Fix 0-Task Plans in Single-Shard Router When Updating a Local Table w…

…ith Reference Table in Subquery (#7897)

This PR fixes an issue #7891 in the Citus planner where an `UPDATE` on a
local table with a subquery referencing a reference table could produce
a 0-task plan. Historically, the planner sometimes failed to detect that
both the target and referenced tables were effectively “local,”
assigning `INVALID_SHARD_ID `and yielding a no-op plan.

### Root Cause

- In the Citus router logic (`PlanRouterQuery`), we relied on `shardId`
to determine whether a query should be routed to a single shard.
- If `shardId == INVALID_SHARD_ID`, but we also had not marked the query
as a “local table modification,” the code path would produce zero tasks.
- Local + reference tables do not require multi-shard routing. Failing
to detect this “purely local” scenario caused Citus to incorrectly route
to zero tasks.

### Changes

**Enhanced Local Table Detection**

- Updated `IsLocalTableModification` and related checks to consider both
local and reference tables as “local” for planning, preventing the
0-task scenario.
- Expanded `ContainsOnlyLocalOrReferenceTables` to return true if there
are no fully distributed tables in the query.

**Added Regress Test**

- Introduced a new regress test (`issue_7891.sql`) which reproduces the
scenario.
- Verifies we get a valid single- or local-task plan rather than a
0-task plan.

src/backend/distributed/planner/merge_planner.c

@@ -1583,7 +1583,7 @@ IsLocalTableModification(Oid targetRelationId, Query *query, uint64 shardId,
 		return true;
 	}
 
-	if (shardId == INVALID_SHARD_ID && ContainsOnlyLocalTables(rteProperties))
+	if (shardId == INVALID_SHARD_ID && ContainsOnlyLocalOrReferenceTables(rteProperties))
 	{
 		return true;
 	}

src/backend/distributed/planner/multi_router_planner.c

@@ -2556,13 +2556,15 @@ AllShardsColocated(List *relationShardList)
 
 
 /*
- * ContainsOnlyLocalTables returns true if there is only
- * local tables and not any distributed or reference table.
+ * ContainsOnlyLocalOrReferenceTables returns true if there are no distributed
+ * tables in the query. In other words, the query might reference only local
+ * tables and/or reference tables, but no fully distributed tables.
  */
 bool
-ContainsOnlyLocalTables(RTEListProperties *rteProperties)
+ContainsOnlyLocalOrReferenceTables(RTEListProperties *rteProperties)
 {
-	return !rteProperties->hasDistributedTable && !rteProperties->hasReferenceTable;
+	/* If hasDistributedTable is false, then all tables are either local or reference. */
+	return !rteProperties->hasDistributedTable;
 }
 
 

src/include/distributed/multi_router_planner.h

@@ -118,7 +118,7 @@ extern bool HasDangerousJoinUsing(List *rtableList, Node *jtnode);
 extern Job * RouterJob(Query *originalQuery,
 					   PlannerRestrictionContext *plannerRestrictionContext,
 					   DeferredErrorMessage **planningError);
-extern bool ContainsOnlyLocalTables(RTEListProperties *rteProperties);
+extern bool ContainsOnlyLocalOrReferenceTables(RTEListProperties *rteProperties);
 extern RangeTblEntry * ExtractSourceResultRangeTableEntry(Query *query);
 
 #endif /* MULTI_ROUTER_PLANNER_H */

src/test/regress/expected/issue_7891.out

@@ -0,0 +1,211 @@
+-- This test validates that the query planner correctly handles nested subqueries involving both a
+-- local table (t4_pg) and a reference table (t2_ref). The steps are as follows:
+--
+-- 1. A dedicated schema (issue_7891) is created, and three tables (t2_ref, t4_pg, t6_pg) are set up.
+-- 2. The table t2_ref is designated as a reference table using the create_reference_table() function.
+-- 3. Sample data is inserted into all tables.
+-- 4. An UPDATE is executed on t6_pg. The update uses an EXISTS clause with a nested subquery:
+--    - The outer subquery iterates over every row in t4_pg.
+--    - The inner subquery selects c15 from t2_ref.
+-- 5. The update should occur if the nested subquery returns any row, effectively updating t6_pg's vkey to 43.
+-- 6. The final state of t6_pg is displayed to confirm that the update was applied.
+--
+-- Note: This test was originally designed to detect a planner bug where the nested structure might
+-- lead to an incorrect plan (such as a 0-task plan), ensuring proper handling of reference and local tables.
+-- https://github.com/citusdata/citus/issues/7891
+CREATE SCHEMA issue_7891;
+SET search_path TO issue_7891;
+-- Create tables
+CREATE TABLE t2_ref (
+    vkey INT,
+    pkey INT,
+    c15 TIMESTAMP
+);
+CREATE TABLE t2_ref2 (
+    vkey INT,
+    pkey INT,
+    c15 TIMESTAMP
+);
+CREATE TABLE t4_pg (
+    vkey INT,
+    pkey INT,
+    c22 NUMERIC,
+    c23 TEXT,
+    c24 TIMESTAMP
+);
+CREATE TABLE t6_pg (
+    vkey INT,
+    pkey INT,
+    c26 TEXT
+);
+-- Mark t2_ref and t2_ref2 as a reference table
+SELECT create_reference_table('t2_ref');
+ create_reference_table
+---------------------------------------------------------------------
+
+(1 row)
+
+SELECT create_reference_table('t2_ref2');
+ create_reference_table
+---------------------------------------------------------------------
+
+(1 row)
+
+-- Insert sample data
+INSERT INTO t6_pg (vkey, pkey, c26) VALUES
+    (2, 12000, 'initial'),
+    (3, 13000, 'will_be_deleted'),
+    (4, 14000, 'to_merge');
+INSERT INTO t4_pg (vkey, pkey, c22, c23, c24)
+    VALUES (5, 15000, 0.0, ']]?', MAKE_TIMESTAMP(2071, 10, 26, 16, 20, 5));
+INSERT INTO t2_ref (vkey, pkey, c15)
+    VALUES (14, 24000, NULL::timestamp);
+-- Show initial data
+SELECT 't6_pg before' AS label, * FROM t6_pg;
+    label     | vkey | pkey  |       c26
+---------------------------------------------------------------------
+ t6_pg before |    2 | 12000 | initial
+ t6_pg before |    3 | 13000 | will_be_deleted
+ t6_pg before |    4 | 14000 | to_merge
+(3 rows)
+
+SELECT 't4_pg data' AS label, * FROM t4_pg;
+   label    | vkey | pkey  | c22 | c23 |           c24
+---------------------------------------------------------------------
+ t4_pg data |    5 | 15000 | 0.0 | ]]? | Mon Oct 26 16:20:05 2071
+(1 row)
+
+SELECT 't2_ref data' AS label, * FROM t2_ref;
+    label    | vkey | pkey  | c15
+---------------------------------------------------------------------
+ t2_ref data |   14 | 24000 |
+(1 row)
+
+--
+-- The problematic query: update t6_pg referencing t4_pg and sub-subquery on t2_ref.
+-- Historically might produce a 0-task plan if the planner incorrectly fails to
+-- treat t4_pg/t2_ref as local/reference.
+--
+-- The outer subquery iterates over every row in table t4_pg.
+UPDATE t6_pg
+   SET vkey = 43
+ WHERE EXISTS (
+   SELECT (SELECT c15 FROM t2_ref)
+   FROM t4_pg
+);
+SELECT 't6_pg after' AS label, * FROM t6_pg;
+    label    | vkey | pkey  |       c26
+---------------------------------------------------------------------
+ t6_pg after |   43 | 12000 | initial
+ t6_pg after |   43 | 13000 | will_be_deleted
+ t6_pg after |   43 | 14000 | to_merge
+(3 rows)
+
+--
+--    DELETE with a similar nested subquery approach
+--    Here, let's delete any rows for which t4_pg is non-empty (like a trivial check).
+--    We'll specifically target the row with c26='will_be_deleted' to confirm it's removed.
+--
+DELETE FROM t6_pg
+ WHERE EXISTS (
+   SELECT (SELECT c15 FROM t2_ref)
+   FROM t4_pg
+ )
+ AND c26 = 'will_be_deleted';
+SELECT 't6_pg after DELETE' AS label, * FROM t6_pg;
+       label        | vkey | pkey  |   c26
+---------------------------------------------------------------------
+ t6_pg after DELETE |   43 | 12000 | initial
+ t6_pg after DELETE |   43 | 14000 | to_merge
+(2 rows)
+
+--
+--    We'll merge from t4_pg into t6_pg. The merge will update c26 for pkey=14000.
+--
+-- Anticipate an error indicating non-IMMUTABLE functions are not supported in MERGE statements on distributed tables.
+-- Retain this comment to highlight the current limitation.
+--
+MERGE INTO t6_pg AS tgt
+USING t4_pg AS src
+ON (tgt.pkey = 14000)
+WHEN MATCHED THEN
+    UPDATE SET c26 = 'merged_' || (SELECT pkey FROM t2_ref WHERE pkey=24000 LIMIT 1)
+WHEN NOT MATCHED THEN
+    INSERT (vkey, pkey, c26)
+    VALUES (99, src.pkey, 'inserted_via_merge');
+ERROR:  non-IMMUTABLE functions are not yet supported in MERGE sql with distributed tables
+MERGE INTO t2_ref AS tgt
+USING t4_pg AS src
+  ON (tgt.pkey = src.pkey)
+WHEN MATCHED THEN
+  UPDATE SET c15 = '2088-01-01 00:00:00'::timestamp
+WHEN NOT MATCHED THEN
+  INSERT (vkey, pkey, c15)
+  VALUES (src.vkey, src.pkey, '2099-12-31 23:59:59'::timestamp);
+ERROR:  Reference table as target is not allowed in MERGE command
+-- Show the final state of t2_ref:
+SELECT 't2_ref after MERGE (using t4_pg)' AS label, * FROM t2_ref;
+              label               | vkey | pkey  | c15
+---------------------------------------------------------------------
+ t2_ref after MERGE (using t4_pg) |   14 | 24000 |
+(1 row)
+
+MERGE INTO t2_ref2 AS tgt
+USING t2_ref AS src
+  ON (tgt.pkey = src.pkey)
+WHEN MATCHED THEN
+  UPDATE SET c15 = '2077-07-07 07:07:07'::timestamp
+WHEN NOT MATCHED THEN
+  INSERT (vkey, pkey, c15)
+  VALUES (src.vkey, src.pkey, '2066-06-06 06:06:06'::timestamp);
+ERROR:  Reference table as target is not allowed in MERGE command
+-- Show the final state of t2_ref2:
+SELECT 't2_ref2 after MERGE (using t2_ref)' AS label, * FROM t2_ref2;
+ label | vkey | pkey | c15
+---------------------------------------------------------------------
+(0 rows)
+
+MERGE INTO t6_pg AS tgt
+USING t4_pg AS src
+  ON (tgt.pkey = src.pkey)
+WHEN MATCHED THEN
+  UPDATE SET c26 = 'merged_value'
+WHEN NOT MATCHED THEN
+  INSERT (vkey, pkey, c26)
+  VALUES (src.vkey, src.pkey, 'inserted_via_merge');
+SELECT 't6_pg after MERGE' AS label, * FROM t6_pg;
+       label       | vkey | pkey  |        c26
+---------------------------------------------------------------------
+ t6_pg after MERGE |   43 | 12000 | initial
+ t6_pg after MERGE |   43 | 14000 | to_merge
+ t6_pg after MERGE |    5 | 15000 | inserted_via_merge
+(3 rows)
+
+--
+--   Update the REFERENCE table itself and verify the change
+--   This is to ensure that the reference table is correctly handled.
+UPDATE t2_ref
+   SET c15 = '2099-01-01 00:00:00'::timestamp
+ WHERE pkey = 24000;
+SELECT 't2_ref after self-update' AS label, * FROM t2_ref;
+          label           | vkey | pkey  |           c15
+---------------------------------------------------------------------
+ t2_ref after self-update |   14 | 24000 | Thu Jan 01 00:00:00 2099
+(1 row)
+
+UPDATE t2_ref
+   SET c15 = '2099-01-01 00:00:00'::timestamp
+ WHERE EXISTS (
+    SELECT 1
+    FROM t4_pg
+ );
+ERROR:  relation t4_pg is not distributed
+SELECT 't2_ref after UPDATE' AS label, * FROM t2_ref;
+        label        | vkey | pkey  |           c15
+---------------------------------------------------------------------
+ t2_ref after UPDATE |   14 | 24000 | Thu Jan 01 00:00:00 2099
+(1 row)
+
+-- Cleanup
+SET client_min_messages TO WARNING;
+DROP SCHEMA issue_7891 CASCADE;

src/test/regress/multi_schedule

@@ -103,7 +103,7 @@ test: multi_dropped_column_aliases foreign_key_restriction_enforcement
 test: binary_protocol
 test: alter_table_set_access_method
 test: alter_distributed_table
-test: issue_5248 issue_5099 issue_5763 issue_6543 issue_6758 issue_7477
+test: issue_5248 issue_5099 issue_5763 issue_6543 issue_6758 issue_7477 issue_7891
 test: object_propagation_debug
 test: undistribute_table
 test: run_command_on_all_nodes