Stabilize table_checks across PG15–PG18: switch to pg_constraint, remove dupes, exclude NOT NULL (#8140)

DESCRIPTION: Stabilize table_checks across PG15–PG18: switch to
pg_constraint, remove dupes, exclude NOT NUL

fixes #8138
fixes #8131 

**Problem**

```diff
diff -dU10 -w /__w/citus/citus/src/test/regress/expected/multi_create_table_constraints.out /__w/citus/citus/src/test/regress/results/multi_create_table_constraints.out
--- /__w/citus/citus/src/test/regress/expected/multi_create_table_constraints.out.modified	2025-08-18 12:26:51.991598284 +0000
+++ /__w/citus/citus/src/test/regress/results/multi_create_table_constraints.out.modified	2025-08-18 12:26:52.004598519 +0000
@@ -403,22 +403,30 @@
     relid = 'check_example_partition_col_key_365068'::regclass;
     Column     |  Type   |  Definition   
 ---------------+---------+---------------
  partition_col | integer | partition_col
 (1 row)
 
 SELECT "Constraint", "Definition" FROM table_checks WHERE relid='public.check_example_365068'::regclass;
              Constraint              |            Definition             
 -------------------------------------+-----------------------------------
  check_example_other_col_check       | CHECK other_col >= 100
+ check_example_other_col_check       | CHECK other_col >= 100
+ check_example_other_col_check       | CHECK other_col >= 100
+ check_example_other_col_check       | CHECK other_col >= 100
+ check_example_other_col_check       | CHECK other_col >= 100
  check_example_other_other_col_check | CHECK abs(other_other_col) >= 100
-(2 rows)
+ check_example_other_other_col_check | CHECK abs(other_other_col) >= 100
+ check_example_other_other_col_check | CHECK abs(other_other_col) >= 100
+ check_example_other_other_col_check | CHECK abs(other_other_col) >= 100
+ check_example_other_other_col_check | CHECK abs(other_other_col) >= 100
+(10 rows)
```

On PostgreSQL 18, `NOT NULL` is represented as a cataloged constraint
and surfaces through `information_schema.check_constraints`.
14e87ffa5c
Our helper view `table_checks` (built on
`information_schema.check_constraints` + `constraint_column_usage`)
started returning:

* Extra `…_not_null` rows (noise for our tests)
* Duplicate rows for real CHECKs due to the one-to-many join via
`constraint_column_usage`
* Occasional literal formatting differences (e.g., dates) coming from
the information\_schema deparser

### What changed

1. **Rewrite `table_checks` to use system catalogs directly**
We now select only expression-based, table-level constraints—excluding
NOT NULL—by filtering on `contype <> 'n'` and requiring `conbin IS NOT
NULL`. This yields the same effective set as real CHECKs while remaining
future-proof against non-CHECK constraint types.

```sql
CREATE OR REPLACE VIEW table_checks AS
SELECT
  c.conname AS "Constraint",
  'CHECK ' ||
  -- drop a single pair of outer parens if the deparser adds them
  regexp_replace(pg_get_expr(c.conbin, c.conrelid, true), '^\((.*)\)$', '\1')
    AS "Definition",
  c.conrelid AS relid
FROM pg_catalog.pg_constraint AS c
WHERE c.contype <> 'n'         -- drop NOT NULL (PG18)
  AND c.conbin IS NOT NULL     -- only expression-bearing constraints (i.e., CHECKs)
  AND c.conrelid <> 0          -- table-level only (exclude domains)
ORDER BY "Constraint", "Definition";
```

Why this filter?

* `contype <> 'n'` excludes PG18’s NOT NULL rows.
* `conbin IS NOT NULL` restricts to expression-backed constraints
(CHECKs); PK/UNIQUE/FK/EXCLUSION don’t have `conbin`.
* `conrelid <> 0` removes domain constraints.

2. **Add a PG18-specific regression test for `contype = 'n'`**
   New test (`pg18_not_null_constraints`) verifies:

* Coordinator tables have `n` rows for NOT NULL (columns `a`, `c`),
* A worker shard has matching `n` rows,
* Dropping a NOT NULL on the coordinator propagates to shards (count
goes from 2 → 1),
* `table_checks` *never* reports NOT NULL, but does report a real CHECK
added for the test.

---

### Why this works (PG15–PG18)

* **Stable source of truth:** Directly reads `pg_constraint` instead of
`information_schema`.
* **No duplicates:** Eliminates the `constraint_column_usage` join,
removing multiplicity.
* **No NOT NULL noise:** PG18’s `contype = 'n'` is filtered out by
design.
* **Deterministic text:** Uses `pg_get_expr` and strips a single outer
set of parentheses for consistent output.

---

### Impact on tests

* Removes spurious `…_not_null` entries and duplicate `checky_…` rows
(e.g., in `multi_name_lengths` and similar).
* Existing expected files stabilize without adding brittle
normalizations.
* New PG18 test asserts correct catalog behavior and Citus propagation
while remaining a no-op on earlier PG versions.

---

src/test/regress/expected/multi_test_catalog_views.out

@@ -70,15 +70,19 @@ SELECT "name" AS "Column",
 	   "relid"
 FROM table_attrs;
 
-CREATE VIEW table_checks AS
+CREATE OR REPLACE VIEW table_checks AS
-SELECT cc.constraint_name AS "Constraint",
+SELECT
-       ('CHECK ' || regexp_replace(check_clause, '^\((.*)\)$', '\1')) AS "Definition",
+  c.conname AS "Constraint",
-       format('%I.%I', ccu.table_schema, ccu.table_name)::regclass::oid AS relid
+  'CHECK ' ||
-FROM information_schema.check_constraints cc,
+  -- drop a single pair of outer parens if the deparser adds them
-     information_schema.constraint_column_usage ccu
+  regexp_replace(pg_get_expr(c.conbin, c.conrelid, true), '^\((.*)\)$', '\1')
-WHERE cc.constraint_schema = ccu.constraint_schema AND
+    AS "Definition",
-      cc.constraint_name = ccu.constraint_name
+  c.conrelid AS relid
-ORDER BY cc.constraint_name ASC;
+FROM pg_catalog.pg_constraint AS c
+WHERE c.contype <> 'n'         -- drop NOT NULL
+  AND c.conbin IS NOT NULL     -- only things with an expression (i.e., CHECKs)
+  AND c.conrelid <> 0          -- table-level (exclude domain checks)
+ORDER BY "Constraint", "Definition";
 
 CREATE VIEW index_attrs AS
 WITH indexoid AS (

src/test/regress/expected/pg18.out

@@ -0,0 +1,163 @@
+--
+-- PG18
+--
+SHOW server_version \gset
+SELECT substring(:'server_version', '\d+')::int >= 18 AS server_version_ge_18
+\gset
+\if :server_version_ge_18
+\else
+\q
+\endif
+-- PG17-specific tests go here.
+--
+-- Purpose: Verify PG18 behavior that NOT NULL constraints are materialized
+--          as pg_constraint rows with contype = 'n' on both coordinator and
+--          worker shards. Also confirm our helper view (table_checks) does
+--          NOT surface NOT NULL entries.
+-- https://github.com/postgres/postgres/commit/14e87ffa5c543b5f30ead7413084c25f7735039f
+CREATE SCHEMA pg18_nn;
+SET search_path TO pg18_nn;
+-- Local control table
+DROP TABLE IF EXISTS nn_local CASCADE;
+NOTICE:  table "nn_local" does not exist, skipping
+CREATE TABLE nn_local(
+    a int NOT NULL,
+    b int,
+    c text NOT NULL
+);
+-- Distributed table
+DROP TABLE IF EXISTS nn_dist CASCADE;
+NOTICE:  table "nn_dist" does not exist, skipping
+CREATE TABLE nn_dist(
+    a int NOT NULL,
+    b int,
+    c text NOT NULL
+);
+SELECT create_distributed_table('nn_dist', 'a');
+ create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
+-- Coordinator: count NOT NULL constraint rows
+SELECT 'local_n_count' AS label, contype, count(*)
+FROM pg_constraint
+WHERE conrelid = 'pg18_nn.nn_local'::regclass
+GROUP BY contype
+ORDER BY contype;
+     label     | contype | count
+---------------------------------------------------------------------
+ local_n_count | n       |     2
+(1 row)
+
+SELECT 'dist_n_count' AS label, contype, count(*)
+FROM pg_constraint
+WHERE conrelid = 'pg18_nn.nn_dist'::regclass
+GROUP BY contype
+ORDER BY contype;
+    label     | contype | count
+---------------------------------------------------------------------
+ dist_n_count | n       |     2
+(1 row)
+
+-- Our helper view should exclude NOT NULL
+SELECT 'table_checks_local_count' AS label, count(*)
+FROM public.table_checks
+WHERE relid = 'pg18_nn.nn_local'::regclass;
+          label           | count
+---------------------------------------------------------------------
+ table_checks_local_count |     0
+(1 row)
+
+SELECT 'table_checks_dist_count' AS label, count(*)
+FROM public.table_checks
+WHERE relid = 'pg18_nn.nn_dist'::regclass;
+          label          | count
+---------------------------------------------------------------------
+ table_checks_dist_count |     0
+(1 row)
+
+-- Add a real CHECK to ensure table_checks still reports real checks
+ALTER TABLE nn_dist ADD CONSTRAINT nn_dist_check CHECK (b IS DISTINCT FROM 42);
+SELECT 'table_checks_dist_with_real_check' AS label, count(*)
+FROM public.table_checks
+WHERE relid = 'pg18_nn.nn_dist'::regclass;
+               label               | count
+---------------------------------------------------------------------
+ table_checks_dist_with_real_check |     1
+(1 row)
+
+-- === Worker checks ===
+\c - - - :worker_1_port
+SET client_min_messages TO WARNING;
+SET search_path TO pg18_nn;
+-- Pick one heap shard of nn_dist in our schema
+SELECT format('%I.%I', n.nspname, c.relname) AS shard_regclass
+FROM pg_class c
+JOIN pg_namespace n ON n.oid = c.relnamespace
+WHERE n.nspname = 'pg18_nn'
+  AND c.relname LIKE 'nn_dist_%'
+  AND c.relkind = 'r'
+ORDER BY c.relname
+LIMIT 1
+\gset
+-- Expect: 2 NOT NULL rows (a,c) + 1 CHECK row on the shard
+SELECT 'worker_shard_n_count' AS label, contype, count(*)
+FROM pg_constraint
+WHERE conrelid = :'shard_regclass'::regclass
+GROUP BY contype
+ORDER BY contype;
+        label         | contype | count
+---------------------------------------------------------------------
+ worker_shard_n_count | c       |     1
+ worker_shard_n_count | n       |     2
+(2 rows)
+
+-- table_checks on shard should hide NOT NULL
+SELECT 'table_checks_worker_shard_count' AS label, count(*)
+FROM public.table_checks
+WHERE relid = :'shard_regclass'::regclass;
+              label              | count
+---------------------------------------------------------------------
+ table_checks_worker_shard_count |     1
+(1 row)
+
+-- Drop one NOT NULL on coordinator; verify propagation
+\c - - - :master_port
+SET search_path TO pg18_nn;
+ALTER TABLE nn_dist ALTER COLUMN c DROP NOT NULL;
+-- Re-check on worker: NOT NULL count should drop to 1
+\c - - - :worker_1_port
+SET search_path TO pg18_nn;
+SELECT 'worker_shard_n_after_drop' AS label, contype, count(*)
+FROM pg_constraint
+WHERE conrelid = :'shard_regclass'::regclass
+GROUP BY contype
+ORDER BY contype;
+           label           | contype | count
+---------------------------------------------------------------------
+ worker_shard_n_after_drop | c       |     1
+ worker_shard_n_after_drop | n       |     1
+(2 rows)
+
+-- And on coordinator
+\c - - - :master_port
+SET search_path TO pg18_nn;
+SELECT 'dist_n_after_drop' AS label, contype, count(*)
+FROM pg_constraint
+WHERE conrelid = 'pg18_nn.nn_dist'::regclass
+GROUP BY contype
+ORDER BY contype;
+       label       | contype | count
+---------------------------------------------------------------------
+ dist_n_after_drop | c       |     1
+ dist_n_after_drop | n       |     1
+(2 rows)
+
+-- cleanup
+RESET client_min_messages;
+RESET search_path;
+DROP SCHEMA pg18_nn CASCADE;
+NOTICE:  drop cascades to 2 other objects
+DETAIL:  drop cascades to table pg18_nn.nn_local
+drop cascades to table pg18_nn.nn_dist

src/test/regress/expected/pg18_0.out

@@ -0,0 +1,9 @@
+--
+-- PG18
+--
+SHOW server_version \gset
+SELECT substring(:'server_version', '\d+')::int >= 18 AS server_version_ge_18
+\gset
+\if :server_version_ge_18
+\else
+\q

src/test/regress/multi_schedule

@@ -68,6 +68,7 @@ test: pg14
 test: pg15
 test: pg15_jsonpath detect_conn_close
 test: pg17 pg17_json
+test: pg18
 test: drop_column_partitioned_table
 test: tableam
 

src/test/regress/sql/multi_test_catalog_views.sql

@@ -71,15 +71,19 @@ SELECT "name" AS "Column",
 	   "relid"
 FROM table_attrs;
 
-CREATE VIEW table_checks AS
+CREATE OR REPLACE VIEW table_checks AS
-SELECT cc.constraint_name AS "Constraint",
+SELECT
-       ('CHECK ' || regexp_replace(check_clause, '^\((.*)\)$', '\1')) AS "Definition",
+  c.conname AS "Constraint",
-       format('%I.%I', ccu.table_schema, ccu.table_name)::regclass::oid AS relid
+  'CHECK ' ||
-FROM information_schema.check_constraints cc,
+  -- drop a single pair of outer parens if the deparser adds them
-     information_schema.constraint_column_usage ccu
+  regexp_replace(pg_get_expr(c.conbin, c.conrelid, true), '^\((.*)\)$', '\1')
-WHERE cc.constraint_schema = ccu.constraint_schema AND
+    AS "Definition",
-      cc.constraint_name = ccu.constraint_name
+  c.conrelid AS relid
-ORDER BY cc.constraint_name ASC;
+FROM pg_catalog.pg_constraint AS c
+WHERE c.contype <> 'n'         -- drop NOT NULL
+  AND c.conbin IS NOT NULL     -- only things with an expression (i.e., CHECKs)
+  AND c.conrelid <> 0          -- table-level (exclude domain checks)
+ORDER BY "Constraint", "Definition";
 
 CREATE VIEW index_attrs AS
 WITH indexoid AS (

src/test/regress/sql/pg18.sql

@@ -0,0 +1,129 @@
+--
+-- PG18
+--
+SHOW server_version \gset
+SELECT substring(:'server_version', '\d+')::int >= 18 AS server_version_ge_18
+\gset
+
+\if :server_version_ge_18
+\else
+\q
+\endif
+
+-- PG17-specific tests go here.
+--
+
+-- Purpose: Verify PG18 behavior that NOT NULL constraints are materialized
+--          as pg_constraint rows with contype = 'n' on both coordinator and
+--          worker shards. Also confirm our helper view (table_checks) does
+--          NOT surface NOT NULL entries.
+-- https://github.com/postgres/postgres/commit/14e87ffa5c543b5f30ead7413084c25f7735039f
+
+CREATE SCHEMA pg18_nn;
+SET search_path TO pg18_nn;
+
+-- Local control table
+DROP TABLE IF EXISTS nn_local CASCADE;
+CREATE TABLE nn_local(
+    a int NOT NULL,
+    b int,
+    c text NOT NULL
+);
+
+-- Distributed table
+DROP TABLE IF EXISTS nn_dist CASCADE;
+CREATE TABLE nn_dist(
+    a int NOT NULL,
+    b int,
+    c text NOT NULL
+);
+
+SELECT create_distributed_table('nn_dist', 'a');
+
+-- Coordinator: count NOT NULL constraint rows
+SELECT 'local_n_count' AS label, contype, count(*)
+FROM pg_constraint
+WHERE conrelid = 'pg18_nn.nn_local'::regclass
+GROUP BY contype
+ORDER BY contype;
+
+SELECT 'dist_n_count' AS label, contype, count(*)
+FROM pg_constraint
+WHERE conrelid = 'pg18_nn.nn_dist'::regclass
+GROUP BY contype
+ORDER BY contype;
+
+-- Our helper view should exclude NOT NULL
+SELECT 'table_checks_local_count' AS label, count(*)
+FROM public.table_checks
+WHERE relid = 'pg18_nn.nn_local'::regclass;
+
+SELECT 'table_checks_dist_count' AS label, count(*)
+FROM public.table_checks
+WHERE relid = 'pg18_nn.nn_dist'::regclass;
+
+-- Add a real CHECK to ensure table_checks still reports real checks
+ALTER TABLE nn_dist ADD CONSTRAINT nn_dist_check CHECK (b IS DISTINCT FROM 42);
+
+SELECT 'table_checks_dist_with_real_check' AS label, count(*)
+FROM public.table_checks
+WHERE relid = 'pg18_nn.nn_dist'::regclass;
+
+-- === Worker checks ===
+\c - - - :worker_1_port
+SET client_min_messages TO WARNING;
+SET search_path TO pg18_nn;
+
+-- Pick one heap shard of nn_dist in our schema
+SELECT format('%I.%I', n.nspname, c.relname) AS shard_regclass
+FROM pg_class c
+JOIN pg_namespace n ON n.oid = c.relnamespace
+WHERE n.nspname = 'pg18_nn'
+  AND c.relname LIKE 'nn_dist_%'
+  AND c.relkind = 'r'
+ORDER BY c.relname
+LIMIT 1
+\gset
+
+-- Expect: 2 NOT NULL rows (a,c) + 1 CHECK row on the shard
+SELECT 'worker_shard_n_count' AS label, contype, count(*)
+FROM pg_constraint
+WHERE conrelid = :'shard_regclass'::regclass
+GROUP BY contype
+ORDER BY contype;
+
+-- table_checks on shard should hide NOT NULL
+SELECT 'table_checks_worker_shard_count' AS label, count(*)
+FROM public.table_checks
+WHERE relid = :'shard_regclass'::regclass;
+
+-- Drop one NOT NULL on coordinator; verify propagation
+\c - - - :master_port
+SET search_path TO pg18_nn;
+
+ALTER TABLE nn_dist ALTER COLUMN c DROP NOT NULL;
+
+-- Re-check on worker: NOT NULL count should drop to 1
+\c - - - :worker_1_port
+SET search_path TO pg18_nn;
+
+SELECT 'worker_shard_n_after_drop' AS label, contype, count(*)
+FROM pg_constraint
+WHERE conrelid = :'shard_regclass'::regclass
+GROUP BY contype
+ORDER BY contype;
+
+-- And on coordinator
+\c - - - :master_port
+SET search_path TO pg18_nn;
+
+SELECT 'dist_n_after_drop' AS label, contype, count(*)
+FROM pg_constraint
+WHERE conrelid = 'pg18_nn.nn_dist'::regclass
+GROUP BY contype
+ORDER BY contype;
+
+-- cleanup
+RESET client_min_messages;
+RESET search_path;
+DROP SCHEMA pg18_nn CASCADE;