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Repair error noticed by Roberto Cornacchia: selectivity code

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was rejecting negative attnums as bogus, which of course they are not.
Add code to get_attdisbursion to produce a useful value for OID attribute,
since VACUUM does not store stats for system attributes.
Also, repair bug that's been in eqjoinsel for a long time: it was taking
the max of the two columns' disbursions, whereas it should use the min.
This commit is contained in:
Tom Lane
1999-09-09 02:36:04 +00:00
parent 45500964f6
commit 51db6455ea
5 changed files with 67 additions and 38 deletions
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32
src/backend/utils/adt/selfuncs.c
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@ -10,7 +10,7 @@
*
*
* IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/utils/adt/selfuncs.c,v 1.39 1999/08/21 00:56:18 tgl Exp $
* $Header: /cvsroot/pgsql/src/backend/utils/adt/selfuncs.c,v 1.40 1999/09/09 02:35:58 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -31,7 +31,7 @@
#include "utils/syscache.h"
/* N is not a valid var/constant or relation id */
#define NONVALUE(N) ((N) == -1)
#define NONVALUE(N) ((N) == 0)
/* are we looking at a functional index selectivity request? */
#define FunctionalSelectivity(nIndKeys,attNum) ((attNum)==InvalidAttrNumber)
@ -170,7 +170,9 @@ eqsel(Oid opid,
else
{
/* No VACUUM ANALYZE stats available, so make a guess using
* the disbursion stat (if we have that, which is unlikely...)
* the disbursion stat (if we have that, which is unlikely
* for a normal attribute; but for a system attribute we may
* be able to estimate it).
*/
selec = get_attdisbursion(relid, attno, 0.01);
}
@ -366,7 +368,7 @@ eqjoinsel(Oid opid,
float64 result;
float64data num1,
num2,
max;
min;
result = (float64) palloc(sizeof(float64data));
if (NONVALUE(attno1) || NONVALUE(relid1) ||
@ -376,11 +378,23 @@ eqjoinsel(Oid opid,
{
num1 = get_attdisbursion(relid1, attno1, 0.01);
num2 = get_attdisbursion(relid2, attno2, 0.01);
max = (num1 > num2) ? num1 : num2;
if (max <= 0)
*result = 1.0;
else
*result = max;
/*
* The join selectivity cannot be more than num2, since each
* tuple in table 1 could match no more than num2 fraction of
* tuples in table 2 (and that's only if the table-1 tuple
* matches the most common value in table 2, so probably it's
* less). By the same reasoning it is not more than num1.
* The min is therefore an upper bound.
*
* XXX can we make a better estimate here? Using the nullfrac
* statistic might be helpful, for example. Assuming the operator
* is strict (does not succeed for null inputs) then the selectivity
* couldn't be more than (1-nullfrac1)*(1-nullfrac2), which might
* be usefully small if there are many nulls. How about applying
* the operator to the most common values?
*/
min = (num1 < num2) ? num1 : num2;
*result = min;
}
return result;
}