Add explicit alias to avoid debug output diff in pg15 (#6183)

src/test/regress/expected/local_table_join.out

@@ -1357,7 +1357,7 @@ select typdefault from (
         where typdefault > 'a'
         limit 1) as subq_0
     where (
-      select true from pg_catalog.pg_am limit 1
+      select true as bool from pg_catalog.pg_am limit 1
     )
   ) as subq_1
 ) as subq_2;
@@ -1387,7 +1387,7 @@ select typdefault from (
         where typdefault > 'a'
         limit 1) as subq_0
     where (
-      select true from pg_catalog.pg_am limit 1
+      select true as bool from pg_catalog.pg_am limit 1
     )
   ) as subq_1
 ) as subq_2;

src/test/regress/sql/local_table_join.sql

@@ -359,7 +359,7 @@ select typdefault from (
         where typdefault > 'a'
         limit 1) as subq_0
     where (
-      select true from pg_catalog.pg_am limit 1
+      select true as bool from pg_catalog.pg_am limit 1
     )
   ) as subq_1
 ) as subq_2;
@@ -376,7 +376,7 @@ select typdefault from (
         where typdefault > 'a'
         limit 1) as subq_0
     where (
-      select true from pg_catalog.pg_am limit 1
+      select true as bool from pg_catalog.pg_am limit 1
     )
   ) as subq_1
 ) as subq_2;