Make Vars be outer-join-aware.

Traditionally we used the same Var struct to represent the value
of a table column everywhere in parse and plan trees.  This choice
predates our support for SQL outer joins, and it's really a pretty
bad idea with outer joins, because the Var's value can depend on
where it is in the tree: it might go to NULL above an outer join.
So expression nodes that are equal() per equalfuncs.c might not
represent the same value, which is a huge correctness hazard for
the planner.

To improve this, decorate Var nodes with a bitmapset showing
which outer joins (identified by RTE indexes) may have nulled
them at the point in the parse tree where the Var appears.
This allows us to trust that equal() Vars represent the same value.
A certain amount of klugery is still needed to cope with cases
where we re-order two outer joins, but it's possible to make it
work without sacrificing that core principle.  PlaceHolderVars
receive similar decoration for the same reason.

In the planner, we include these outer join bitmapsets into the relids
that an expression is considered to depend on, and in consequence also
add outer-join relids to the relids of join RelOptInfos.  This allows
us to correctly perceive whether an expression can be calculated above
or below a particular outer join.

This change affects FDWs that want to plan foreign joins.  They *must*
follow suit when labeling foreign joins in order to match with the
core planner, but for many purposes (if postgres_fdw is any guide)
they'd prefer to consider only base relations within the join.
To support both requirements, redefine ForeignScan.fs_relids as
base+OJ relids, and add a new field fs_base_relids that's set up by
the core planner.

Large though it is, this commit just does the minimum necessary to
install the new mechanisms and get check-world passing again.
Follow-up patches will perform some cleanup.  (The README additions
and comments mention some stuff that will appear in the follow-up.)

Patch by me; thanks to Richard Guo for review.

Discussion: https://postgr.es/m/830269.1656693747@sss.pgh.pa.us

1 files changed, 38 insertions, 2 deletions

diff --git a/src/backend/parser/parse_relation.c b/src/backend/parser/parse_relation.c
index b490541f03d..52b4a6e89df 100644
--- a/src/backend/parser/parse_relation.c
+++ b/src/backend/parser/parse_relation.c
@@ -763,6 +763,9 @@ scanNSItemForColumn(ParseState *pstate, ParseNamespaceItem *nsitem,
 	}
 	var->location = location;
 
+	/* Mark Var if it's nulled by any outer joins */
+	markNullableIfNeeded(pstate, var);
+
 	/* Require read access to the column */
 	markVarForSelectPriv(pstate, var);
 
@@ -1024,6 +1027,35 @@ searchRangeTableForCol(ParseState *pstate, const char *alias, const char *colnam
 }
 
 /*
+ * markNullableIfNeeded
+ *		If the RTE referenced by the Var is nullable by outer join(s)
+ *		at this point in the query, set var->varnullingrels to show that.
+ */
+void
+markNullableIfNeeded(ParseState *pstate, Var *var)
+{
+	int			rtindex = var->varno;
+	Bitmapset  *relids;
+
+	/* Find the appropriate pstate */
+	for (int lv = 0; lv < var->varlevelsup; lv++)
+		pstate = pstate->parentParseState;
+
+	/* Find currently-relevant join relids for the Var's rel */
+	if (rtindex > 0 && rtindex <= list_length(pstate->p_nullingrels))
+		relids = (Bitmapset *) list_nth(pstate->p_nullingrels, rtindex - 1);
+	else
+		relids = NULL;
+
+	/*
+	 * Merge with any already-declared nulling rels.  (Typically there won't
+	 * be any, but let's get it right if there are.)
+	 */
+	if (relids != NULL)
+		var->varnullingrels = bms_union(var->varnullingrels, relids);
+}
+
+/*
  * markRTEForSelectPriv
  *	   Mark the specified column of the RTE with index rtindex
  *	   as requiring SELECT privilege
@@ -3087,7 +3119,7 @@ expandTupleDesc(TupleDesc tupdesc, Alias *eref, int count, int offset,
  * the list elements mustn't be modified.
  */
 List *
-expandNSItemVars(ParseNamespaceItem *nsitem,
+expandNSItemVars(ParseState *pstate, ParseNamespaceItem *nsitem,
 				 int sublevels_up, int location,
 				 List **colnames)
 {
@@ -3123,6 +3155,10 @@ expandNSItemVars(ParseNamespaceItem *nsitem,
 			var->varnosyn = nscol->p_varnosyn;
 			var->varattnosyn = nscol->p_varattnosyn;
 			var->location = location;
+
+			/* ... and update varnullingrels */
+			markNullableIfNeeded(pstate, var);
+
 			result = lappend(result, var);
 			if (colnames)
 				*colnames = lappend(*colnames, colnameval);
@@ -3158,7 +3194,7 @@ expandNSItemAttrs(ParseState *pstate, ParseNamespaceItem *nsitem,
 			   *var;
 	List	   *te_list = NIL;
 
-	vars = expandNSItemVars(nsitem, sublevels_up, location, &names);
+	vars = expandNSItemVars(pstate, nsitem, sublevels_up, location, &names);
 
 	/*
 	 * Require read access to the table.  This is normally redundant with the