Remove unnecessary read

* Add benchmark for SelectValueTo

bench_test.go

@@ -2,6 +2,7 @@ package pgx_test
 
 import (
 	"github.com/JackC/pgx"
+	"io/ioutil"
 	"math/rand"
 	"testing"
 )
@@ -41,6 +42,7 @@ func createNarrowTestData(b *testing.B, conn *pgx.Connection) {
 
 	mustPrepare(b, conn, "getNarrowById", "select * from narrow where id=$1")
 	mustPrepare(b, conn, "getMultipleNarrowById", "select * from narrow where id between $1 and $2")
+	mustPrepare(b, conn, "getMultipleNarrowByIdAsJSON", "select json_agg(row_to_json(narrow)) from narrow where id between $1 and $2")
 
 	narrowTestDataLoaded = true
 }
@@ -124,6 +126,38 @@ func BenchmarkSelectRowsPreparedNarrow(b *testing.B) {
 	}
 }
 
+func BenchmarkSelectValuePreparedNarrow(b *testing.B) {
+	conn := getSharedConnection(b)
+	createNarrowTestData(b, conn)
+
+	// Get random ids outside of timing
+	ids := make([]int32, b.N)
+	for i := 0; i < b.N; i++ {
+		ids[i] = 1 + rand.Int31n(9999)
+	}
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		mustSelectValue(b, conn, "getMultipleNarrowByIdAsJSON", ids[i], ids[i]+10)
+	}
+}
+
+func BenchmarkSelectValueToPreparedNarrow(b *testing.B) {
+	conn := getSharedConnection(b)
+	createNarrowTestData(b, conn)
+
+	// Get random ids outside of timing
+	ids := make([]int32, b.N)
+	for i := 0; i < b.N; i++ {
+		ids[i] = 1 + rand.Int31n(9999)
+	}
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		mustSelectValueTo(b, conn, ioutil.Discard, "getMultipleNarrowByIdAsJSON", ids[i], ids[i]+10)
+	}
+}
+
 func createJoinsTestData(b *testing.B, conn *pgx.Connection) {
 	if testJoinsDataLoaded {
 		return

connection.go

@@ -13,6 +13,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"net"
 	"os/user"
 	"time"
@@ -393,14 +394,8 @@ func (c *Connection) SelectValueTo(w io.Writer, sql string, arguments ...interfa
 }
 
 func (c *Connection) rxDataRowValueTo(w io.Writer, bodySize int32) (err error) {
-	buf := c.getBuf()
-	if _, err = io.CopyN(buf, c.reader, 6); err != nil {
-		c.die(err)
-		return
-	}
-
 	var columnCount int16
-	err = binary.Read(buf, binary.BigEndian, &columnCount)
+	err = binary.Read(c.reader, binary.BigEndian, &columnCount)
 	if err != nil {
 		c.die(err)
 		return
@@ -408,7 +403,7 @@ func (c *Connection) rxDataRowValueTo(w io.Writer, bodySize int32) (err error) {
 
 	if columnCount != 1 {
 		// Read the rest of the data row so it can be discarded
-		if _, err = io.CopyN(buf, c.reader, int64(bodySize-6)); err != nil {
+		if _, err = io.CopyN(ioutil.Discard, c.reader, int64(bodySize-2)); err != nil {
 			c.die(err)
 			return
 		}
@@ -417,7 +412,7 @@ func (c *Connection) rxDataRowValueTo(w io.Writer, bodySize int32) (err error) {
 	}
 
 	var valueSize int32
-	err = binary.Read(buf, binary.BigEndian, &valueSize)
+	err = binary.Read(c.reader, binary.BigEndian, &valueSize)
 	if err != nil {
 		c.die(err)
 		return

helper_test.go

@@ -2,6 +2,7 @@ package pgx_test
 
 import (
 	"github.com/JackC/pgx"
+	"io"
 	"testing"
 )
 
@@ -56,3 +57,9 @@ func mustSelectValue(t testing.TB, conn *pgx.Connection, sql string, arguments .
 	}
 	return
 }
+
+func mustSelectValueTo(t testing.TB, conn *pgx.Connection, w io.Writer, sql string, arguments ...interface{}) {
+	if err := conn.SelectValueTo(w, sql, arguments...); err != nil {
+		t.Fatalf("SelectValueTo unexpectedly failed with %v: %v", sql, err)
+	}
+}