Optimization: statements reuse previous column name

Author: methane

connection.go

@@ -231,7 +231,7 @@ func (mc *mysqlConn) Prepare(query string) (driver.Stmt, error) {
 
 		if columnCount > 0 {
 			if mc.extCapabilities&clientCacheMetadata != 0 {
-				if stmt.columns, err = mc.readColumns(int(columnCount)); err != nil {
+				if stmt.columns, err = mc.readColumns(int(columnCount), nil); err != nil {
 					return nil, err
 				}
 			} else {
@@ -448,7 +448,7 @@ func (mc *mysqlConn) query(query string, args []driver.Value) (*textRows, error)
 	}
 
 	// Columns
-	rows.rs.columns, err = mc.readColumns(resLen)
+	rows.rs.columns, err = mc.readColumns(resLen, nil)
 	return rows, err
 }
 

packets.go

@@ -702,8 +702,11 @@ func (mc *okHandler) handleOkPacket(data []byte) error {
 
 // Read Packets as Field Packets until EOF-Packet or an Error appears
 // http://dev.mysql.com/doc/internals/en/com-query-response.html#packet-Protocol::ColumnDefinition41
-func (mc *mysqlConn) readColumns(count int) ([]mysqlField, error) {
+func (mc *mysqlConn) readColumns(count int, old []mysqlField) ([]mysqlField, error) {
 	columns := make([]mysqlField, count)
+	if len(old) != count {
+		old = nil
+	}
 
 	for i := range count {
 		data, err := mc.readPacket()
@@ -731,7 +734,12 @@ func (mc *mysqlConn) readColumns(count int) ([]mysqlField, error) {
 				return nil, err
 			}
 			pos += n
-			columns[i].tableName = string(tableName)
+			if old != nil && old[i].tableName == string(tableName) {
+				// avoid allocating new string
+				columns[i].tableName = old[i].tableName
+			} else {
+				columns[i].tableName = string(tableName)
+			}
 		} else {
 			n, err = skipLengthEncodedString(data[pos:])
 			if err != nil {
@@ -752,7 +760,12 @@ func (mc *mysqlConn) readColumns(count int) ([]mysqlField, error) {
 		if err != nil {
 			return nil, err
 		}
-		columns[i].name = string(name)
+		if old != nil && old[i].name == string(name) {
+			// avoid allocating new string
+			columns[i].name = old[i].name
+		} else {
+			columns[i].name = string(name)
+		}
 		pos += n
 
 		// Original name [len coded string]

rows.go

@@ -186,7 +186,7 @@ func (rows *binaryRows) NextResultSet() error {
 		return err
 	}
 
-	rows.rs.columns, err = rows.mc.readColumns(resLen)
+	rows.rs.columns, err = rows.mc.readColumns(resLen, nil)
 	return err
 }
 
@@ -208,7 +208,7 @@ func (rows *textRows) NextResultSet() (err error) {
 		return err
 	}
 
-	rows.rs.columns, err = rows.mc.readColumns(resLen)
+	rows.rs.columns, err = rows.mc.readColumns(resLen, nil)
 	return err
 }
 

statement.go

@@ -74,7 +74,7 @@ func (stmt *mysqlStmt) Exec(args []driver.Value) (driver.Result, error) {
 		// Columns
 		if metadataFollows && stmt.mc.extCapabilities&clientCacheMetadata != 0 {
 			// we can not skip column metadata because next stmt.Query() may use it.
-			if stmt.columns, err = mc.readColumns(resLen); err != nil {
+			if stmt.columns, err = mc.readColumns(resLen, stmt.columns); err != nil {
 				return nil, err
 			}
 		} else {
@@ -125,7 +125,7 @@ func (stmt *mysqlStmt) query(args []driver.Value) (*binaryRows, error) {
 	if resLen > 0 {
 		rows.mc = mc
 		if metadataFollows {
-			if rows.rs.columns, err = mc.readColumns(resLen); err != nil {
+			if rows.rs.columns, err = mc.readColumns(resLen, stmt.columns); err != nil {
 				return nil, err
 			}
 			stmt.columns = rows.rs.columns