Adds another case to merge routes for information_schema queries

go/vt/sqlparser/ast_funcs.go

@@ -668,6 +668,13 @@ func (node TableIdent) String() string {
 	return node.v
 }
 
+// String returns the unescaped table name with qualifier. It must
+// not be used for SQL generation. Use sqlparser.String
+// instead.
+func (node TableName) String() string {
+	return strings.ToLower(node.Qualifier.String() + "." + node.Name.String())
+}
+
 // CompliantName returns a compliant id name
 // that can be used for a bind var.
 func (node TableIdent) CompliantName() string {

go/vt/vtgate/planbuilder/route.go

@@ -440,15 +440,28 @@ func (rb *route) JoinCanMerge(pb *primitiveBuilder, rrb *route, ajoin *sqlparser
 		if where == nil {
 			return true
 		}
-		hasRuntimeRoutingPredicates := false
+		var tableWithRoutingPredicates sqlparser.TableNames
 		_ = sqlparser.Walk(func(node sqlparser.SQLNode) (kontinue bool, err error) {
 			col, ok := node.(*sqlparser.ColName)
 			if ok {
-				hasRuntimeRoutingPredicates = hasRuntimeRoutingPredicates || isTableNameCol(col) || isDbNameCol(col)
+				hasRuntimeRoutingPredicates := isTableNameCol(col) || isDbNameCol(col)
+				if hasRuntimeRoutingPredicates {
+					for _, table := range tableWithRoutingPredicates {
+						if table.String() == col.Qualifier.String() {
+							return true, nil
+						}
+					}
+					tableWithRoutingPredicates = append(tableWithRoutingPredicates, col.Qualifier)
+				}
 			}
-			return !hasRuntimeRoutingPredicates, nil
+			return true, nil
 		}, where)
-		return !hasRuntimeRoutingPredicates
+		// Routes can be merged if only tables from information schema are accessed and only 1 table is used in the predicates that are used for routing
+		// TODO :- Even if more table are present in the routing, we can merge if they agree
+		if rb.ContainsOnlyInformationSchema() && rrb.ContainsOnlyInformationSchema() && len(tableWithRoutingPredicates) <= 1 {
+			return true
+		}
+		return len(tableWithRoutingPredicates) == 0
 	}
 	if ajoin == nil {
 		return false
@@ -820,3 +833,8 @@ func queryTimeout(d sqlparser.CommentDirectives) int {
 	}
 	return 0
 }
+
+// ContainsOnlyInformationSchema returns if the route only contains information_schema tables
+func (rb *route) ContainsOnlyInformationSchema() bool {
+	return rb.eroute.Opcode == engine.SelectDBA
+}

go/vt/vtgate/planbuilder/testdata/select_cases.txt

@@ -1696,3 +1696,22 @@ Gen4 plan same as above
     ]
   }
 }
+
+# Select from information schema query with two tables that route should be merged
+"SELECT DELETE_RULE, UPDATE_RULE FROM  INFORMATION_SCHEMA.KEY_COLUMN_USAGE AS KCU INNER JOIN INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS AS RC ON KCU.CONSTRAINT_NAME = RC.CONSTRAINT_NAME WHERE KCU.TABLE_SCHEMA = 'test' AND KCU.TABLE_NAME = 'data_type_table' AND KCU.COLUMN_NAME = 'id' AND KCU.REFERENCED_TABLE_SCHEMA = 'test' AND KCU.CONSTRAINT_NAME = 'data_type_table_id_fkey' ORDER BY KCU.CONSTRAINT_NAME, KCU.COLUMN_NAME"
+{
+  "QueryType": "SELECT",
+  "Original": "SELECT DELETE_RULE, UPDATE_RULE FROM  INFORMATION_SCHEMA.KEY_COLUMN_USAGE AS KCU INNER JOIN INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS AS RC ON KCU.CONSTRAINT_NAME = RC.CONSTRAINT_NAME WHERE KCU.TABLE_SCHEMA = 'test' AND KCU.TABLE_NAME = 'data_type_table' AND KCU.COLUMN_NAME = 'id' AND KCU.REFERENCED_TABLE_SCHEMA = 'test' AND KCU.CONSTRAINT_NAME = 'data_type_table_id_fkey' ORDER BY KCU.CONSTRAINT_NAME, KCU.COLUMN_NAME",
+  "Instructions": {
+    "OperatorType": "Route",
+    "Variant": "SelectDBA",
+    "Keyspace": {
+      "Name": "main",
+      "Sharded": false
+    },
+    "FieldQuery": "select DELETE_RULE, UPDATE_RULE from INFORMATION_SCHEMA.KEY_COLUMN_USAGE as KCU join INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS as RC on KCU.CONSTRAINT_NAME = RC.CONSTRAINT_NAME where 1 != 1",
+    "Query": "select DELETE_RULE, UPDATE_RULE from INFORMATION_SCHEMA.KEY_COLUMN_USAGE as KCU join INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS as RC on KCU.CONSTRAINT_NAME = RC.CONSTRAINT_NAME where KCU.TABLE_SCHEMA = :__vtschemaname and KCU.TABLE_NAME = :__vttablename and KCU.COLUMN_NAME = 'id' and KCU.REFERENCED_TABLE_SCHEMA = 'test' and KCU.CONSTRAINT_NAME = 'data_type_table_id_fkey' order by KCU.CONSTRAINT_NAME asc, KCU.COLUMN_NAME asc",
+    "SysTableTableName": "VARBINARY(\"data_type_table\")",
+    "SysTableTableSchema": "VARBINARY(\"test\")"
+  }
+}