diff options
Diffstat (limited to 'src')
| -rw-r--r-- | src/backend/utils/adt/selfuncs.c | 31 |
1 files changed, 21 insertions, 10 deletions
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c index 588d991fa57..987f2154459 100644 --- a/src/backend/utils/adt/selfuncs.c +++ b/src/backend/utils/adt/selfuncs.c @@ -4146,14 +4146,18 @@ estimate_hashagg_tablesize(PlannerInfo *root, Path *path, */ /* - * Find applicable ndistinct statistics for the given list of VarInfos (which - * must all belong to the given rel), and update *ndistinct to the estimate of - * the MVNDistinctItem that best matches. If a match it found, *varinfos is - * updated to remove the list of matched varinfos. + * Find the best matching ndistinct extended statistics for the given list of + * GroupVarInfos. * - * Varinfos that aren't for simple Vars are ignored. + * Callers must ensure that the given GroupVarInfos all belong to 'rel' and + * the GroupVarInfos list does not contain any duplicate Vars or expressions. * - * Return true if we're able to find a match, false otherwise. + * When statistics are found that match > 1 of the given GroupVarInfo, the + * *ndistinct parameter is set according to the ndistinct estimate and a new + * list is built with the matching GroupVarInfos removed, which is output via + * the *varinfos parameter before returning true. When no matching stats are + * found, false is returned and the *varinfos and *ndistinct parameters are + * left untouched. */ static bool estimate_multivariate_ndistinct(PlannerInfo *root, RelOptInfo *rel, @@ -4234,15 +4238,22 @@ estimate_multivariate_ndistinct(PlannerInfo *root, RelOptInfo *rel, } } + /* + * The ndistinct extended statistics contain estimates for a minimum + * of pairs of columns which the statistics are defined on and + * certainly not single columns. Here we skip unless we managed to + * match to at least two columns. + */ if (nshared_vars + nshared_exprs < 2) continue; /* - * Does this statistics object match more columns than the currently - * best object? If so, use this one instead. + * Check if these statistics are a better match than the previous best + * match and if so, take note of the StatisticExtInfo. * - * XXX This should break ties using name of the object, or something - * like that, to make the outcome stable. + * The statslist is sorted by statOid, so the StatisticExtInfo we + * select as the best match is deterministic even when multiple sets + * of statistics match equally as well. */ if ((nshared_exprs > nmatches_exprs) || (((nshared_exprs == nmatches_exprs)) && (nshared_vars > nmatches_vars))) |