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)
{
Expression skip = select.Skip;
Expression take = select.Take;
Expression skipPlusTake = PartialEvaluator.Eval(Expression.Add(skip, take));
select = select.SetTake(skipPlusTake).SetSkip(null);
select = select.AddRedundantSelect(language, new TableAlias());
select = select.SetTake(take);
// propogate order-bys to new layer
select = (SelectExpression)OrderByRewriter.Rewrite(language, select);
IEnumerable <OrderExpression> inverted = select.OrderBy.Select(ob => new OrderExpression(ob.OrderType == OrderType.Ascending ? OrderType.Descending : OrderType.Ascending, ob.Expression));
select = select.SetOrderBy(inverted);
select = select.AddRedundantSelect(language, new TableAlias());
select = select.SetTake(Expression.Constant(0)); // temporary
select = (SelectExpression)OrderByRewriter.Rewrite(language, select);
IEnumerable <OrderExpression> 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(language, new TableAlias());
}
QueryType colType = language.TypeSystem.GetColumnType(typeof(int));
newSelect = newSelect.AddColumn(new ColumnDeclaration("_rownum", new RowNumberExpression(select.OrderBy), colType));
// add layer for WHERE clause that references new rownum column
newSelect = newSelect.AddRedundantSelect(language, new TableAlias());
newSelect = newSelect.RemoveColumn(newSelect.Columns.Single(c => c.Name == "_rownum"));
TableAlias newAlias = ((SelectExpression)newSelect.From).Alias;
var rnCol = new ColumnExpression(typeof(int), colType, newAlias, "_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);
}
protected override Expression VisitProjection(ProjectionExpression proj)
{
if (isTopLevel)
{
isTopLevel = false;
currentSelect = proj.Select;
Expression projector = Visit(proj.Projector);
if (projector != proj.Projector || currentSelect != proj.Select)
{
return(new ProjectionExpression(currentSelect, projector, proj.Aggregator));
}
return(proj);
}
if (proj.IsSingleton && CanJoinOnServer(currentSelect))
{
var newAlias = new TableAlias();
currentSelect = currentSelect.AddRedundantSelect(language, newAlias);
// remap any references to the outer select to the new alias;
var source = (SelectExpression)ColumnMapper.Map(proj.Select, newAlias, currentSelect.Alias);
// add outer-join test
ProjectionExpression pex = language.AddOuterJoinTest(new ProjectionExpression(source, proj.Projector));
ProjectedColumns pc = ColumnProjector.ProjectColumns(language, pex.Projector, currentSelect.Columns, currentSelect.Alias, newAlias, proj.Select.Alias);
var join = new JoinExpression(JoinType.OuterApply, currentSelect.From, pex.Select, null);
currentSelect = new SelectExpression(currentSelect.Alias, pc.Columns, join, null);
return(Visit(pc.Projector));
}
bool saveTop = isTopLevel;
SelectExpression saveSelect = currentSelect;
isTopLevel = true;
currentSelect = null;
Expression result = base.VisitProjection(proj);
isTopLevel = saveTop;
currentSelect = saveSelect;
return(result);
}
