Optimization: statements reuse previous column name #1711

Original file line number	Diff line number	Diff line change
Expand Up		@@ -490,14 +490,13 @@ func BenchmarkReceiveMetadata(b *testing.B) {
		valuePtrs[j] = &values[j]
		}

	b.ReportAllocs()
	b.ResetTimer()

		// Prepare a SELECT query to retrieve metadata
		stmt := tb.checkStmt(db.Prepare("SELECT * FROM large_integer_table LIMIT 1"))
		defer stmt.Close()

		// Benchmark metadata retrieval
	b.ReportAllocs()
	b.ResetTimer()
		for range b.N {
		rows := tb.checkRows(stmt.Query())

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4 changes: 2 additions & 2 deletions connection.go

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Original file line number	Diff line number	Diff line change
Expand Up		@@ -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 {
Expand Down Expand Up		@@ -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
		}

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19 changes: 16 additions & 3 deletions packets.go

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Original file line number	Diff line number	Diff line change
Expand Up		@@ -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()
Expand Down Expand Up		@@ -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 {
Expand All		@@ -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]
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4 changes: 2 additions & 2 deletions rows.go

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Original file line number	Diff line number	Diff line change
Expand Up		@@ -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
		}

Expand All		@@ -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
		}

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4 changes: 2 additions & 2 deletions statement.go

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Original file line number	Diff line number	Diff line change
Expand Up		@@ -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 {
Expand Down Expand Up		@@ -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
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