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(new TableAlias()); } var colType = SqlType.Int32; newSelect = newSelect.AddColumn(new ColumnDeclaration(columnName, new RowNumberExpression(select.OrderBy), colType)); // add layer for WHERE clause that references new rownum column newSelect = newSelect.AddRedundantSelect(new TableAlias()); newSelect = newSelect.RemoveColumn(newSelect.Columns.Single(c => c.Name == columnName)); var newAlias = ((SelectExpression)newSelect.From).Alias; ColumnExpression rnCol = new ColumnExpression(typeof(int), colType, newAlias, columnName); 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 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(new TableAlias()); select = select.SetTake(take); // propogate order-bys to new layer select = (SelectExpression)OrderByRewriter.Rewrite(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(new TableAlias()); select = select.SetTake(Expression.Constant(0)); // temporary select = (SelectExpression)OrderByRewriter.Rewrite(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); }