Fix estimate_array_length error with set-operation array coercions

When a nested set operation's output type doesn't match the parent's
expected type, recurse_set_operations builds a projection target list
using generate_setop_tlist with varno 0.  If the required type
coercion involves an ArrayCoerceExpr, estimate_array_length could be
called on such a Var, and would pass it to examine_variable, which
errors in find_base_rel because varno 0 has no valid relation entry.

Fix by skipping the statistics lookup for Vars with varno 0.

Bug introduced by commit 9391f7152.  Back-patch to v17, where
estimate_array_length was taught to use statistics.

Reported-by: Justin Pryzby <pryzby@telsasoft.com>
Author: Tender Wang <tndrwang@gmail.com>
Reviewed-by: Richard Guo <guofenglinux@gmail.com>
Discussion: https://postgr.es/m/adjW8rfPDkplC7lF@pryzbyj2023
Backpatch-through: 17

src/backend/utils/adt/selfuncs.c

@@ -2165,6 +2165,18 @@ estimate_array_length(PlannerInfo *root, Node *arrayexpr)
 		AttStatsSlot sslot;
 		double		nelem = 0;
 
+		/*
+		 * Skip calling examine_variable for Var with varno 0, which has no
+		 * valid relation entry and would error in find_base_rel.  Such a Var
+		 * can appear when a nested set operation's output type doesn't match
+		 * the parent's expected type, because recurse_set_operations builds a
+		 * projection target list using generate_setop_tlist with varno 0, and
+		 * if the required type coercion involves an ArrayCoerceExpr, we can
+		 * be called on that Var.
+		 */
+		if (IsA(arrayexpr, Var) && ((Var *) arrayexpr)->varno == 0)
+			return 10;			/* default guess, should match scalararraysel */
+
 		examine_variable(root, arrayexpr, 0, &vardata);
 		if (HeapTupleIsValid(vardata.statsTuple))
 		{

src/test/regress/expected/union.out

@@ -1489,3 +1489,20 @@ on true limit 1;
                ->  Result
 (6 rows)
 
+-- Test handling of Vars with varno 0 in estimate_array_length
+explain (verbose, costs off)
+select null::int[] union all select null::int[] union all select null::bigint[];
+                 QUERY PLAN                  
+---------------------------------------------
+ Append
+   ->  Result
+         Output: (NULL::integer[])
+         ->  Append
+               ->  Result
+                     Output: NULL::integer[]
+               ->  Result
+                     Output: NULL::integer[]
+   ->  Result
+         Output: NULL::bigint[]
+(10 rows)
+

src/test/regress/sql/union.sql

@@ -577,3 +577,7 @@ select * from tenk1 t1
 left join lateral
   (select t1.tenthous from tenk2 t2 union all (values(1)))
 on true limit 1;
+
+-- Test handling of Vars with varno 0 in estimate_array_length
+explain (verbose, costs off)
+select null::int[] union all select null::int[] union all select null::bigint[];