In the planner, replace an empty FROM clause with a dummy RTE.

The fact that "SELECT expression" has no base relations has long been a
thorn in the side of the planner.  It makes it hard to flatten a sub-query
that looks like that, or is a trivial VALUES() item, because the planner
generally uses relid sets to identify sub-relations, and such a sub-query
would have an empty relid set if we flattened it.  prepjointree.c contains
some baroque logic that works around this in certain special cases --- but
there is a much better answer.  We can replace an empty FROM clause with a
dummy RTE that acts like a table of one row and no columns, and then there
are no such corner cases to worry about.  Instead we need some logic to
get rid of useless dummy RTEs, but that's simpler and covers more cases
than what was there before.

For really trivial cases, where the query is just "SELECT expression" and
nothing else, there's a hazard that adding the extra RTE makes for a
noticeable slowdown; even though it's not much processing, there's not
that much for the planner to do overall.  However testing says that the
penalty is very small, close to the noise level.  In more complex queries,
this is able to find optimizations that we could not find before.

The new RTE type is called RTE_RESULT, since the "scan" plan type it
gives rise to is a Result node (the same plan we produced for a "SELECT
expression" query before).  To avoid confusion, rename the old ResultPath
path type to GroupResultPath, reflecting that it's only used in degenerate
grouping cases where we know the query produces just one grouped row.
(It wouldn't work to unify the two cases, because there are different
rules about where the associated quals live during query_planner.)

Note: although this touches readfuncs.c, I don't think a catversion
bump is required, because the added case can't occur in stored rules,
only plans.

Patch by me, reviewed by David Rowley and Mark Dilger

Discussion: https://postgr.es/m/15944.1521127664@sss.pgh.pa.us

6 files changed, 21 insertions, 11 deletions

diff --git a/src/include/nodes/nodes.h b/src/include/nodes/nodes.h
index 10dac60cd35..4808a9e76d4 100644
--- a/src/include/nodes/nodes.h
+++ b/src/include/nodes/nodes.h
@@ -237,7 +237,7 @@ typedef enum NodeTag
 	T_HashPath,
 	T_AppendPath,
 	T_MergeAppendPath,
-	T_ResultPath,
+	T_GroupResultPath,
 	T_MaterialPath,
 	T_UniquePath,
 	T_GatherPath,
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index addc2c2ec7a..4ec8a835417 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -950,7 +950,10 @@ typedef enum RTEKind
 	RTE_TABLEFUNC,				/* TableFunc(.., column list) */
 	RTE_VALUES,					/* VALUES (<exprlist>), (<exprlist>), ... */
 	RTE_CTE,					/* common table expr (WITH list element) */
-	RTE_NAMEDTUPLESTORE			/* tuplestore, e.g. for AFTER triggers */
+	RTE_NAMEDTUPLESTORE,		/* tuplestore, e.g. for AFTER triggers */
+	RTE_RESULT					/* RTE represents an empty FROM clause; such
+								 * RTEs are added by the planner, they're not
+								 * present during parsing or rewriting */
 } RTEKind;
 
 typedef struct RangeTblEntry
diff --git a/src/include/nodes/relation.h b/src/include/nodes/relation.h
index 34300613615..420ca05c30a 100644
--- a/src/include/nodes/relation.h
+++ b/src/include/nodes/relation.h
@@ -324,7 +324,6 @@ typedef struct PlannerInfo
 									 * partitioned table */
 	bool		hasJoinRTEs;	/* true if any RTEs are RTE_JOIN kind */
 	bool		hasLateralRTEs; /* true if any RTEs are marked LATERAL */
-	bool		hasDeletedRTEs; /* true if any RTE was deleted from jointree */
 	bool		hasHavingQual;	/* true if havingQual was non-null */
 	bool		hasPseudoConstantQuals; /* true if any RestrictInfo has
 										 * pseudoconstant = true */
@@ -1345,17 +1344,17 @@ typedef struct MergeAppendPath
 } MergeAppendPath;
 
 /*
- * ResultPath represents use of a Result plan node to compute a variable-free
- * targetlist with no underlying tables (a "SELECT expressions" query).
- * The query could have a WHERE clause, too, represented by "quals".
+ * GroupResultPath represents use of a Result plan node to compute the
+ * output of a degenerate GROUP BY case, wherein we know we should produce
+ * exactly one row, which might then be filtered by a HAVING qual.
  *
  * Note that quals is a list of bare clauses, not RestrictInfos.
  */
-typedef struct ResultPath
+typedef struct GroupResultPath
 {
 	Path		path;
 	List	   *quals;
-} ResultPath;
+} GroupResultPath;
 
 /*
  * MaterialPath represents use of a Material plan node, i.e., caching of
diff --git a/src/include/optimizer/cost.h b/src/include/optimizer/cost.h
index e7005b4a0cf..623f733d7df 100644
--- a/src/include/optimizer/cost.h
+++ b/src/include/optimizer/cost.h
@@ -105,6 +105,8 @@ extern void cost_ctescan(Path *path, PlannerInfo *root,
 			 RelOptInfo *baserel, ParamPathInfo *param_info);
 extern void cost_namedtuplestorescan(Path *path, PlannerInfo *root,
 						 RelOptInfo *baserel, ParamPathInfo *param_info);
+extern void cost_resultscan(Path *path, PlannerInfo *root,
+				RelOptInfo *baserel, ParamPathInfo *param_info);
 extern void cost_recursive_union(Path *runion, Path *nrterm, Path *rterm);
 extern void cost_sort(Path *path, PlannerInfo *root,
 		  List *pathkeys, Cost input_cost, double tuples, int width,
@@ -196,6 +198,7 @@ extern void set_cte_size_estimates(PlannerInfo *root, RelOptInfo *rel,
 					   double cte_rows);
 extern void set_tablefunc_size_estimates(PlannerInfo *root, RelOptInfo *rel);
 extern void set_namedtuplestore_size_estimates(PlannerInfo *root, RelOptInfo *rel);
+extern void set_result_size_estimates(PlannerInfo *root, RelOptInfo *rel);
 extern void set_foreign_size_estimates(PlannerInfo *root, RelOptInfo *rel);
 extern PathTarget *set_pathtarget_cost_width(PlannerInfo *root, PathTarget *target);
 extern double compute_bitmap_pages(PlannerInfo *root, RelOptInfo *baserel,
diff --git a/src/include/optimizer/pathnode.h b/src/include/optimizer/pathnode.h
index bd905d3328e..aaaf3f4ff50 100644
--- a/src/include/optimizer/pathnode.h
+++ b/src/include/optimizer/pathnode.h
@@ -75,8 +75,10 @@ extern MergeAppendPath *create_merge_append_path(PlannerInfo *root,
 						 List *pathkeys,
 						 Relids required_outer,
 						 List *partitioned_rels);
-extern ResultPath *create_result_path(PlannerInfo *root, RelOptInfo *rel,
-				   PathTarget *target, List *resconstantqual);
+extern GroupResultPath *create_group_result_path(PlannerInfo *root,
+						 RelOptInfo *rel,
+						 PathTarget *target,
+						 List *havingqual);
 extern MaterialPath *create_material_path(RelOptInfo *rel, Path *subpath);
 extern UniquePath *create_unique_path(PlannerInfo *root, RelOptInfo *rel,
 				   Path *subpath, SpecialJoinInfo *sjinfo);
@@ -105,6 +107,8 @@ extern Path *create_ctescan_path(PlannerInfo *root, RelOptInfo *rel,
 					Relids required_outer);
 extern Path *create_namedtuplestorescan_path(PlannerInfo *root, RelOptInfo *rel,
 								Relids required_outer);
+extern Path *create_resultscan_path(PlannerInfo *root, RelOptInfo *rel,
+					   Relids required_outer);
 extern Path *create_worktablescan_path(PlannerInfo *root, RelOptInfo *rel,
 						  Relids required_outer);
 extern ForeignPath *create_foreignscan_path(PlannerInfo *root, RelOptInfo *rel,
@@ -275,7 +279,6 @@ extern Relids min_join_parameterization(PlannerInfo *root,
 						  Relids joinrelids,
 						  RelOptInfo *outer_rel,
 						  RelOptInfo *inner_rel);
-extern RelOptInfo *build_empty_join_rel(PlannerInfo *root);
 extern RelOptInfo *fetch_upper_rel(PlannerInfo *root, UpperRelationKind kind,
 				Relids relids);
 extern Relids find_childrel_parents(PlannerInfo *root, RelOptInfo *rel);
diff --git a/src/include/optimizer/prep.h b/src/include/optimizer/prep.h
index 62d45dd1426..a03a024ce93 100644
--- a/src/include/optimizer/prep.h
+++ b/src/include/optimizer/prep.h
@@ -21,11 +21,13 @@
 /*
  * prototypes for prepjointree.c
  */
+extern void replace_empty_jointree(Query *parse);
 extern void pull_up_sublinks(PlannerInfo *root);
 extern void inline_set_returning_functions(PlannerInfo *root);
 extern void pull_up_subqueries(PlannerInfo *root);
 extern void flatten_simple_union_all(PlannerInfo *root);
 extern void reduce_outer_joins(PlannerInfo *root);
+extern void remove_useless_result_rtes(PlannerInfo *root);
 extern Relids get_relids_in_jointree(Node *jtnode, bool include_joins);
 extern Relids get_relids_for_join(PlannerInfo *root, int joinrelid);