[release-18.0] fix: handle info_schema routing (#15899) (#15905)

Signed-off-by: Andres Taylor <andres@planetscale.com>
Co-authored-by: Andrés Taylor <andres@planetscale.com>

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

@@ -87,5 +87,26 @@
         "SingleShardOnly": true
       }
     }
+  },
+  {
+    "comment": "describe info_schema table",
+    "query": "describe information_schema.administrable_role_authorizations",
+    "plan": {
+      "QueryType": "EXPLAIN",
+      "Original": "describe information_schema.administrable_role_authorizations",
+      "Instructions": {
+        "OperatorType": "Send",
+        "Keyspace": {
+          "Name": "main",
+          "Sharded": false
+        },
+        "TargetDestination": "AnyShard()",
+        "Query": "explain information_schema.administrable_role_authorizations",
+        "SingleShardOnly": true
+      },
+      "TablesUsed": [
+        "main.administrable_role_authorizations"
+      ]
+    }
   }
 ]

go/vt/vtgate/planbuilder/vexplain.go

@@ -23,6 +23,8 @@ import (
 
 	"vitess.io/vitess/go/vt/vtgate/planbuilder/plancontext"
 
+	"vitess.io/vitess/go/vt/vtgate/vindexes"
+
 	"vitess.io/vitess/go/sqltypes"
 	"vitess.io/vitess/go/vt/key"
 	querypb "vitess.io/vitess/go/vt/proto/query"
@@ -63,24 +65,37 @@ func buildVExplainPlan(ctx context.Context, vexplainStmt *sqlparser.VExplainStmt
 }
 
 func explainTabPlan(explain *sqlparser.ExplainTab, vschema plancontext.VSchema) (*planResult, error) {
-	_, _, ks, _, destination, err := vschema.FindTableOrVindex(explain.Table)
-	if err != nil {
-		return nil, err
+	var keyspace *vindexes.Keyspace
+	var destination key.Destination
+
+	if sqlparser.SystemSchema(explain.Table.Qualifier.String()) {
+		var err error
+		keyspace, err = vschema.AnyKeyspace()
+		if err != nil {
+			return nil, err
+		}
+	} else {
+		var err error
+		var ks string
+		_, _, ks, _, destination, err = vschema.FindTableOrVindex(explain.Table)
+		if err != nil {
+			return nil, err
+		}
+		explain.Table.Qualifier = sqlparser.NewIdentifierCS("")
+
+		keyspace, err = vschema.FindKeyspace(ks)
+		if err != nil {
+			return nil, err
+		}
+		if keyspace == nil {
+			return nil, vterrors.VT14004(ks)
+		}
 	}
-	explain.Table.Qualifier = sqlparser.NewIdentifierCS("")
 
 	if destination == nil {
 		destination = key.DestinationAnyShard{}
 	}
 
-	keyspace, err := vschema.FindKeyspace(ks)
-	if err != nil {
-		return nil, err
-	}
-	if keyspace == nil {
-		return nil, vterrors.VT14004(ks)
-	}
-
 	return newPlanResult(&engine.Send{
 		Keyspace:          keyspace,
 		TargetDestination: destination,