/// <summary>双Top分页，因为没有使用not in，性能比NotIn要好。语句必须有排序，不需额外指定排序列</summary>
/// <param name="builder"></param>
/// <param name="startRowIndex"></param>
/// <param name="maximumRows"></param>
/// <param name="queryCountCallback">查询总记录数的委托，近供DoubleTop使用</param>
/// <returns></returns>
static SelectBuilder DoubleTop(SelectBuilder builder, Int64 startRowIndex, Int64 maximumRows, Func <SelectBuilder, Int64> queryCountCallback)
{
if (builder.Keys == null)
{
throw new ArgumentNullException("Key", "DoubleTop分页算法要求指定排序列！" + builder.ToString());
}
// 采用DoubleTop分页，最后一页可能有问题，需要特殊处理
if (queryCountCallback != null)
{
// 查询总记录数，计算是否最后一页
var count = queryCountCallback(builder);
// 数据不足
if (count <= startRowIndex)
{
return(null);
}
// 刚好相等的就不必处理了
if (startRowIndex + maximumRows > count)
{
maximumRows = count - startRowIndex;
}
}
// 分页标准 Select (20,10,ID Desc)
// 1，按原始排序取20+10行，此时目标页位于底部
// 2，倒序过来取10行，得到目标页，但是顺序是反的
// 3，再倒序一次
// 显然，原始语句必须有排序，否则无法倒序。另外，也不能处理maximumRows<1的情况
// Select * From (Select Top 10 * From (Select Top 20+10 * From Table Order By ID Desc) Order By ID Asc) Order By ID Desc
// 找到排序，优先采用排序字句来做双Top排序
var orderby = builder.OrderBy ?? builder.KeyOrder;
var keys = SelectBuilder.Split(orderby, out var isdescs);
// 把排序反过来
var isdescs2 = new Boolean[keys.Length];
for (var i = 0; i < keys.Length; i++)
{
if (isdescs != null && isdescs.Length > i)
{
isdescs2[i] = !isdescs[i];
}
else
{
isdescs2[i] = true;
}
}
var reversekeyorder = SelectBuilder.Join(keys, isdescs2);
// 构建Select Top 20 * From Table Order By ID Asc
var builder1 = builder.Clone().AppendColumn(keys).Top(startRowIndex + maximumRows);
// 必须加一个排序，否则会被优化掉而导致出错
if (String.IsNullOrEmpty(builder1.OrderBy))
{
builder1.OrderBy = builder1.KeyOrder;
}
var builder2 = builder1.AsChild("XCode_T0", true).Top(maximumRows);
// 要反向排序
builder2.OrderBy = reversekeyorder;
var builder3 = builder2.AsChild("XCode_T1", true);
// 结果列处理
builder3.Column = builder.Column;
// 如果结果列包含有“.”，即有形如tab1.id、tab2.name之类的列时设为获取子查询的全部列
if ((!string.IsNullOrEmpty(builder3.Column)) && builder3.Column.Contains("."))
{
builder3.Column = "*";
}
// 让结果正向排序
builder3.OrderBy = orderby;
return(builder3);
}
