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Further fallout from the MergeAppend patch.

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Fix things so that top-N sorting can be used in child Sort nodes of a
MergeAppend node, when there is a LIMIT and no intervening joins or
grouping.  Actually doing this on the executor side isn't too bad,
but it's a bit messier to get the planner to cost it properly.
Per gripe from Robert Haas.

In passing, fix an oversight in the original top-N-sorting patch:
query_planner should not assume that a LIMIT can be used to make an
explicit sort cheaper when there will be grouping or aggregation in
between.  Possibly this should be back-patched, but I'm not sure the
mistake is serious enough to be a real problem in practice.
This commit is contained in:
Tom Lane
2010-11-18 00:30:10 -05:00
parent 45768d10e3
commit 6fbc323c80
7 changed files with 103 additions and 21 deletions
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18
src/backend/optimizer/plan/planner.c
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@ -968,6 +968,7 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
{
/* No set operations, do regular planning */
List *sub_tlist;
double sub_limit_tuples;
AttrNumber *groupColIdx = NULL;
bool need_tlist_eval = true;
QualCost tlist_cost;
@ -1119,13 +1120,28 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
else
root->query_pathkeys = NIL;
/*
* Figure out whether there's a hard limit on the number of rows that
* query_planner's result subplan needs to return. Even if we know a
* hard limit overall, it doesn't apply if the query has any
* grouping/aggregation operations.
*/
if (parse->groupClause ||
parse->distinctClause ||
parse->hasAggs ||
parse->hasWindowFuncs ||
root->hasHavingQual)
sub_limit_tuples = -1.0;
else
sub_limit_tuples = limit_tuples;
/*
* Generate the best unsorted and presorted paths for this Query (but
* note there may not be any presorted path). query_planner will also
* estimate the number of groups in the query, and canonicalize all
* the pathkeys.
*/
query_planner(root, sub_tlist, tuple_fraction, limit_tuples,
query_planner(root, sub_tlist, tuple_fraction, sub_limit_tuples,
&cheapest_path, &sorted_path, &dNumGroups);
/*