Fix Assert failure induced by commit 215b43cdc.

I'd somehow talked myself into believing that set_append_rel_size
doesn't need to worry about getting back an AND clause when it applies
eval_const_expressions to the result of adjust_appendrel_attrs (that is,
transposing the appendrel parent's restriction clauses for one child).
But that is nonsense, and Andreas Seltenreich's fuzz tester soon
turned up a counterexample.  Put back the make_ands_implicit step
that was there before, and add a regression test covering the case.

Report: https://postgr.es/m/878tq6vja6.fsf@ansel.ydns.eu

src/backend/optimizer/path/allpaths.c

@@ -896,7 +896,7 @@ set_append_rel_size(PlannerInfo *root, RelOptInfo *rel,
 		{
 			RestrictInfo *rinfo = (RestrictInfo *) lfirst(lc);
 			Node	   *childqual;
-			bool		pseudoconstant;
+			ListCell   *lc2;
 
 			Assert(IsA(rinfo, RestrictInfo));
 			childqual = adjust_appendrel_attrs(root,
@@ -916,10 +916,16 @@ set_append_rel_size(PlannerInfo *root, RelOptInfo *rel,
 				/* Restriction reduces to constant TRUE, so drop it */
 				continue;
 			}
+			/* might have gotten an AND clause, if so flatten it */
+			foreach(lc2, make_ands_implicit((Expr *) childqual))
+			{
+				Node	   *onecq = (Node *) lfirst(lc2);
+				bool		pseudoconstant;
+
 				/* check for pseudoconstant (no Vars or volatile functions) */
 				pseudoconstant =
-				!contain_vars_of_level(childqual, 0) &&
+					!contain_vars_of_level(onecq, 0) &&
-				!contain_volatile_functions(childqual);
+					!contain_volatile_functions(onecq);
 				if (pseudoconstant)
 				{
 					/* tell createplan.c to check for gating quals */
@@ -927,7 +933,7 @@ set_append_rel_size(PlannerInfo *root, RelOptInfo *rel,
 				}
 				/* reconstitute RestrictInfo with appropriate properties */
 				childquals = lappend(childquals,
-								 make_restrictinfo((Expr *) childqual,
+									 make_restrictinfo((Expr *) onecq,
 													   rinfo->is_pushed_down,
 													rinfo->outerjoin_delayed,
 													   pseudoconstant,
@@ -936,6 +942,7 @@ set_append_rel_size(PlannerInfo *root, RelOptInfo *rel,
 				/* track minimum security level among child quals */
 				cq_min_security = Min(cq_min_security, rinfo->security_level);
 			}
+		}
 
 		/*
 		 * In addition to the quals inherited from the parent, we might have

src/test/regress/expected/union.out

@@ -720,3 +720,33 @@ select * from
 
 drop table t3;
 drop function expensivefunc(int);
+-- Test handling of appendrel quals that const-simplify into an AND
+explain (costs off)
+select * from
+  (select *, 0 as x from int8_tbl a
+   union all
+   select *, 1 as x from int8_tbl b) ss
+where (x = 0) or (q1 >= q2 and q1 <= q2);
+                 QUERY PLAN                  
+---------------------------------------------
+ Append
+   ->  Seq Scan on int8_tbl a
+   ->  Seq Scan on int8_tbl b
+         Filter: ((q1 >= q2) AND (q1 <= q2))
+(4 rows)
+
+select * from
+  (select *, 0 as x from int8_tbl a
+   union all
+   select *, 1 as x from int8_tbl b) ss
+where (x = 0) or (q1 >= q2 and q1 <= q2);
+        q1        |        q2         | x 
+------------------+-------------------+---
+              123 |               456 | 0
+              123 |  4567890123456789 | 0
+ 4567890123456789 |               123 | 0
+ 4567890123456789 |  4567890123456789 | 0
+ 4567890123456789 | -4567890123456789 | 0
+ 4567890123456789 |  4567890123456789 | 1
+(6 rows)
+

src/test/regress/sql/union.sql

@@ -322,3 +322,16 @@ select * from
 
 drop table t3;
 drop function expensivefunc(int);
+
+-- Test handling of appendrel quals that const-simplify into an AND
+explain (costs off)
+select * from
+  (select *, 0 as x from int8_tbl a
+   union all
+   select *, 1 as x from int8_tbl b) ss
+where (x = 0) or (q1 >= q2 and q1 <= q2);
+select * from
+  (select *, 0 as x from int8_tbl a
+   union all
+   select *, 1 as x from int8_tbl b) ss
+where (x = 0) or (q1 >= q2 and q1 <= q2);