restore v19 behavior: push LIMIT to each shard for non-scatter multi-shard DML (#838)

* feat: add scatter-update-limit-passthru vtgate flag

When enabled, multi-shard UPDATE/DELETE with LIMIT on non-scatter
routes (e.g. WHERE id IN (...) on a vindex column) pushes the LIMIT
to each shard directly instead of converting to DMLWithInput which
requires schema tracking. Scatter routes are unaffected.

This restores v19 behavior for targeted multi-shard DML with LIMIT
behind an opt-in flag.

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>
Signed-off-by: Tanjin Xu <tanjin.xu@slack-corp.com>

* fix: return false instead of panic for test mock IsScatterUpdateLimitPassthruEnabled

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>
Signed-off-by: Tanjin Xu <tanjin.xu@slack-corp.com>

* feat: add scatter-update-limit-passthru to help text files

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>

Signed-off-by: Tanjin Xu <tanjin.xu@slack-corp.com>

* fix: remove scatter-update-limit-passthru from vttestserver help text

The flag is only registered for vtgate and vtcombo, not vttestserver.

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>
Signed-off-by: Tanjin Xu <tanjin.xu@slack-corp.com>

* fix: correct spacing in scatter-update-limit-passthru help text

Add one extra space to align the description column consistently
with other flags in vtgate.txt and vtcombo.txt.

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>
Signed-off-by: Tanjin Xu <tanjin.xu@slack-corp.com>

* remove scatter-update-limit-passthru flag, enable by default

The behavior is now always-on: non-scatter multi-shard DML with LIMIT
pushes the LIMIT to each shard directly. Scatter DML with LIMIT still
uses DMLWithInput as before.

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>
Signed-off-by: Tanjin Xu <tanjin.xu@slack-corp.com>

* remove scatter-update-limit-passthru from help text files

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>
Signed-off-by: Tanjin Xu <tanjin.xu@slack-corp.com>

* fix e2e test: use col instead of val for user_extra

The user_extra table does not have a val column in the e2e schema.

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>
Signed-off-by: Tanjin Xu <tanjin.xu@slack-corp.com>

---------

Signed-off-by: Tanjin Xu <tanjin.xu@slack-corp.com>
Co-authored-by: Claude <svc-devxp-claude@slack-corp.com>

Author: tanjinx

go/vt/vtgate/planbuilder/operators/query_planning.go

@@ -525,6 +525,9 @@ func tryPushingDownLimitInRoute(ctx *plancontext.PlanningContext, in *Limit, src
 	}
 
 	if sqlparser.IsDMLStatement(ctx.Statement) {
+		if src.Routing.OpCode() != engine.Scatter {
+			return Swap(in, src, "push limit under route")
+		}
 		return setUpperLimit(in)
 	}
 

go/vt/vtgate/planbuilder/testdata/dml_cases.json

@@ -7430,5 +7430,57 @@
       ]
     },
     "skip_e2e": true
+  },
+  {
+    "comment": "update with IN clause on vindex and limit pushes limit to each shard",
+    "query": "update user_extra set col = 1 where user_id in (1, 2) limit 5",
+    "plan": {
+      "Type": "MultiShard",
+      "QueryType": "UPDATE",
+      "Original": "update user_extra set col = 1 where user_id in (1, 2) limit 5",
+      "Instructions": {
+        "OperatorType": "Update",
+        "Variant": "IN",
+        "Keyspace": {
+          "Name": "user",
+          "Sharded": true
+        },
+        "Query": "update user_extra set col = 1 where user_id in ::__vals limit 5",
+        "Table": "user_extra",
+        "Values": [
+          "(1, 2)"
+        ],
+        "Vindex": "user_index"
+      },
+      "TablesUsed": [
+        "user.user_extra"
+      ]
+    }
+  },
+  {
+    "comment": "delete with IN clause on vindex and limit pushes limit to each shard",
+    "query": "delete from user_extra where user_id in (1, 2) limit 5",
+    "plan": {
+      "Type": "MultiShard",
+      "QueryType": "DELETE",
+      "Original": "delete from user_extra where user_id in (1, 2) limit 5",
+      "Instructions": {
+        "OperatorType": "Delete",
+        "Variant": "IN",
+        "Keyspace": {
+          "Name": "user",
+          "Sharded": true
+        },
+        "Query": "delete from user_extra where user_id in ::__vals limit 5",
+        "Table": "user_extra",
+        "Values": [
+          "(1, 2)"
+        ],
+        "Vindex": "user_index"
+      },
+      "TablesUsed": [
+        "user.user_extra"
+      ]
+    }
   }
 ]