Query optimization: rewrite GROUP BY with LIMIT to DISTINCT.

[alexey-milovidov]

Use case
If query contains GROUP BY but no aggregate functions and also contains LIMIT, remove GROUP BY and add DISTINCT.

SELECT a, b, c FROM t GROUP BY a, b, c LIMIT 10

->

SELECT DISTINCT a, b, c FROM t LIMIT 10

This optimization is viable, because DISTINCT can output records without waiting for full result and it can stop earlier in presence of LIMIT.

But if LIMIT is not present, DISTINCT is less efficient than GROUP BY due to lack of some specializations.

PS. Advanced version:

SELECT a, b, c, any(x), any(y) FROM t GROUP BY a, b, c LIMIT 10

->

SELECT a, b, c, x, y FROM t LIMIT 1 BY a, b, c