database/sql: Add optional ColumnsCounter interface

src/pkg/database/sql/driver/driver.go

 // Copyright 2011 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 // Package driver defines interfaces to be implemented by database
 // drivers as used by package sql.
 //
 // Most code should use package sql.
 package driver
 
@@ skipping 155 matching lines @@
         // size as the Columns() are wide.
         //
         // The dest slice may be populated only with
         // a driver Value type, but excluding string.
         // All string values must be converted to []byte.
         //
         // Next should return io.EOF when there are no more rows.
         Next(dest []Value) error
 }
 
+// ColumnsCounter may be optionally implemented by Rows if the number
+// of columns is known without building a slice of all column names.
+// If implemented, the number of columns of the result is inferred
+// from ColumnsCount instead of the length of the slice returned by
+// Columns.
+type ColumnsCounter interface {

[ahormann, 2013/12/17 15:01:20]

I'd name it NumColumns or ColumnCount for consistency with Columns.
It's more of an attribute access than an action to me.

+        ColumnsCount() int
+}
+
 // Tx is a transaction.
 type Tx interface {
         Commit() error
         Rollback() error
 }
 
 // RowsAffected implements Result for an INSERT or UPDATE operation
 // which mutates a number of rows.
 type RowsAffected int64
 
@@ skipping 16 matching lines @@
 
 var _ Result = noRows{}
 
 func (noRows) LastInsertId() (int64, error) {
         return 0, errors.New("no LastInsertId available after DDL statement")
 }
 
 func (noRows) RowsAffected() (int64, error) {
         return 0, errors.New("no RowsAffected available after DDL statement")
 }