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Change patternsel() so that instead of switching from a pure

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pattern-examination heuristic method to purely histogram-driven selectivity at
histogram size 100, we compute both estimates and use a weighted average.
The weight put on the heuristic estimate decreases linearly with histogram
size, dropping to zero for 100 or more histogram entries.
Likewise in ltreeparentsel().  After a patch by Greg Stark, though I
reorganized the logic a bit to give the caller of histogram_selectivity()
more control.
This commit is contained in:
Tom Lane
2008-03-09 00:32:09 +00:00
parent 422495d0da
commit f4230d2937
3 changed files with 75 additions and 35 deletions
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27
contrib/ltree/ltree_op.c
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@ -1,7 +1,7 @@
/*
* op function for ltree
* Teodor Sigaev <teodor@stack.net>
* $PostgreSQL: pgsql/contrib/ltree/ltree_op.c,v 1.16 2007/02/28 22:44:38 tgl Exp $
* $PostgreSQL: pgsql/contrib/ltree/ltree_op.c,v 1.17 2008/03/09 00:32:09 tgl Exp $
*/
#include "ltree.h"
@ -609,6 +609,7 @@ ltreeparentsel(PG_FUNCTION_ARGS)
double mcvsum;
double mcvsel;
double nullfrac;
int hist_size;
fmgr_info(get_opcode(operator), &contproc);
@ -626,21 +627,31 @@ ltreeparentsel(PG_FUNCTION_ARGS)
*/
selec = histogram_selectivity(&vardata, &contproc,
constval, varonleft,
100, 1);
10, 1, &hist_size);
if (selec < 0)
{
/* Nope, fall back on default */
selec = DEFAULT_PARENT_SEL;
}
else
else if (hist_size < 100)
{
/* Yes, but don't believe extremely small or large estimates. */
if (selec < 0.0001)
selec = 0.0001;
else if (selec > 0.9999)
selec = 0.9999;
/*
* For histogram sizes from 10 to 100, we combine the
* histogram and default selectivities, putting increasingly
* more trust in the histogram for larger sizes.
*/
double hist_weight = hist_size / 100.0;
selec = selec * hist_weight +
DEFAULT_PARENT_SEL * (1.0 - hist_weight);
}
/* In any case, don't believe extremely small or large estimates. */
if (selec < 0.0001)
selec = 0.0001;
else if (selec > 0.9999)
selec = 0.9999;
if (HeapTupleIsValid(vardata.statsTuple))
nullfrac = ((Form_pg_statistic) GETSTRUCT(vardata.statsTuple))->stanullfrac;
else