Fix planner problems with LATERAL references in PlaceHolderVars.

The planner largely failed to consider the possibility that a
PlaceHolderVar's expression might contain a lateral reference to a Var
coming from somewhere outside the PHV's syntactic scope.  We had a previous
report of a problem in this area, which I tried to fix in a quick-hack way
in commit 4da6439bd8, but Antonin Houska
pointed out that there were still some problems, and investigation turned
up other issues.  This patch largely reverts that commit in favor of a more
thoroughly thought-through solution.  The new theory is that a PHV's
ph_eval_at level cannot be higher than its original syntactic level.  If it
contains lateral references, those don't change the ph_eval_at level, but
rather they create a lateral-reference requirement for the ph_eval_at join
relation.  The code in joinpath.c needs to handle that.

Another issue is that createplan.c wasn't handling nested PlaceHolderVars
properly.

In passing, push knowledge of lateral-reference checks for join clauses
into join_clause_is_movable_to.  This is mainly so that FDWs don't need
to deal with it.

This patch doesn't fix the original join-qual-placement problem reported by
Jeremy Evans (and indeed, one of the new regression test cases shows the
wrong answer because of that).  But the PlaceHolderVar problems need to be
fixed before that issue can be addressed, so committing this separately
seems reasonable.

src/include/nodes/relation.h

@@ -339,6 +339,7 @@ typedef struct PlannerInfo
  *					   Vars and PlaceHolderVars)
  *		lateral_relids - required outer rels for LATERAL, as a Relids set
  *						 (for child rels this can be more than lateral_vars)
+ *		lateral_referencers - relids of rels that reference this one laterally
  *		indexlist - list of IndexOptInfo nodes for relation's indexes
  *					(always NIL if it's not a table)
  *		pages - number of disk pages in relation (zero if not a table)
@@ -432,6 +433,7 @@ typedef struct RelOptInfo
 	int32	   *attr_widths;	/* array indexed [min_attr .. max_attr] */
 	List	   *lateral_vars;	/* LATERAL Vars and PHVs referenced by rel */
 	Relids		lateral_relids; /* minimum parameterization of rel */
+	Relids		lateral_referencers;	/* rels that reference me laterally */
 	List	   *indexlist;		/* list of IndexOptInfo */
 	BlockNumber pages;			/* size estimates derived from pg_class */
 	double		tuples;
@@ -1344,30 +1346,38 @@ typedef struct SpecialJoinInfo
 /*
  * "Lateral join" info.
  *
- * Lateral references in subqueries constrain the join order in a way that's
- * somewhat like outer joins, though different in detail.  We construct one or
- * more LateralJoinInfos for each RTE with lateral references, and add them to
- * the PlannerInfo node's lateral_info_list.
+ * Lateral references constrain the join order in a way that's somewhat like
+ * outer joins, though different in detail.  We construct a LateralJoinInfo
+ * for each lateral cross-reference, placing them in the PlannerInfo node's
+ * lateral_info_list.
  *
- * lateral_rhs is the relid of a baserel with lateral references, and
- * lateral_lhs is a set of relids of baserels it references, all of which
- * must be present on the LHS to compute a parameter needed by the RHS.
- * Typically, lateral_lhs is a singleton, but it can include multiple rels
- * if the RHS references a PlaceHolderVar with a multi-rel ph_eval_at level.
- * We disallow joining to only part of the LHS in such cases, since that would
- * result in a join tree with no convenient place to compute the PHV.
+ * For unflattened LATERAL RTEs, we generate LateralJoinInfo(s) in which
+ * lateral_rhs is the relid of the LATERAL baserel, and lateral_lhs is a set
+ * of relids of baserels it references, all of which must be present on the
+ * LHS to compute a parameter needed by the RHS.  Typically, lateral_lhs is
+ * a singleton, but it can include multiple rels if the RHS references a
+ * PlaceHolderVar with a multi-rel ph_eval_at level.  We disallow joining to
+ * only part of the LHS in such cases, since that would result in a join tree
+ * with no convenient place to compute the PHV.
  *
  * When an appendrel contains lateral references (eg "LATERAL (SELECT x.col1
  * UNION ALL SELECT y.col2)"), the LateralJoinInfos reference the parent
  * baserel not the member otherrels, since it is the parent relid that is
  * considered for joining purposes.
+ *
+ * If any LATERAL RTEs were flattened into the parent query, it is possible
+ * that the query now contains PlaceHolderVars containing lateral references,
+ * representing expressions that need to be evaluated at particular spots in
+ * the jointree but contain lateral references to Vars from elsewhere.	These
+ * give rise to LateralJoinInfos in which lateral_rhs is the evaluation point
+ * of a PlaceHolderVar and lateral_lhs is the set of lateral rels it needs.
  */
 
 typedef struct LateralJoinInfo
 {
 	NodeTag		type;
-	Index		lateral_rhs;	/* a baserel containing lateral refs */
-	Relids		lateral_lhs;	/* some base relids it references */
+	Relids		lateral_lhs;	/* rels needed to compute a lateral value */
+	Relids		lateral_rhs;	/* rel where lateral value is needed */
 } LateralJoinInfo;
 
 /*
@@ -1465,6 +1475,10 @@ typedef struct AppendRelInfo
  * then allow it to bubble up like a Var until the ph_needed join level.
  * ph_needed has the same definition as attr_needed for a regular Var.
  *
+ * The PlaceHolderVar's expression might contain LATERAL references to vars
+ * coming from outside its syntactic scope.  If so, those rels are *not*
+ * included in ph_eval_at, but they are recorded in ph_lateral.
+ *
  * Notice that when ph_eval_at is a join rather than a single baserel, the
  * PlaceHolderInfo may create constraints on join order: the ph_eval_at join
  * has to be formed below any outer joins that should null the PlaceHolderVar.
@@ -1481,6 +1495,7 @@ typedef struct PlaceHolderInfo
 	Index		phid;			/* ID for PH (unique within planner run) */
 	PlaceHolderVar *ph_var;		/* copy of PlaceHolderVar tree */
 	Relids		ph_eval_at;		/* lowest level we can evaluate value at */
+	Relids		ph_lateral;		/* relids of contained lateral refs, if any */
 	Relids		ph_needed;		/* highest level the value is needed at */
 	int32		ph_width;		/* estimated attribute width */
 } PlaceHolderInfo;

src/include/optimizer/restrictinfo.h

@@ -41,7 +41,7 @@ extern List *extract_actual_clauses(List *restrictinfo_list,
 extern void extract_actual_join_clauses(List *restrictinfo_list,
 							List **joinquals,
 							List **otherquals);
-extern bool join_clause_is_movable_to(RestrictInfo *rinfo, Index baserelid);
+extern bool join_clause_is_movable_to(RestrictInfo *rinfo, RelOptInfo *baserel);
 extern bool join_clause_is_movable_into(RestrictInfo *rinfo,
 							Relids currentrelids,
 							Relids current_and_outer);