protected override Expression VisitSelect(SelectExpression select)
{
// select * from table order by x skip s take t
// =>
// select * from (select top s * from (select top s + t from table order by x) order by -x) order by x
select = (SelectExpression)base.VisitSelect(select);
if (select.Skip != null && select.Take != null && select.OrderBy.Count > 0)
{
var skip = select.Skip;
var take = select.Take;
var skipPlusTake = PartialEvaluator.Eval(Expression.Add(skip, take));
select = select.SetTake(skipPlusTake).SetSkip(null);
select = select.AddRedundantSelect(this.language, new TableAlias());
select = select.SetTake(take);
// propogate order-bys to new layer
select = (SelectExpression)OrderByRewriter.Rewrite(this.language, select);
var inverted = select.OrderBy.Select(ob => new OrderExpression(
ob.OrderType == OrderType.Ascending ? OrderType.Descending : OrderType.Ascending,
ob.Expression
));
select = select.SetOrderBy(inverted);
select = select.AddRedundantSelect(this.language, new TableAlias());
select = select.SetTake(Expression.Constant(0)); // temporary
select = (SelectExpression)OrderByRewriter.Rewrite(this.language, select);
var reverted = select.OrderBy.Select(ob => new OrderExpression(
ob.OrderType == OrderType.Ascending ? OrderType.Descending : OrderType.Ascending,
ob.Expression
));
select = select.SetOrderBy(reverted);
select = select.SetTake(null);
}
return(select);
}
protected override Expression VisitSelect(SelectExpression select)
{
select = (SelectExpression)base.VisitSelect(select);
if (select.Skip != null)
{
SelectExpression newSelect = select.SetSkip(null).SetTake(null);
bool canAddColumn = !select.IsDistinct && (select.GroupBy == null || select.GroupBy.Count == 0);
if (!canAddColumn)
{
newSelect = newSelect.AddRedundantSelect(this.language, new TableAlias());
}
var colType = this.language.TypeSystem.GetColumnType(typeof(int));
newSelect = newSelect.AddColumn(new ColumnDeclaration("o_rownum", new RowNumberExpression(select.OrderBy), colType));
// add layer for WHERE clause that references new rownum column
newSelect = newSelect.AddRedundantSelect(this.language, new TableAlias());
newSelect = newSelect.RemoveColumn(newSelect.Columns.Single(c => c.Name == "o_rownum"));
var newAlias = ((SelectExpression)newSelect.From).Alias;
ColumnExpression rnCol = new ColumnExpression(typeof(int), colType, newAlias, "o_rownum");
Expression where;
if (select.Take != null)
{
where = new BetweenExpression(rnCol, Expression.Add(select.Skip, Expression.Constant(1)), Expression.Add(select.Skip, select.Take));
}
else
{
where = rnCol.GreaterThan(select.Skip);
}
if (newSelect.Where != null)
{
where = newSelect.Where.And(where);
}
newSelect = newSelect.SetWhere(where);
select = newSelect;
}
return(select);
}
