diff options
Diffstat (limited to 'src/backend/optimizer/plan/subselect.c')
| -rw-r--r-- | src/backend/optimizer/plan/subselect.c | 33 |
1 files changed, 21 insertions, 12 deletions
diff --git a/src/backend/optimizer/plan/subselect.c b/src/backend/optimizer/plan/subselect.c index a5df56d9204..16f4a95a785 100644 --- a/src/backend/optimizer/plan/subselect.c +++ b/src/backend/optimizer/plan/subselect.c @@ -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); |