Comment 1 for bug 1001659

It is very difficult made GROUP BY in TREE VIEW or in GRAPH VIEW, if field calculate from other fields.
For example we have table with following fields:
date (Date of production)
prod_qty (Total Quantity produced items)
reject_qty (Quantity of rejected items)
quality (Calculated field: quality = reject_qty/prod_qty)

If we try GROUP BY 'date' then for some date we get:

PROD_QTY = SUM(prod_qty of the same date)
REJECT_QTY = SUM(reject_qty of the same date)
QUALITY = SUM(quality of the same date)

But for agregate QUALITY = REJECT_QTY/PROD_QTY = SUM(reject_qty of the same date)/SUM(prod_qty of the same date)
Because it no the same as SUM(quality of the same date), need add extra code (for example rewrite function read_group())

It will be more cool if we have functional attribute for field, that will describe custom agregate function other than SUM().