diff options
Diffstat (limited to 'src/backend/optimizer/path')
| -rw-r--r-- | src/backend/optimizer/path/allpaths.c | 20 |
1 files changed, 13 insertions, 7 deletions
diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c index 9c6436eb72f..41233b98373 100644 --- a/src/backend/optimizer/path/allpaths.c +++ b/src/backend/optimizer/path/allpaths.c @@ -1810,9 +1810,11 @@ add_paths_to_append_rel(PlannerInfo *root, RelOptInfo *rel, * We generate a path for each ordering (pathkey list) appearing in * all_child_pathkeys. * - * We consider both cheapest-startup and cheapest-total cases, ie, for each - * interesting ordering, collect all the cheapest startup subpaths and all the - * cheapest total paths, and build a suitable path for each case. + * We consider the cheapest-startup and cheapest-total cases, and also the + * cheapest-fractional case when not all tuples need to be retrieved. For each + * interesting ordering, we collect all the cheapest startup subpaths, all the + * cheapest total paths, and, if applicable, all the cheapest fractional paths, + * and build a suitable path for each case. * * We don't currently generate any parameterized ordered paths here. While * it would not take much more code here to do so, it's very unclear that it @@ -1977,14 +1979,18 @@ generate_orderedappend_paths(PlannerInfo *root, RelOptInfo *rel, double path_fraction = root->tuple_fraction; /* - * Merge Append considers only live children relations. Dummy - * relations must be filtered out before. + * We should not have a dummy child relation here. However, + * we cannot use childrel->rows to compute the tuple fraction, + * as childrel can be an upper relation with an unset row + * estimate. Instead, we use the row estimate from the + * cheapest_total path, which should already have been forced + * to a sane value. */ - Assert(childrel->rows > 0); + Assert(cheapest_total->rows > 0); /* Convert absolute limit to a path fraction */ if (path_fraction >= 1.0) - path_fraction /= childrel->rows; + path_fraction /= cheapest_total->rows; cheapest_fractional = get_cheapest_fractional_path_for_pathkeys(childrel->pathlist, |