diff options
| -rw-r--r-- | src/backend/optimizer/path/pathkeys.c | 29 |
1 files changed, 16 insertions, 13 deletions
diff --git a/src/backend/optimizer/path/pathkeys.c b/src/backend/optimizer/path/pathkeys.c index 18f88700981..e2fdcd3163c 100644 --- a/src/backend/optimizer/path/pathkeys.c +++ b/src/backend/optimizer/path/pathkeys.c @@ -587,22 +587,25 @@ pathkey_sort_cost_comparator(const void *_a, const void *_b) /* * get_cheapest_group_keys_order - * Reorders the group pathkeys/clauses to minimize the comparison cost. + * Reorders the group pathkeys / clauses to minimize the comparison cost. * - * Given a list of pathkeys, we try to reorder them in a way that minimizes - * the CPU cost of sorting. This depends mainly on the cost of comparator - * function (the pathkeys may use different data types) and the number of - * distinct values in each column (which affects the number of comparator - * calls for the following pathkeys). + * Given the list of pathkeys in '*group_pathkeys', we try to arrange these + * in an order that minimizes the sort costs that will be incurred by the + * GROUP BY. The costs mainly depend on the cost of the sort comparator + * function(s) and the number of distinct values in each column of the GROUP + * BY clause (*group_clauses). Sorting on subsequent columns is only required + * for tiebreak situations where two values sort equally. * * In case the input is partially sorted, only the remaining pathkeys are - * considered. - * - * Returns true if the keys/clauses have been reordered (or might have been), - * and a new list is returned through an argument. The list is a new copy - * and may be freed using list_free. - * - * Returns false if no reordering was possible. + * considered. 'n_preordered' denotes how many of the leading *group_pathkeys + * the input is presorted by. + * + * Returns true and sets *group_pathkeys and *group_clauses to the newly + * ordered versions of the lists that were passed in via these parameters. + * If no reordering was deemed necessary then we return false, in which case + * the *group_pathkeys and *group_clauses lists are left untouched. The + * original *group_pathkeys and *group_clauses parameter values are never + * destructively modified in place. */ static bool get_cheapest_group_keys_order(PlannerInfo *root, double nrows, |