the table rows are wide).  Per a discussion in early December with
Neil Conway.  I also fixed int_aggregate.c to check this, thereby
turning it into something approaching a supportable technique instead
of being a crude hack.
This commit is contained in:
Tom Lane
2005-03-12 20:25:06 +00:00
parent de004e44e2
commit fa5e44017a
6 changed files with 159 additions and 70 deletions
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21
src/backend/executor/nodeAgg.c
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@ -40,12 +40,28 @@
* is used to run finalize functions and compute the output tuple;
* this context can be reset once per output tuple.
*
* Beginning in PostgreSQL 8.1, the executor's AggState node is passed as
* the fmgr "context" value in all transfunc and finalfunc calls. It is
* not really intended that the transition functions will look into the
* AggState node, but they can use code like
* if (fcinfo->context && IsA(fcinfo->context, AggState))
* to verify that they are being called by nodeAgg.c and not as ordinary
* SQL functions. The main reason a transition function might want to know
* that is that it can avoid palloc'ing a fixed-size pass-by-ref transition
* value on every call: it can instead just scribble on and return its left
* input. Ordinarily it is completely forbidden for functions to modify
* pass-by-ref inputs, but in the aggregate case we know the left input is
* either the initial transition value or a previous function result, and
* in either case its value need not be preserved. See int8inc() for an
* example. Notice that advance_transition_function() is coded to avoid a
* data copy step when the previous transition value pointer is returned.
*
*
* Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/executor/nodeAgg.c,v 1.128 2005/01/28 19:33:56 tgl Exp $
* $PostgreSQL: pgsql/src/backend/executor/nodeAgg.c,v 1.129 2005/03/12 20:25:06 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -363,7 +379,7 @@ advance_transition_function(AggState *aggstate,
*/
/* MemSet(&fcinfo, 0, sizeof(fcinfo)); */
fcinfo.context = NULL;
fcinfo.context = (void *) aggstate;
fcinfo.resultinfo = NULL;
fcinfo.isnull = false;
@ -541,6 +557,7 @@ finalize_aggregate(AggState *aggstate,
FunctionCallInfoData fcinfo;
MemSet(&fcinfo, 0, sizeof(fcinfo));
fcinfo.context = (void *) aggstate;
fcinfo.flinfo = &peraggstate->finalfn;
fcinfo.nargs = 1;
fcinfo.arg[0] = pergroupstate->transValue;