Postpone extParam/allParam calculations until the very end of planning.

Until now we computed these Param ID sets at the end of subquery_planner,
but that approach depends on subquery_planner returning a concrete Plan
tree.  We would like to switch over to returning one or more Paths for a
subquery, and in that representation the necessary details aren't fully
fleshed out (not to mention that we don't really want to do this work for
Paths that end up getting discarded).  Hence, refactor so that we can
compute the param ID sets at the end of planning, just before
set_plan_references is run.

The main change necessary to make this work is that we need to capture
the set of outer-level Param IDs available to the current query level
before exiting subquery_planner, since the outer levels' plan_params lists
are transient.  (That's not going to pose a problem for returning Paths,
since all the work involved in producing that data is part of expression
preprocessing, which will continue to happen before Paths are produced.)
On the plus side, this change gets rid of several existing kluges.

Eventually I'd like to get rid of SS_finalize_plan altogether in favor of
doing this work during set_plan_references, but that will require some
complex rejiggering because SS_finalize_plan needs to visit subplans and
initplans before the main plan.  So leave that idea for another day.

src/test/regress/expected/join.out

@@ -4889,6 +4889,63 @@ select * from
   0 | 9998 |  0
 (1 row)
 
+-- check proper extParam/allParam handling (this isn't exactly a LATERAL issue,
+-- but we can make the test case much more compact with LATERAL)
+explain (verbose, costs off)
+select * from (values (0), (1)) v(id),
+lateral (select * from int8_tbl t1,
+         lateral (select * from
+                    (select * from int8_tbl t2
+                     where q1 = any (select q2 from int8_tbl t3
+                                     where q2 = (select greatest(t1.q1,t2.q2))
+                                       and (select v.id=0)) offset 0) ss2) ss
+         where t1.q1 = ss.q2) ss0;
+                           QUERY PLAN                            
+-----------------------------------------------------------------
+ Nested Loop
+   Output: "*VALUES*".column1, t1.q1, t1.q2, ss2.q1, ss2.q2
+   ->  Seq Scan on public.int8_tbl t1
+         Output: t1.q1, t1.q2
+   ->  Nested Loop
+         Output: "*VALUES*".column1, ss2.q1, ss2.q2
+         ->  Values Scan on "*VALUES*"
+               Output: "*VALUES*".column1
+         ->  Subquery Scan on ss2
+               Output: ss2.q1, ss2.q2
+               Filter: (t1.q1 = ss2.q2)
+               ->  Seq Scan on public.int8_tbl t2
+                     Output: t2.q1, t2.q2
+                     Filter: (SubPlan 3)
+                     SubPlan 3
+                       ->  Result
+                             Output: t3.q2
+                             One-Time Filter: $4
+                             InitPlan 1 (returns $2)
+                               ->  Result
+                                     Output: GREATEST($0, t2.q2)
+                             InitPlan 2 (returns $4)
+                               ->  Result
+                                     Output: ($3 = 0)
+                             ->  Seq Scan on public.int8_tbl t3
+                                   Output: t3.q1, t3.q2
+                                   Filter: (t3.q2 = $2)
+(27 rows)
+
+select * from (values (0), (1)) v(id),
+lateral (select * from int8_tbl t1,
+         lateral (select * from
+                    (select * from int8_tbl t2
+                     where q1 = any (select q2 from int8_tbl t3
+                                     where q2 = (select greatest(t1.q1,t2.q2))
+                                       and (select v.id=0)) offset 0) ss2) ss
+         where t1.q1 = ss.q2) ss0;
+ id |        q1        |        q2         |        q1        |        q2        
+----+------------------+-------------------+------------------+------------------
+  0 | 4567890123456789 |               123 | 4567890123456789 | 4567890123456789
+  0 | 4567890123456789 |  4567890123456789 | 4567890123456789 | 4567890123456789
+  0 | 4567890123456789 | -4567890123456789 | 4567890123456789 | 4567890123456789
+(3 rows)
+
 -- test some error cases where LATERAL should have been used but wasn't
 select f1,g from int4_tbl a, (select f1 as g) ss;
 ERROR:  column "f1" does not exist

src/test/regress/sql/join.sql

@@ -1542,6 +1542,27 @@ select * from
            where f1 = any (select unique1 from tenk1
                            where unique2 = v.x offset 0)) ss;
 
+-- check proper extParam/allParam handling (this isn't exactly a LATERAL issue,
+-- but we can make the test case much more compact with LATERAL)
+explain (verbose, costs off)
+select * from (values (0), (1)) v(id),
+lateral (select * from int8_tbl t1,
+         lateral (select * from
+                    (select * from int8_tbl t2
+                     where q1 = any (select q2 from int8_tbl t3
+                                     where q2 = (select greatest(t1.q1,t2.q2))
+                                       and (select v.id=0)) offset 0) ss2) ss
+         where t1.q1 = ss.q2) ss0;
+
+select * from (values (0), (1)) v(id),
+lateral (select * from int8_tbl t1,
+         lateral (select * from
+                    (select * from int8_tbl t2
+                     where q1 = any (select q2 from int8_tbl t3
+                                     where q2 = (select greatest(t1.q1,t2.q2))
+                                       and (select v.id=0)) offset 0) ss2) ss
+         where t1.q1 = ss.q2) ss0;
+
 -- test some error cases where LATERAL should have been used but wasn't
 select f1,g from int4_tbl a, (select f1 as g) ss;
 select f1,g from int4_tbl a, (select a.f1 as g) ss;