diff options
Diffstat (limited to 'src/backend/optimizer/path/joinpath.c')
| -rw-r--r-- | src/backend/optimizer/path/joinpath.c | 31 |
1 files changed, 31 insertions, 0 deletions
diff --git a/src/backend/optimizer/path/joinpath.c b/src/backend/optimizer/path/joinpath.c index 4c30c655640..d9d48827a9a 100644 --- a/src/backend/optimizer/path/joinpath.c +++ b/src/backend/optimizer/path/joinpath.c @@ -504,6 +504,37 @@ get_resultcache_path(PlannerInfo *root, RelOptInfo *innerrel, return NULL; /* + * Result Cache normally marks cache entries as complete when it runs out + * of tuples to read from its subplan. However, with unique joins, Nested + * Loop will skip to the next outer tuple after finding the first matching + * inner tuple. This means that we may not read the inner side of the + * join to completion which leaves no opportunity to mark the cache entry + * as complete. To work around that, when the join is unique we + * automatically mark cache entries as complete after fetching the first + * tuple. This works when the entire join condition is parameterized. + * Otherwise, when the parameterization is only a subset of the join + * condition, we can't be sure which part of it causes the join to be + * unique. This means there are no guarantees that only 1 tuple will be + * read. We cannot mark the cache entry as complete after reading the + * first tuple without that guarantee. This means the scope of Result + * Cache's usefulness is limited to only outer rows that have no join + * partner as this is the only case where Nested Loop would exhaust the + * inner scan of a unique join. Since the scope is limited to that, we + * just don't bother making a result cache path in this case. + * + * Lateral vars needn't be considered here as they're not considered when + * determining if the join is unique. + * + * XXX this could be enabled if the remaining join quals were made part of + * the inner scan's filter instead of the join filter. Maybe it's worth + * considering doing that? + */ + if (extra->inner_unique && + list_length(inner_path->param_info->ppi_clauses) < + list_length(extra->restrictlist)) + return NULL; + + /* * We can't use a result cache if there are volatile functions in the * inner rel's target list or restrict list. A cache hit could reduce the * number of calls to these functions. |