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Change get_variable_numdistinct's API to flag default estimates explicitly. 

Formerly, callers tested for DEFAULT_NUM_DISTINCT, which had the problem
that a perfectly solid estimate might be mistaken for a content-free
default.
This commit is contained in:
Tom Lane 2011-09-04 15:41:49 -04:00
parent 1cb108efb0
commit 4c2777d0b7
2 changed files with 45 additions and 28 deletions
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70
src/backend/utils/adt/selfuncs.c
View File

@ -244,6 +244,7 @@ var_eq_const(VariableStatData *vardata, Oid operator,
bool varonleft)
{
double selec;
bool isdefault;
/*
* If the constant is NULL, assume operator is strict and return zero, ie,
@ -344,7 +345,7 @@ var_eq_const(VariableStatData *vardata, Oid operator,
* all the not-common values share this remaining fraction
* equally, so we divide by the number of other distinct values.
*/
otherdistinct = get_variable_numdistinct(vardata) - nnumbers;
otherdistinct = get_variable_numdistinct(vardata, &isdefault) - nnumbers;
if (otherdistinct > 1)
selec /= otherdistinct;
@ -366,7 +367,7 @@ var_eq_const(VariableStatData *vardata, Oid operator,
* of distinct values and assuming they are equally common. (The guess
* is unlikely to be very good, but we do know a few special cases.)
*/
selec = 1.0 / get_variable_numdistinct(vardata);
selec = 1.0 / get_variable_numdistinct(vardata, &isdefault);
}
/* result should be in range, but make sure... */
@ -384,6 +385,7 @@ var_eq_non_const(VariableStatData *vardata, Oid operator,
bool varonleft)
{
double selec;
bool isdefault;
/*
* If we matched the var to a unique index, assume there is exactly one
@ -414,7 +416,7 @@ var_eq_non_const(VariableStatData *vardata, Oid operator,
* idea?)
*/
selec = 1.0 - stats->stanullfrac;
ndistinct = get_variable_numdistinct(vardata);
ndistinct = get_variable_numdistinct(vardata, &isdefault);
if (ndistinct > 1)
selec /= ndistinct;
@ -441,7 +443,7 @@ var_eq_non_const(VariableStatData *vardata, Oid operator,
* of distinct values and assuming they are equally common. (The guess
* is unlikely to be very good, but we do know a few special cases.)
*/
selec = 1.0 / get_variable_numdistinct(vardata);
selec = 1.0 / get_variable_numdistinct(vardata, &isdefault);
}
/* result should be in range, but make sure... */
@ -2071,6 +2073,8 @@ eqjoinsel_inner(Oid operator,
double selec;
double nd1;
double nd2;
bool isdefault1;
bool isdefault2;
Form_pg_statistic stats1 = NULL;
Form_pg_statistic stats2 = NULL;
bool have_mcvs1 = false;
@ -2084,8 +2088,8 @@ eqjoinsel_inner(Oid operator,
float4 *numbers2 = NULL;
int nnumbers2 = 0;
nd1 = get_variable_numdistinct(vardata1);
nd2 = get_variable_numdistinct(vardata2);
nd1 = get_variable_numdistinct(vardata1, &isdefault1);
nd2 = get_variable_numdistinct(vardata2, &isdefault2);
if (HeapTupleIsValid(vardata1->statsTuple))
{
@ -2296,6 +2300,8 @@ eqjoinsel_semi(Oid operator,
double selec;
double nd1;
double nd2;
bool isdefault1;
bool isdefault2;
Form_pg_statistic stats1 = NULL;
bool have_mcvs1 = false;
Datum *values1 = NULL;
@ -2308,8 +2314,8 @@ eqjoinsel_semi(Oid operator,
float4 *numbers2 = NULL;
int nnumbers2 = 0;
nd1 = get_variable_numdistinct(vardata1);
nd2 = get_variable_numdistinct(vardata2);
nd1 = get_variable_numdistinct(vardata1, &isdefault1);
nd2 = get_variable_numdistinct(vardata2, &isdefault2);
/*
* We clamp nd2 to be not more than what we estimate the inner relation's
@ -2441,7 +2447,7 @@ eqjoinsel_semi(Oid operator,
* nd2 is default, punt and assume half of the uncertain rows have
* join partners.
*/
if (nd1 != DEFAULT_NUM_DISTINCT && nd2 != DEFAULT_NUM_DISTINCT)
if (!isdefault1 && !isdefault2)
{
nd1 -= nmatches;
nd2 -= nmatches;
@ -2464,7 +2470,7 @@ eqjoinsel_semi(Oid operator,
*/
double nullfrac1 = stats1 ? stats1->stanullfrac : 0.0;
if (nd1 != DEFAULT_NUM_DISTINCT && nd2 != DEFAULT_NUM_DISTINCT)
if (!isdefault1 && !isdefault2)
{
if (nd1 <= nd2 || nd2 < 0)
selec = 1.0 - nullfrac1;
@ -2955,9 +2961,10 @@ add_unique_group_var(PlannerInfo *root, List *varinfos,
{
GroupVarInfo *varinfo;
double ndistinct;
bool isdefault;
ListCell *lc;
ndistinct = get_variable_numdistinct(vardata);
ndistinct = get_variable_numdistinct(vardata, &isdefault);
/* cannot use foreach here because of possible list_delete */
lc = list_head(varinfos);
@ -3292,14 +3299,23 @@ estimate_hash_bucketsize(PlannerInfo *root, Node *hashkey, double nbuckets)
stanullfrac,
mcvfreq,
avgfreq;
bool isdefault;
float4 *numbers;
int nnumbers;
examine_variable(root, hashkey, 0, &vardata);
/* Get number of distinct values and fraction that are null */
ndistinct = get_variable_numdistinct(&vardata);
/* Get number of distinct values */
ndistinct = get_variable_numdistinct(&vardata, &isdefault);
/* If ndistinct isn't real, punt and return 0.1, per comments above */
if (isdefault)
{
ReleaseVariableStats(vardata);
return (Selectivity) 0.1;
}
/* Get fraction that are null */
if (HeapTupleIsValid(vardata.statsTuple))
{
Form_pg_statistic stats;
@ -3308,19 +3324,7 @@ estimate_hash_bucketsize(PlannerInfo *root, Node *hashkey, double nbuckets)
stanullfrac = stats->stanullfrac;
}
else
{
/*
* Believe a default ndistinct only if it came from stats. Otherwise
* punt and return 0.1, per comments above.
*/
if (ndistinct == DEFAULT_NUM_DISTINCT)
{
ReleaseVariableStats(vardata);
return (Selectivity) 0.1;
}
stanullfrac = 0.0;
}
/* Compute avg freq of all distinct data values in raw relation */
avgfreq = (1.0 - stanullfrac) / ndistinct;
@ -4414,16 +4418,20 @@ examine_simple_variable(PlannerInfo *root, Var *var,
* Estimate the number of distinct values of a variable.
*
* vardata: results of examine_variable
* *isdefault: set to TRUE if the result is a default rather than based on
* anything meaningful.
*
* NB: be careful to produce an integral result, since callers may compare
* the result to exact integer counts.
*/
double
get_variable_numdistinct(VariableStatData *vardata)
get_variable_numdistinct(VariableStatData *vardata, bool *isdefault)
{
double stadistinct;
double ntuples;
*isdefault = false;
/*
* Determine the stadistinct value to use. There are cases where we can
* get an estimate even without a pg_statistic entry, or can get a better
@ -4496,10 +4504,16 @@ get_variable_numdistinct(VariableStatData *vardata)
* Otherwise we need to get the relation size; punt if not available.
*/
if (vardata->rel == NULL)
{
*isdefault = true;
return DEFAULT_NUM_DISTINCT;
}
ntuples = vardata->rel->tuples;
if (ntuples <= 0.0)
{
*isdefault = true;
return DEFAULT_NUM_DISTINCT;
}
/*
* If we had a relative estimate, use that.
@ -4509,11 +4523,13 @@ get_variable_numdistinct(VariableStatData *vardata)
/*
* With no data, estimate ndistinct = ntuples if the table is small, else
* use default.
* use default. We use DEFAULT_NUM_DISTINCT as the cutoff for "small"
* so that the behavior isn't discontinuous.
*/
if (ntuples < DEFAULT_NUM_DISTINCT)
return ntuples;
*isdefault = true;
return DEFAULT_NUM_DISTINCT;
}

3
src/include/utils/selfuncs.h
View File

@ -121,7 +121,8 @@ extern void get_join_variables(PlannerInfo *root, List *args,
VariableStatData *vardata1,
VariableStatData *vardata2,
bool *join_is_reversed);
extern double get_variable_numdistinct(VariableStatData *vardata);
extern double get_variable_numdistinct(VariableStatData *vardata,
bool *isdefault);
extern double mcv_selectivity(VariableStatData *vardata, FmgrInfo *opproc,
Datum constval, bool varonleft,
double *sumcommonp);