Improve planner's ability to recognize cases where an IN's RHS is unique.

If the right-hand side of a semijoin is unique, then we can treat it like a
normal join (or another way to say that is: we don't need to explicitly
unique-ify the data before doing it as a normal join).  We were recognizing
such cases when the RHS was a sub-query with appropriate DISTINCT or GROUP
BY decoration, but there's another way: if the RHS is a plain relation with
unique indexes, we can check if any of the indexes prove the output is
unique.  Most of the infrastructure for that was there already in the join
removal code, though I had to rearrange it a bit.  Per reflection about a
recent example in pgsql-performance.

1 files changed, 25 insertions, 2 deletions

diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index 6aa34412def..1e7aac95ef4 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -1021,8 +1021,8 @@ create_unique_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
 	pathnode->path.parent = rel;
 
 	/*
-	 * Treat the output as always unsorted, since we don't necessarily have
-	 * pathkeys to represent it.
+	 * Assume the output is unsorted, since we don't necessarily have pathkeys
+	 * to represent it.  (This might get overridden below.)
 	 */
 	pathnode->path.pathkeys = NIL;
 
@@ -1031,6 +1031,29 @@ create_unique_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
 	pathnode->uniq_exprs = uniq_exprs;
 
 	/*
+	 * If the input is a relation and it has a unique index that proves the
+	 * uniq_exprs are unique, then we don't need to do anything.  Note that
+	 * relation_has_unique_index_for automatically considers restriction
+	 * clauses for the rel, as well.
+	 */
+	if (rel->rtekind == RTE_RELATION && all_btree &&
+		relation_has_unique_index_for(root, rel, NIL,
+									  uniq_exprs, in_operators))
+	{
+		pathnode->umethod = UNIQUE_PATH_NOOP;
+		pathnode->rows = rel->rows;
+		pathnode->path.startup_cost = subpath->startup_cost;
+		pathnode->path.total_cost = subpath->total_cost;
+		pathnode->path.pathkeys = subpath->pathkeys;
+
+		rel->cheapest_unique_path = (Path *) pathnode;
+
+		MemoryContextSwitchTo(oldcontext);
+
+		return pathnode;
+	}
+
+	/*
 	 * If the input is a subquery whose output must be unique already, then we
 	 * don't need to do anything.  The test for uniqueness has to consider
 	 * exactly which columns we are extracting; for example "SELECT DISTINCT