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Account for SRFs in targetlists in planner rowcount estimates.

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We made use of the ROWS estimate for set-returning functions used in FROM,
but not for those used in SELECT targetlists; which is a bit of an
oversight considering there are common usages that require the latter
approach.  Improve that.  (I had initially thought it might be worth
folding this into cost_qual_eval, but after investigation concluded that
that wouldn't be very helpful, so just do it separately.)  Per complaint
from David Johnston.

Back-patch to 9.2, but not further, for fear of destabilizing plan choices
in existing releases.
This commit is contained in:
Tom Lane
2012-07-21 17:45:07 -04:00
parent ed0af33247
commit 31c7c642b6
7 changed files with 120 additions and 48 deletions
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42
src/backend/optimizer/util/clauses.c
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@@ -661,10 +661,12 @@ find_window_functions_walker(Node *node, WindowFuncLists *lists)
/*
* expression_returns_set_rows
* Estimate the number of rows in a set result.
* Estimate the number of rows returned by a set-returning expression.
* The result is 1 if there are no set-returning functions.
*
* We use the product of the rowcount estimates of all the functions in
* the given tree. The result is 1 if there are no set-returning functions.
* the given tree (this corresponds to the behavior of ExecMakeFunctionResult
* for nested set-returning functions).
*
* Note: keep this in sync with expression_returns_set() in nodes/nodeFuncs.c.
*/
@@ -674,7 +676,7 @@ expression_returns_set_rows(Node *clause)
double result = 1;
(void) expression_returns_set_rows_walker(clause, &result);
return result;
return clamp_row_est(result);
}
static bool
@@ -736,6 +738,40 @@ expression_returns_set_rows_walker(Node *node, double *count)
(void *) count);
}
/*
* tlist_returns_set_rows
* Estimate the number of rows returned by a set-returning targetlist.
* The result is 1 if there are no set-returning functions.
*
* Here, the result is the largest rowcount estimate of any of the tlist's
* expressions, not the product as you would get from naively applying
* expression_returns_set_rows() to the whole tlist. The behavior actually
* implemented by ExecTargetList produces a number of rows equal to the least
* common multiple of the expression rowcounts, so that the product would be
* a worst-case estimate that is typically not realistic. Taking the max as
* we do here is a best-case estimate that might not be realistic either,
* but it's probably closer for typical usages. We don't try to compute the
* actual LCM because we're working with very approximate estimates, so their
* LCM would be unduly noisy.
*/
double
tlist_returns_set_rows(List *tlist)
{
double result = 1;
ListCell *lc;
foreach(lc, tlist)
{
TargetEntry *tle = (TargetEntry *) lfirst(lc);
double colresult;
colresult = expression_returns_set_rows((Node *) tle->expr);
if (result < colresult)
result = colresult;
}
return result;
}
/*****************************************************************************
* Subplan clause manipulation