Add prefer simple protocol support to stdlib

Test code partially taken from james-lawrence
(7471e7f9eb0f33a05e2f0cf06db8714850880d72)

stdlib/sql.go

@@ -158,7 +158,7 @@ func (d *Driver) Open(name string) (driver.Conn, error) {
 		}
 	}
 
-	c := &Conn{conn: conn, driver: d}
+	c := &Conn{conn: conn, driver: d, connConfig: connConfig}
 	return c, nil
 }
 
@@ -210,9 +210,10 @@ func UnregisterDriverConfig(c *DriverConfig) {
 }
 
 type Conn struct {
-	conn    *pgx.Conn
-	psCount int64 // Counter used for creating unique prepared statement names
-	driver  *Driver
+	conn       *pgx.Conn
+	psCount    int64 // Counter used for creating unique prepared statement names
+	driver     *Driver
+	connConfig pgx.ConnConfig
 }
 
 func (c *Conn) Prepare(query string) (driver.Stmt, error) {
@@ -303,14 +304,24 @@ func (c *Conn) Query(query string, argsV []driver.Value) (driver.Rows, error) {
 		return nil, driver.ErrBadConn
 	}
 
-	ps, err := c.conn.Prepare("", query)
+	if !c.connConfig.PreferSimpleProtocol {
+		ps, err := c.conn.Prepare("", query)
+		if err != nil {
+			return nil, err
+		}
+
+		restrictBinaryToDatabaseSqlTypes(ps)
+		return c.queryPrepared("", argsV)
+	}
+
+	rows, err := c.conn.Query(query, valueToInterface(argsV)...)
 	if err != nil {
 		return nil, err
 	}
 
-	restrictBinaryToDatabaseSqlTypes(ps)
-
-	return c.queryPrepared("", argsV)
+	// Preload first row because otherwise we won't know what columns are available when database/sql asks.
+	more := rows.Next()
+	return &Rows{rows: rows, skipNext: true, skipNextMore: more}, nil
 }
 
 func (c *Conn) QueryContext(ctx context.Context, query string, argsV []driver.NamedValue) (driver.Rows, error) {
@@ -318,14 +329,24 @@ func (c *Conn) QueryContext(ctx context.Context, query string, argsV []driver.Na
 		return nil, driver.ErrBadConn
 	}
 
-	ps, err := c.conn.PrepareEx(ctx, "", query, nil)
+	if !c.connConfig.PreferSimpleProtocol {
+		ps, err := c.conn.PrepareEx(ctx, "", query, nil)
+		if err != nil {
+			return nil, err
+		}
+
+		restrictBinaryToDatabaseSqlTypes(ps)
+		return c.queryPreparedContext(ctx, "", argsV)
+	}
+
+	rows, err := c.conn.QueryEx(ctx, query, nil, namedValueToInterface(argsV)...)
 	if err != nil {
 		return nil, err
 	}
 
-	restrictBinaryToDatabaseSqlTypes(ps)
-
-	return c.queryPreparedContext(ctx, "", argsV)
+	// Preload first row because otherwise we won't know what columns are available when database/sql asks.
+	more := rows.Next()
+	return &Rows{rows: rows, skipNext: true, skipNextMore: more}, nil
 }
 
 func (c *Conn) queryPrepared(name string, argsV []driver.Value) (driver.Rows, error) {
@@ -408,8 +429,10 @@ func (s *Stmt) QueryContext(ctx context.Context, argsV []driver.NamedValue) (dri
 }
 
 type Rows struct {
-	rows   *pgx.Rows
-	values []interface{}
+	rows         *pgx.Rows
+	values       []interface{}
+	skipNext     bool
+	skipNextMore bool
 }
 
 func (r *Rows) Columns() []string {
@@ -486,7 +509,14 @@ func (r *Rows) Next(dest []driver.Value) error {
 		}
 	}
 
-	more := r.rows.Next()
+	var more bool
+	if r.skipNext {
+		more = r.skipNextMore
+		r.skipNext = false
+	} else {
+		more = r.rows.Next()
+	}
+
 	if !more {
 		if r.rows.Err() == nil {
 			return io.EOF