Move resolution of AlternativeSubPlan choices to the planner.

When commit bd3daddaf introduced AlternativeSubPlans, I had some
ambitions towards allowing the choice of subplan to change during
execution.  That has not happened, or even been thought about, in the
ensuing twelve years; so it seems like a failed experiment.  So let's
rip that out and resolve the choice of subplan at the end of planning
(in setrefs.c) rather than during executor startup.  This has a number
of positive benefits:

* Removal of a few hundred lines of executor code, since
AlternativeSubPlans need no longer be supported there.

* Removal of executor-startup overhead (particularly, initialization
of subplans that won't be used).

* Removal of incidental costs of having a larger plan tree, such as
tree-scanning and copying costs in the plancache; not to mention
setrefs.c's own costs of processing the discarded subplans.

* EXPLAIN no longer has to print a weird (and undocumented)
representation of an AlternativeSubPlan choice; it sees only the
subplan actually used.  This should mean less confusion for users.

* Since setrefs.c knows which subexpression of a plan node it's
working on at any instant, it's possible to adjust the estimated
number of executions of the subplan based on that.  For example,
we should usually estimate more executions of a qual expression
than a targetlist expression.  The implementation used here is
pretty simplistic, because we don't want to expend a lot of cycles
on the issue; but it's better than ignoring the point entirely,
as the executor had to.

That last point might possibly result in shifting the choice
between hashed and non-hashed EXISTS subplans in a few cases,
but in general this patch isn't meant to change planner choices.
Since we're doing the resolution so late, it's really impossible
to change any plan choices outside the AlternativeSubPlan itself.

Patch by me; thanks to David Rowley for review.

Discussion: https://postgr.es/m/1992952.1592785225@sss.pgh.pa.us

6 files changed, 4 insertions, 27 deletions

diff --git a/src/include/executor/execExpr.h b/src/include/executor/execExpr.h
index dbe8649a576..b792de1bc95 100644
--- a/src/include/executor/execExpr.h
+++ b/src/include/executor/execExpr.h
@@ -218,7 +218,6 @@ typedef enum ExprEvalOp
 	EEOP_GROUPING_FUNC,
 	EEOP_WINDOW_FUNC,
 	EEOP_SUBPLAN,
-	EEOP_ALTERNATIVE_SUBPLAN,
 
 	/* aggregation related nodes */
 	EEOP_AGG_STRICT_DESERIALIZE,
@@ -589,13 +588,6 @@ typedef struct ExprEvalStep
 			SubPlanState *sstate;
 		}			subplan;
 
-		/* for EEOP_ALTERNATIVE_SUBPLAN */
-		struct
-		{
-			/* out-of-line state, created by nodeSubplan.c */
-			AlternativeSubPlanState *asstate;
-		}			alternative_subplan;
-
 		/* for EEOP_AGG_*DESERIALIZE */
 		struct
 		{
@@ -734,8 +726,6 @@ extern void ExecEvalXmlExpr(ExprState *state, ExprEvalStep *op);
 extern void ExecEvalGroupingFunc(ExprState *state, ExprEvalStep *op);
 extern void ExecEvalSubPlan(ExprState *state, ExprEvalStep *op,
 							ExprContext *econtext);
-extern void ExecEvalAlternativeSubPlan(ExprState *state, ExprEvalStep *op,
-									   ExprContext *econtext);
 extern void ExecEvalWholeRowVar(ExprState *state, ExprEvalStep *op,
 								ExprContext *econtext);
 extern void ExecEvalSysVar(ExprState *state, ExprEvalStep *op,
diff --git a/src/include/executor/nodeSubplan.h b/src/include/executor/nodeSubplan.h
index 83e90b3d07b..b629af1f5fb 100644
--- a/src/include/executor/nodeSubplan.h
+++ b/src/include/executor/nodeSubplan.h
@@ -18,12 +18,8 @@
 
 extern SubPlanState *ExecInitSubPlan(SubPlan *subplan, PlanState *parent);
 
-extern AlternativeSubPlanState *ExecInitAlternativeSubPlan(AlternativeSubPlan *asplan, PlanState *parent);
-
 extern Datum ExecSubPlan(SubPlanState *node, ExprContext *econtext, bool *isNull);
 
-extern Datum ExecAlternativeSubPlan(AlternativeSubPlanState *node, ExprContext *econtext, bool *isNull);
-
 extern void ExecReScanSetParamPlan(SubPlanState *node, PlanState *parent);
 
 extern void ExecSetParamPlan(SubPlanState *node, ExprContext *econtext);
diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index a5ab1aed14d..ef448d67c77 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -877,18 +877,6 @@ typedef struct SubPlanState
 	ExprState  *cur_eq_comp;	/* equality comparator for LHS vs. table */
 } SubPlanState;
 
-/* ----------------
- *		AlternativeSubPlanState node
- * ----------------
- */
-typedef struct AlternativeSubPlanState
-{
-	NodeTag		type;
-	AlternativeSubPlan *subplan;	/* expression plan node */
-	List	   *subplans;		/* SubPlanStates of alternative subplans */
-	int			active;			/* list index of the one we're using */
-} AlternativeSubPlanState;
-
 /*
  * DomainConstraintState - one item to check during CoerceToDomain
  *
diff --git a/src/include/nodes/nodes.h b/src/include/nodes/nodes.h
index 381d84b4e4f..7ddd8c011bf 100644
--- a/src/include/nodes/nodes.h
+++ b/src/include/nodes/nodes.h
@@ -213,7 +213,6 @@ typedef enum NodeTag
 	T_WindowFuncExprState,
 	T_SetExprState,
 	T_SubPlanState,
-	T_AlternativeSubPlanState,
 	T_DomainConstraintState,
 
 	/*
diff --git a/src/include/nodes/pathnodes.h b/src/include/nodes/pathnodes.h
index 485d1b06c91..dbe86e7af65 100644
--- a/src/include/nodes/pathnodes.h
+++ b/src/include/nodes/pathnodes.h
@@ -347,6 +347,7 @@ struct PlannerInfo
 	bool		hasHavingQual;	/* true if havingQual was non-null */
 	bool		hasPseudoConstantQuals; /* true if any RestrictInfo has
 										 * pseudoconstant = true */
+	bool		hasAlternativeSubPlans; /* true if we've made any of those */
 	bool		hasRecursion;	/* true if planning a recursive WITH item */
 
 	/* These fields are used only when hasRecursion is true: */
diff --git a/src/include/nodes/primnodes.h b/src/include/nodes/primnodes.h
index d73be2ad46c..fd65ee8f9c5 100644
--- a/src/include/nodes/primnodes.h
+++ b/src/include/nodes/primnodes.h
@@ -736,6 +736,9 @@ typedef struct SubPlan
 /*
  * AlternativeSubPlan - expression node for a choice among SubPlans
  *
+ * This is used only transiently during planning: by the time the plan
+ * reaches the executor, all AlternativeSubPlan nodes have been removed.
+ *
  * The subplans are given as a List so that the node definition need not
  * change if there's ever more than two alternatives.  For the moment,
  * though, there are always exactly two; and the first one is the fast-start