Optimization: statements reuse previous column name (#1711)

#1708 added `[]mysqlField` cache to stmt. It was used only for MariaDB cached metadata.

This commit allows MySQL to also benefit from the metadata cache. If the
column names are the same as the cached metadata, it reuses them instead
of allocating new strings.

goos: darwin
goarch: arm64
pkg: github.com/go-sql-driver/mysql
cpu: Apple M1 Pro
                  │ master.txt  │           reuse.txt           │
                  │   sec/op    │   sec/op     vs base          │
ReceiveMetadata-8   1.273m ± 2%   1.269m ± 2%  ~ (p=1.000 n=10)

                  │  master.txt  │              reuse.txt              │
                  │     B/op     │     B/op      vs base               │
ReceiveMetadata-8   88.17Ki ± 0%   80.39Ki ± 0%  -8.82% (p=0.000 n=10)

                  │  master.txt  │             reuse.txt              │
                  │  allocs/op   │ allocs/op   vs base                │
ReceiveMetadata-8   1015.00 ± 0%   16.00 ± 0%  -98.42% (p=0.000 n=10)

Author: methane

benchmark_test.go

@@ -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())
 

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