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, 8 insertions, 0 deletions

diff --git a/src/include/parser/parse_node.h b/src/include/parser/parse_node.h
index 1a3792236a1..f589112d5e5 100644
--- a/src/include/parser/parse_node.h
+++ b/src/include/parser/parse_node.h
@@ -118,6 +118,13 @@ typedef Node *(*CoerceParamHook) (ParseState *pstate, Param *param,
  * This is one-for-one with p_rtable, but contains NULLs for non-join
  * RTEs, and may be shorter than p_rtable if the last RTE(s) aren't joins.
  *
+ * p_nullingrels: list of Bitmapsets associated with p_rtable entries, each
+ * containing the set of outer-join RTE indexes that can null that relation
+ * at the current point in the parse tree.  This is one-for-one with p_rtable,
+ * but may be shorter than p_rtable, in which case the missing entries are
+ * implicitly empty (NULL).  That rule allows us to save work when the query
+ * contains no outer joins.
+ *
  * p_joinlist: list of join items (RangeTblRef and JoinExpr nodes) that
  * will become the fromlist of the query's top-level FromExpr node.
  *
@@ -187,6 +194,7 @@ struct ParseState
 	List	   *p_rteperminfos; /* list of RTEPermissionInfo nodes for each
 								 * RTE_RELATION entry in rtable */
 	List	   *p_joinexprs;	/* JoinExprs for RTE_JOIN p_rtable entries */
+	List	   *p_nullingrels;	/* Bitmapsets showing nulling outer joins */
 	List	   *p_joinlist;		/* join items so far (will become FromExpr
 								 * node's fromlist) */
 	List	   *p_namespace;	/* currently-referenceable RTEs (List of