diff options
Diffstat (limited to 'src/backend/optimizer/path/joinpath.c')
| -rw-r--r-- | src/backend/optimizer/path/joinpath.c | 41 |
1 files changed, 31 insertions, 10 deletions
diff --git a/src/backend/optimizer/path/joinpath.c b/src/backend/optimizer/path/joinpath.c index c02c9052262..b9f64ccc5ec 100644 --- a/src/backend/optimizer/path/joinpath.c +++ b/src/backend/optimizer/path/joinpath.c @@ -117,13 +117,14 @@ add_paths_to_joinrel(PlannerInfo *root, /* * Decide whether it's sensible to generate parameterized paths for this * joinrel, and if so, which relations such paths should require. There - * is no need to create a parameterized result path unless there is a join - * order restriction that prevents joining one of our input rels directly - * to the parameter source rel instead of joining to the other input rel. - * This restriction reduces the number of parameterized paths we have to - * deal with at higher join levels, without compromising the quality of - * the resulting plan. We express the restriction as a Relids set that - * must overlap the parameterization of any proposed join path. + * is usually no need to create a parameterized result path unless there + * is a join order restriction that prevents joining one of our input rels + * directly to the parameter source rel instead of joining to the other + * input rel. (But see exception in try_nestloop_path.) This restriction + * reduces the number of parameterized paths we have to deal with at + * higher join levels, without compromising the quality of the resulting + * plan. We express the restriction as a Relids set that must overlap the + * parameterization of any proposed join path. */ foreach(lc, root->join_info_list) { @@ -291,9 +292,29 @@ try_nestloop_path(PlannerInfo *root, if (required_outer && !bms_overlap(required_outer, param_source_rels)) { - /* Waste no memory when we reject a path here */ - bms_free(required_outer); - return; + /* + * We override the param_source_rels heuristic to accept nestloop + * paths in which the outer rel satisfies some but not all of the + * inner path's parameterization. This is necessary to get good plans + * for star-schema scenarios, in which a parameterized path for a + * large table may require parameters from multiple small tables that + * will not get joined directly to each other. We can handle that by + * stacking nestloops that have the small tables on the outside; but + * this breaks the rule the param_source_rels heuristic is based on, + * namely that parameters should not be passed down across joins + * unless there's a join-order-constraint-based reason to do so. So + * ignore the param_source_rels restriction when this case applies. + */ + Relids outerrelids = outer_path->parent->relids; + Relids innerparams = PATH_REQ_OUTER(inner_path); + + if (!(bms_overlap(innerparams, outerrelids) && + bms_nonempty_difference(innerparams, outerrelids))) + { + /* Waste no memory when we reject a path here */ + bms_free(required_outer); + return; + } } /* |