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Fix some bogosities in the code that deals with estimating the fraction

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of tuples we are going to retrieve from a sub-SELECT.  Must have been
half asleep when I did this code the first time :-(
This commit is contained in:
Tom Lane
2000-03-14 02:23:15 +00:00
parent a1642089bf
commit 6217a8c7ba
3 changed files with 42 additions and 29 deletions
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33
src/backend/optimizer/plan/subselect.c
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@@ -7,7 +7,7 @@
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/optimizer/plan/subselect.c,v 1.30 2000/03/11 23:53:41 tgl Exp $
* $Header: /cvsroot/pgsql/src/backend/optimizer/plan/subselect.c,v 1.31 2000/03/14 02:23:15 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -134,25 +134,34 @@ make_subplan(SubLink *slink)
PlannerInitPlan = NULL;
PlannerQueryLevel++; /* we becomes child */
PlannerQueryLevel++; /* we become child */
/*
* For an EXISTS subplan, tell lower-level planner to expect that
* only the first tuple will be retrieved. For ALL, ANY, and MULTIEXPR
* subplans, we will be able to stop evaluating if the test condition
* fails, so very often not all the tuples will be retrieved; for lack
* of a better idea, specify 50% retrieval. For EXPR_SUBLINK use default
* behavior.
* only the first tuple will be retrieved. For ALL and ANY subplans,
* we will be able to stop evaluating if the test condition fails,
* so very often not all the tuples will be retrieved; for lack of a
* better idea, specify 50% retrieval. For EXPR and MULTIEXPR subplans,
* use default behavior (we're only expecting one row out, anyway).
*
* NOTE: if you change these numbers, also change cost_qual_eval_walker
* in costsize.c.
* NOTE: if you change these numbers, also change cost_qual_eval_walker()
* in path/costsize.c.
*
* XXX If an ALL/ANY subplan is uncorrelated, we may decide to materialize
* its result below. In that case it would've been better to specify
* full retrieval. At present, however, we can only detect correlation
* or lack of it after we've made the subplan :-(. Perhaps detection
* of correlation should be done as a separate step. Meanwhile, we don't
* want to be too optimistic about the percentage of tuples retrieved,
* for fear of selecting a plan that's bad for the materialization case.
*/
if (slink->subLinkType == EXISTS_SUBLINK)
tuple_fraction = 1.0; /* just like a LIMIT 1 */
else if (slink->subLinkType == EXPR_SUBLINK)
tuple_fraction = -1.0; /* default behavior */
else
else if (slink->subLinkType == ALL_SUBLINK ||
slink->subLinkType == ANY_SUBLINK)
tuple_fraction = 0.5; /* 50% */
else
tuple_fraction = -1.0; /* default behavior */
node->plan = plan = union_planner(subquery, tuple_fraction);