PG18: Add _plan_json() test helper and switch columnar index tests to JSON plan checks (#8299)

fixes #8263

* Introduces `columnar_test_helpers._plan_json(q text) -> jsonb`, which
runs `EXPLAIN (FORMAT JSON, COSTS OFF, ANALYZE OFF)` and returns the
plan as `jsonb`. This lets us assert on plan structure instead of text
matching.

* Updates `columnar_indexes.sql` tests to detect whether any index-based
scan is used by searching the JSON plan’s `"Node Type"` (e.g., `Index
Scan`, `Index Only Scan`, `Bitmap Index Scan`).

## Notable changes

* New helper in `src/test/regress/sql/columnar_test_helpers.sql`:

* `EXECUTE format('EXPLAIN (FORMAT JSON, COSTS OFF, ANALYZE OFF) %s', q)
INTO j;`
  * Returns the `jsonb` plan for downstream assertions.
  
  ```sql
  SELECT NOT jsonb_path_exists(
columnar_test_helpers._plan_json('SELECT b FROM columnar_table WHERE b =
30000'),
'$[*].Plan.** ? (@."Node Type" like_regex "^(Index|Bitmap
Index).*Scan$")'
  ) AS uses_no_index_scan;
  ```

to verify no index scan occurs at the partial index boundary (`b =
30000`) and that an index scan is used where expected (`b = 30001`).

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/test/regress/expected/columnar_indexes.out

@@ -176,12 +176,15 @@ SELECT pg_total_relation_size('columnar_table_b_idx') * 5 <
 (1 row)
 
 -- can't use index scan due to partial index boundaries
-EXPLAIN (COSTS OFF) SELECT b FROM columnar_table WHERE b = 30000;
-         QUERY PLAN
+SELECT NOT jsonb_path_exists(
+         columnar_test_helpers._plan_json('SELECT b FROM columnar_table WHERE b = 30000'),
+         -- Regex matches any index-based scan: "Index Scan", "Index Only Scan", "Bitmap Index Scan".
+         '$[*].Plan.** ? (@."Node Type" like_regex "^(Index|Bitmap Index).*Scan$")'
+       ) AS uses_no_index_scan; -- expect: t
+ uses_no_index_scan
 ---------------------------------------------------------------------
- Seq Scan on columnar_table
-   Filter: (b = 30000)
-(2 rows)
+ t
+(1 row)
 
 -- can use index scan
 EXPLAIN (COSTS OFF) SELECT b FROM columnar_table WHERE b = 30001;

src/test/regress/expected/columnar_test_helpers.out

@@ -146,3 +146,11 @@ BEGIN
     RETURN NEXT;
   END LOOP;
 END; $$ language plpgsql;
+CREATE OR REPLACE FUNCTION _plan_json(q text)
+RETURNS jsonb
+LANGUAGE plpgsql AS $$
+DECLARE j jsonb;
+BEGIN
+  EXECUTE format('EXPLAIN (FORMAT JSON, COSTS OFF, ANALYZE OFF) %s', q) INTO j;
+  RETURN j;
+END $$;

src/test/regress/sql/columnar_indexes.sql

@@ -114,7 +114,11 @@ SELECT pg_total_relation_size('columnar_table_b_idx') * 5 <
        pg_total_relation_size('columnar_table_a_idx');
 
 -- can't use index scan due to partial index boundaries
-EXPLAIN (COSTS OFF) SELECT b FROM columnar_table WHERE b = 30000;
+SELECT NOT jsonb_path_exists(
+         columnar_test_helpers._plan_json('SELECT b FROM columnar_table WHERE b = 30000'),
+         -- Regex matches any index-based scan: "Index Scan", "Index Only Scan", "Bitmap Index Scan".
+         '$[*].Plan.** ? (@."Node Type" like_regex "^(Index|Bitmap Index).*Scan$")'
+       ) AS uses_no_index_scan; -- expect: t
 -- can use index scan
 EXPLAIN (COSTS OFF) SELECT b FROM columnar_table WHERE b = 30001;
 

src/test/regress/sql/columnar_test_helpers.sql

@@ -158,3 +158,12 @@ BEGIN
     RETURN NEXT;
   END LOOP;
 END; $$ language plpgsql;
+
+CREATE OR REPLACE FUNCTION _plan_json(q text)
+RETURNS jsonb
+LANGUAGE plpgsql AS $$
+DECLARE j jsonb;
+BEGIN
+  EXECUTE format('EXPLAIN (FORMAT JSON, COSTS OFF, ANALYZE OFF) %s', q) INTO j;
+  RETURN j;
+END $$;