Have the planner replace COUNT(ANY) with COUNT(*), when possible

This adds SupportRequestSimplifyAggref to allow pg_proc.prosupport functions to receive an Aggref and allow them to determine if there is a way that the Aggref call can be optimized. Also added is a support function to allow transformation of COUNT(ANY) into COUNT(*). This is possible to do when the given "ANY" cannot be NULL and also that there are no ORDER BY / DISTINCT clauses within the Aggref. This is a useful transformation to do as it is common that people write COUNT(1), which until now has added unneeded overhead. When counting a NOT NULL column. The overheads can be worse as that might mean deforming more of the tuple, which for large fact tables may be many columns in. It may be possible to add prosupport functions for other aggregates. We could consider if ORDER BY could be dropped for some calls, e.g. the ORDER BY is quite useless in MAX(c ORDER BY c). There is a little bit of passing fallout from adjusting expr_is_nonnullable() to handle Const which results in a plan change in the aggregates.out regression test. Previously, nothing was able to determine that "One-Time Filter: (100 IS NOT NULL)" was always true, therefore useless to include in the plan. Author: David Rowley <dgrowleyml@gmail.com> Reviewed-by: Corey Huinker <corey.huinker@gmail.com> Reviewed-by: Matheus Alcantara <matheusssilv97@gmail.com> Discussion: https://postgr.es/m/CAApHDvqGcPTagXpKfH=CrmHBqALpziThJEDs_MrPqjKVeDF9wA@mail.gmail.com

Author: david-rowley

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -2975,9 +2975,9 @@ select sum(t1.c1), count(t2.c1) from ft1 t1 inner join ft2 t2 on (t1.c1 = t2.c1)
  QUERY PLAN 
 ----------------------------------------------------------------------------------------------------------------------------
  Aggregate
- Output: sum(t1.c1), count(t2.c1)
+ Output: sum(t1.c1), count(*)
  -> Foreign Scan
- Output: t1.c1, t2.c1
+ Output: t1.c1
  Filter: (((((t1.c1 * t2.c1) / (t1.c1 * t2.c1)))::double precision * random()) <= '1'::double precision)
  Relations: (public.ft1 t1) INNER JOIN (public.ft2 t2)
  Remote SQL: SELECT r1."C 1", r2."C 1" FROM ("S 1"."T 1" r1 INNER JOIN "S 1"."T 1" r2 ON (((r2."C 1" = r1."C 1"))))
@@ -3073,12 +3073,12 @@ select c2 * (random() <= 1)::int as c2 from ft2 group by c2 * (random() <= 1)::i
 -- GROUP BY clause in various forms, cardinal, alias and constant expression
 explain (verbose, costs off)
 select count(c2) w, c2 x, 5 y, 7.0 z from ft1 group by 2, y, 9.0::int order by 2;
-  QUERY PLAN 
-------------------------------------------------------------------------------------------------------------
+ QUERY PLAN 
+-----------------------------------------------------------------------------------------------------------
  Foreign Scan
- Output: (count(c2)), c2, 5, 7.0, 9
+ Output: (count(*)), c2, 5, 7.0, 9
  Relations: Aggregate on (public.ft1)
- Remote SQL: SELECT count(c2), c2, 5, 7.0, 9 FROM "S 1"."T 1" GROUP BY 2, 3, 5 ORDER BY c2 ASC NULLS LAST
+ Remote SQL: SELECT count(*), c2, 5, 7.0, 9 FROM "S 1"."T 1" GROUP BY 2, 3, 5 ORDER BY c2 ASC NULLS LAST
 (4 rows)
 
 select count(c2) w, c2 x, 5 y, 7.0 z from ft1 group by 2, y, 9.0::int order by 2;
@@ -3379,8 +3379,8 @@ select distinct (select count(*) filter (where t2.c2 = 6 and t2.c1 < 10) from ft
 -- Inner query is aggregation query
 explain (verbose, costs off)
 select distinct (select count(t1.c1) filter (where t2.c2 = 6 and t2.c1 < 10) from ft1 t1 where t1.c1 = 6) from ft2 t2 where t2.c2 % 6 = 0 order by 1;
-  QUERY PLAN  
-------------------------------------------------------------------------------------------------------------------------------------------------------
+ QUERY PLAN 
+--------------------------------------------------------------------------------------------------------------------------------------------------
  Unique
  Output: ((SubPlan expr_1))
  -> Sort
@@ -3391,9 +3391,9 @@ select distinct (select count(t1.c1) filter (where t2.c2 = 6 and t2.c1 < 10) fro
  Remote SQL: SELECT "C 1", c2 FROM "S 1"."T 1" WHERE (((c2 % 6) = 0))
  SubPlan expr_1
  -> Foreign Scan
- Output: (count(t1.c1) FILTER (WHERE ((t2.c2 = 6) AND (t2.c1 < 10))))
+ Output: (count(*) FILTER (WHERE ((t2.c2 = 6) AND (t2.c1 < 10))))
  Relations: Aggregate on (public.ft1 t1)
- Remote SQL: SELECT count("C 1") FILTER (WHERE (($1::integer = 6) AND ($2::integer < 10))) FROM "S 1"."T 1" WHERE (("C 1" = 6))
+ Remote SQL: SELECT count(*) FILTER (WHERE (($1::integer = 6) AND ($2::integer < 10))) FROM "S 1"."T 1" WHERE (("C 1" = 6))
 (13 rows)
 
 select distinct (select count(t1.c1) filter (where t2.c2 = 6 and t2.c1 < 10) from ft1 t1 where t1.c1 = 6) from ft2 t2 where t2.c2 % 6 = 0 order by 1;

src/backend/optimizer/plan/initsplan.c

@@ -3413,22 +3413,6 @@ add_base_clause_to_rel(PlannerInfo *root, Index relid,
  restrictinfo->security_level);
 }
 
-/*
- * expr_is_nonnullable
- * Check to see if the Expr cannot be NULL
- *
- * Currently we only support simple Vars.
- */
-static bool
-expr_is_nonnullable(PlannerInfo *root, Expr *expr)
-{
-/* For now only check simple Vars */
-if (!IsA(expr, Var))
-return false;
-
-return var_is_nonnullable(root, (Var *) expr, true);
-}
-
 /*
  * restriction_is_always_true
  * Check to see if the RestrictInfo is always true.
@@ -3465,7 +3449,7 @@ restriction_is_always_true(PlannerInfo *root,
 if (nulltest->argisrow)
 return false;
 
-return expr_is_nonnullable(root, nulltest->arg);
+return expr_is_nonnullable(root, nulltest->arg, true);
 }
 
 /* If it's an OR, check its sub-clauses */
@@ -3530,7 +3514,7 @@ restriction_is_always_false(PlannerInfo *root,
 if (nulltest->argisrow)
 return false;
 
-return expr_is_nonnullable(root, nulltest->arg);
+return expr_is_nonnullable(root, nulltest->arg, true);
 }
 
 /* If it's an OR, check its sub-clauses */

src/backend/optimizer/util/clauses.c

@@ -131,6 +131,8 @@ static Expr *simplify_function(Oid funcid,
  Oid result_collid, Oid input_collid, List **args_p,
  bool funcvariadic, bool process_args, bool allow_non_const,
  eval_const_expressions_context *context);
+static Node *simplify_aggref(Aggref *aggref,
+ eval_const_expressions_context *context);
 static List *reorder_function_arguments(List *args, int pronargs,
 HeapTuple func_tuple);
 static List *add_function_defaults(List *args, int pronargs,
@@ -2634,6 +2636,9 @@ eval_const_expressions_mutator(Node *node,
 newexpr->location = expr->location;
 return (Node *) newexpr;
 }
+case T_Aggref:
+node = ece_generic_processing(node);
+return simplify_aggref((Aggref *) node, context);
 case T_OpExpr:
 {
 OpExpr *expr = (OpExpr *) node;
@@ -4200,6 +4205,50 @@ simplify_function(Oid funcid, Oid result_type, int32 result_typmod,
 return newexpr;
 }
 
+/*
+ * simplify_aggref
+ *	Call the Aggref.aggfnoid's prosupport function to allow it to
+ *	determine if simplification of the Aggref is possible. Returns the
+ *	newly simplified node if conversion took place; otherwise, returns the
+ *	original Aggref.
+ *
+ * See SupportRequestSimplifyAggref comments in supportnodes.h for further
+ * details.
+ */
+static Node *
+simplify_aggref(Aggref *aggref, eval_const_expressions_context *context)
+{
+Oidprosupport = get_func_support(aggref->aggfnoid);
+
+if (OidIsValid(prosupport))
+{
+SupportRequestSimplifyAggref req;
+Node *newnode;
+
+/*
+ * Build a SupportRequestSimplifyAggref node to pass to the support
+ * function.
+ */
+req.type = T_SupportRequestSimplifyAggref;
+req.root = context->root;
+req.aggref = aggref;
+
+newnode = (Node *) DatumGetPointer(OidFunctionCall1(prosupport,
+PointerGetDatum(&req)));
+
+/*
+ * We expect the support function to return either a new Node or NULL
+ * (when simplification isn't possible).
+ */
+Assert(newnode != (Node *) aggref || newnode == NULL);
+
+if (newnode != NULL)
+return newnode;
+}
+
+return (Node *) aggref;
+}
+
 /*
  * var_is_nonnullable: check to see if the Var cannot be NULL
  *
@@ -4261,6 +4310,30 @@ var_is_nonnullable(PlannerInfo *root, Var *var, bool use_rel_info)
 return false;
 }
 
+/*
+ * expr_is_nonnullable: check to see if the Expr cannot be NULL
+ *
+ * Returns true iff the given 'expr' cannot produce SQL NULLs.
+ *
+ * If 'use_rel_info' is true, nullability of Vars is checked via the
+ * corresponding RelOptInfo for the given Var. Some callers require
+ * nullability information before RelOptInfos are generated. These should
+ * pass 'use_rel_info' as false.
+ *
+ * For now, we only support Var and Const. Support for other node types may
+ * be possible.
+ */
+bool
+expr_is_nonnullable(PlannerInfo *root, Expr *expr, bool use_rel_info)
+{
+if (IsA(expr, Var))
+return var_is_nonnullable(root, (Var *) expr, use_rel_info);
+if (IsA(expr, Const))
+return !castNode(Const, expr)->constisnull;
+
+return false;
+}
+
 /*
  * expand_function_arguments: convert named-notation args to positional args
  * and/or insert default args, as needed

src/backend/utils/adt/int8.c

@@ -24,7 +24,7 @@
 #include "nodes/supportnodes.h"
 #include "optimizer/optimizer.h"
 #include "utils/builtins.h"
-
+#include "utils/fmgroids.h"
 
 typedef struct
 {
@@ -811,6 +811,53 @@ int8inc_support(PG_FUNCTION_ARGS)
 PG_RETURN_POINTER(req);
 }
 
+if (IsA(rawreq, SupportRequestSimplifyAggref))
+{
+SupportRequestSimplifyAggref *req = (SupportRequestSimplifyAggref *) rawreq;
+Aggref *agg = req->aggref;
+
+/*
+ * Check for COUNT(ANY) and try to convert to COUNT(*). The input
+ * argument cannot be NULL, we can't have an ORDER BY / DISTINCT in
+ * the aggregate, and agglevelsup must be 0.
+ *
+ * Technically COUNT(ANY) must have 1 arg, but be paranoid and check.
+ */
+if (agg->aggfnoid == F_COUNT_ANY && list_length(agg->args) == 1)
+{
+TargetEntry *tle = (TargetEntry *) linitial(agg->args);
+Expr *arg = tle->expr;
+
+/* Check for unsupported cases */
+if (agg->aggdistinct != NIL || agg->aggorder != NIL ||
+agg->agglevelsup != 0)
+PG_RETURN_POINTER(NULL);
+
+/* If the arg isn't NULLable, do the conversion */
+if (expr_is_nonnullable(req->root, arg, false))
+{
+Aggref *newagg;
+
+/* We don't expect these to have been set yet */
+Assert(agg->aggtransno == -1);
+Assert(agg->aggtranstype == InvalidOid);
+
+/* Convert COUNT(ANY) to COUNT(*) by making a new Aggref */
+newagg = makeNode(Aggref);
+memcpy(newagg, agg, sizeof(Aggref));
+newagg->aggfnoid = F_COUNT_;
+
+/* count(*) has no args */
+newagg->aggargtypes = NULL;
+newagg->args = NULL;
+newagg->aggstar = true;
+newagg->location = -1;
+
+PG_RETURN_POINTER(newagg);
+}
+}
+}
+
 PG_RETURN_POINTER(NULL);
 }
 

src/include/nodes/supportnodes.h

@@ -71,6 +71,31 @@ typedef struct SupportRequestSimplify
 FuncExpr *fcall;/* Function call to be simplified */
 } SupportRequestSimplify;
 
+/*
+ * Similar to SupportRequestSimplify but for Aggref node types.
+ *
+ * This supports conversions such as swapping COUNT(1) or COUNT(notnullcol)
+ * for COUNT(*).
+ *
+ * Supporting functions can consult 'root' and the input 'aggref'. When the
+ * implementing support function deems the simplification is possible, it must
+ * create a new Node (probably another Aggref) and not modify the original.
+ * The newly created Node should then be returned to indicate that the
+ * conversion is to take place. When no conversion is possible, a NULL
+ * pointer should be returned.
+ *
+ * It is important to consider that implementing support functions can receive
+ * Aggrefs with agglevelsup > 0. Careful consideration should be given to
+ * whether the simplification is still possible at levels above 0.
+ */
+typedef struct SupportRequestSimplifyAggref
+{
+NodeTagtype;
+
+PlannerInfo *root;/* Planner's infrastructure */
+Aggref *aggref;/* Aggref to be simplified */
+} SupportRequestSimplifyAggref;
+
 /*
  * The InlineInFrom request allows the support function to perform plan-time
  * simplification of a call to its target function that appears in FROM.

src/include/optimizer/optimizer.h

@@ -147,6 +147,9 @@ extern Expr *evaluate_expr(Expr *expr, Oid result_type, int32 result_typmod,
 
 extern bool var_is_nonnullable(PlannerInfo *root, Var *var, bool use_rel_info);
 
+extern bool expr_is_nonnullable(PlannerInfo *root, Expr *expr,
+bool use_rel_info);
+
 extern List *expand_function_arguments(List *args, bool include_out_arguments,
  Oid result_type,
  HeapTuple func_tuple);