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Comment on why planagg.c punts "MIN(x ORDER BY y)".

This commit is contained in:
Noah Misch 2013-07-16 20:14:37 -04:00
parent 4a87f308b3
commit 7a8e9f298e
1 changed files with 16 additions and 2 deletions
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18
src/backend/optimizer/plan/planagg.c
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@ -314,15 +314,29 @@ find_minmax_aggs_walker(Node *node, List **context)
ListCell *l;
Assert(aggref->agglevelsup == 0);
if (list_length(aggref->args) != 1 || aggref->aggorder != NIL)
if (list_length(aggref->args) != 1)
return true; /* it couldn't be MIN/MAX */
/*
* ORDER BY is usually irrelevant for MIN/MAX, but it can change the
* outcome if the aggsortop's operator class recognizes non-identical
* values as equal. For example, 4.0 and 4.00 are equal according to
* numeric_ops, yet distinguishable. If MIN() receives more than one
* value equal to 4.0 and no value less than 4.0, it is unspecified
* which of those equal values MIN() returns. An ORDER BY expression
* that differs for each of those equal values of the argument
* expression makes the result predictable once again. This is a
* niche requirement, and we do not implement it with subquery paths.
*/
if (aggref->aggorder != NIL)
return true;
/* note: we do not care if DISTINCT is mentioned ... */
curTarget = (TargetEntry *) linitial(aggref->args);
aggsortop = fetch_agg_sort_op(aggref->aggfnoid);
if (!OidIsValid(aggsortop))
return true; /* not a MIN/MAX aggregate */
curTarget = (TargetEntry *) linitial(aggref->args);
if (contain_mutable_functions((Node *) curTarget->expr))
return true; /* not potentially indexable */