[SQL] Address review comments discussed async

x-pack/metricbeat/module/sql/query/query.go

@@ -106,26 +106,37 @@ func New(base mb.BaseMetricSet) (mb.MetricSet, error) {
 	return b, nil
 }
 
-// queryDBNames returns the command to list databases present in a server
+// queryDBNames returns the query to list databases present in a server
 // as per the driver name. If the given driver is not supported, queryDBNames
 // returns an empty query.
 func queryDBNames(driver string) string {
 	switch sql.SwitchDriverName(driver) {
-	case "mysql":
-		return "SHOW DATABASES"
 	case "mssql":
 		return "SELECT [name] FROM sys.databases WITH (NOLOCK) WHERE state = 0 AND HAS_DBACCESS([name]) = 1"
-	case "godror":
-		// NOTE: Requires necessary priviledges to access DBA_USERS
-		// Ref: https://stackoverflow.com/a/3005623/5821408
-		return "SELECT * FROM DBA_USERS"
-	case "postgres":
-		return "SELECT datname FROM pg_database"
+		// case "mysql":
+		// 	return "SHOW DATABASES"
+		// case "godror":
+		// 	// NOTE: Requires necessary priviledges to access DBA_USERS
+		// 	// Ref: https://stackoverflow.com/a/3005623/5821408
+		// 	return "SELECT * FROM DBA_USERS"
+		// case "postgres":
+		// 	return "SELECT datname FROM pg_database"
 	}
 
 	return ""
 }
 
+// dbSelector returns the statement to select a named database to run the
+// subsequent statements. If the given driver is not supported, dbSelector
+// returns an empty query.
+func dbSelector(driver, dbName string) string {
+	switch sql.SwitchDriverName(driver) {
+	case "mssql":
+		return fmt.Sprintf("USE [%s];", dbName)
+	}
+	return ""
+}
+
 func (m *MetricSet) fetch(ctx context.Context, db *sql.DbClient, reporter mb.ReporterV2, queries []query) error {
 	merged := make(mapstr.M, 0)
 
@@ -211,6 +222,19 @@ func (m *MetricSet) Fetch(ctx context.Context, reporter mb.ReporterV2) error {
 		return nil
 	}
 
+	// NOTE: Only mssql driver is supported for now because:
+	//
+	// * Difference in queries to fetch the name of the databases
+	// * The statement to select a named database (for subsequent statements
+	// to be executed) may not be generic i.e, USE statement (e.g., USE <db_name>)
+	// works for MSSQL but not Oracle.
+	//
+	// TODO: Add the feature for other drivers when need arises.
+	validQuery := queryDBNames(m.Config.Driver)
+	if validQuery == "" {
+		return fmt.Errorf("fetch from all databases feature is not supported for driver: %s", m.Config.Driver)
+	}
+
 	// Discover all databases in the server and execute given queries on each
 	// of the databases.
 	dbNames, err := db.FetchTableMode(ctx, queryDBNames(m.Config.Driver))
@@ -241,9 +265,11 @@ func (m *MetricSet) Fetch(ctx context.Context, reporter mb.ReporterV2) error {
 			continue
 		}
 
-		// Prefix `USE <db_name>;` to the query
+		// Prefix dbSelector to the query based on the driver
+		// provided.
+		// Example: USE <dbName>; @command (or @query)
 		for i := range qs {
-			qs[i].Query = fmt.Sprintf("USE [%s]; ", dbName) + qs[i].Query
+			qs[i].Query = dbSelector(m.Config.Driver, dbName) + " " + qs[i].Query
 		}
 
 		err = m.fetch(ctx, db, reporter, qs)