public static SelectExpression RemoveRedundantFrom(this SelectExpression select)
{
SelectExpression fromSelect = select.From as SelectExpression;
if (fromSelect != null)
{
return(SubqueryRemover.Remove(select, fromSelect));
}
return(select);
}
protected override Expression VisitProjection(ProjectionExpression proj)
{
proj = (ProjectionExpression)base.VisitProjection(proj);
if (proj.Select.From is SelectExpression)
{
List <SelectExpression> redundant = RedundantSubqueryGatherer.Gather(proj.Select);
if (redundant != null)
{
proj = SubqueryRemover.Remove(proj, redundant);
}
}
return(proj);
}
protected override Expression VisitSelect(SelectExpression select)
{
select = (SelectExpression)base.VisitSelect(select);
// first remove all purely redundant subqueries
List <SelectExpression> redundant = RedundantSubqueryGatherer.Gather(select.From);
if (redundant != null)
{
select = SubqueryRemover.Remove(select, redundant);
}
return(select);
}
protected override Expression VisitSelect(SelectExpression select)
{
bool wasTopLevel = isTopLevel;
isTopLevel = false;
select = (SelectExpression)base.VisitSelect(select);
// next attempt to merge subqueries that would have been removed by the above
// logic except for the existence of a where clause
while (CanMergeWithFrom(select, wasTopLevel))
{
SelectExpression fromSelect = GetLeftMostSelect(select.From);
// remove the redundant subquery
select = SubqueryRemover.Remove(select, fromSelect);
// merge where expressions
Expression where = select.Where;
if (fromSelect.Where != null)
{
if (where != null)
{
where = fromSelect.Where.And(where);
}
else
{
where = fromSelect.Where;
}
}
var orderBy = select.OrderBy != null && select.OrderBy.Count > 0 ? select.OrderBy : fromSelect.OrderBy;
var groupBy = select.GroupBy != null && select.GroupBy.Count > 0 ? select.GroupBy : fromSelect.GroupBy;
Expression skip = select.Skip != null ? select.Skip : fromSelect.Skip;
Expression take = select.Take != null ? select.Take : fromSelect.Take;
bool isDistinct = select.IsDistinct | fromSelect.IsDistinct;
if (where != select.Where ||
orderBy != select.OrderBy ||
groupBy != select.GroupBy ||
isDistinct != select.IsDistinct ||
skip != select.Skip ||
take != select.Take)
{
select = new SelectExpression(select.Alias, select.Columns, select.From, where, orderBy, groupBy, isDistinct, skip, take, select.IsReverse);
}
}
return(select);
}