[PATCH] Remove useless distinct clauses

Hi

In a recent audit, I noticed that application developers have a tendency to
abuse the distinct clause. For instance they use an ORM and add a distinct at
the top level just because they don't know the cost it has, or they don't know
that using EXISTS is a better way to express their queries than doing JOINs
(or worse, they can't do better).

They thus have this kind of queries (considering tbl1 has a PK of course):
SELECT DISTINCT * FROM tbl1;
SELECT DISTINCT * FROM tbl1 ORDER BY a;
SELECT DISTINCT tbl1.* FROM tbl1
JOIN tbl2 ON tbl2.a = tbl1.id;

These can be transformed into:
SELECT * FROM tbl1 ORDER BY *;
SELECT * FROM tbl1 ORDER BY a;
SELECT tbl1.* FROM tbl1 SEMI-JOIN tbl2 ON tbl2.a = tbl1.id ORDER BY tbl1.*;

The attached patch does that.
I added extra safeties in several place just to be sure I don't touch
something I can not handle, but I may have been very candid with the distinct
to sort transformation.
The cost of this optimization is quite low : for queries that don't have any
distinct, it's just one if. If there is a distinct, we iterate once through
every target, then we fetch the PK and iterate through the DISTINCT clause
fields. If it is possible to optimize, we then iterate through the JOINs.

Any comment on this would be more than welcome!

Regards

Pierre