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Push limit through subqueries to underlying sort, where possible.

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Douglas Doole, reviewed by Ashutosh Bapat and by me.  Minor formatting
change by me.

Discussion: http://postgr.es/m/CADE5jYLuugnEEUsyW6Q_4mZFYTxHxaVCQmGAsF0yiY8ZDggi-w@mail.gmail.com
This commit is contained in:
Robert Haas
2017-08-21 14:43:01 -04:00
parent 79ccd7cbd5
commit 1f6d515a67
3 changed files with 124 additions and 0 deletions
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26
src/backend/executor/nodeLimit.c
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@ -308,6 +308,9 @@ recompute_limits(LimitState *node)
* since the MergeAppend surely need read no more than that many tuples from
* any one input. We also have to be prepared to look through a Result,
* since the planner might stick one atop MergeAppend for projection purposes.
* We can also accept one or more levels of subqueries that have no quals or
* SRFs (that is, each subquery is just projecting columns) between the LIMIT
* and any of the above.
*
* This is a bit of a kluge, but we don't have any more-abstract way of
* communicating between the two nodes; and it doesn't seem worth trying
@ -320,6 +323,29 @@ recompute_limits(LimitState *node)
static void
pass_down_bound(LimitState *node, PlanState *child_node)
{
/*
* If the child is a subquery that does no filtering (no predicates)
* and does not have any SRFs in the target list then we can potentially
* push the limit through the subquery. It is possible that we could have
* multiple subqueries, so tunnel through them all.
*/
while (IsA(child_node, SubqueryScanState))
{
SubqueryScanState *subqueryScanState;
subqueryScanState = (SubqueryScanState *) child_node;
/*
* Non-empty predicates or an SRF means we cannot push down the limit.
*/
if (subqueryScanState->ss.ps.qual != NULL ||
expression_returns_set((Node *) child_node->plan->targetlist))
return;
/* Use the child in the following checks */
child_node = subqueryScanState->subplan;
}
if (IsA(child_node, SortState))
{
SortState *sortState = (SortState *) child_node;