Re: Where Clause Optimization

[Øystein Grøvlen]

Mohammed Yehia El Sayed Abdoh Wrote:
> I already created a test case with all
> posdibilities and couldn't note any differences in
> the result set. That's why I am asking

Not sure why you would expect to see any differences in the result set. The whole point of optimizations is to find more efficient ways to execute the query without changing the result.

That HAVING is merged with WHERE means that if you have a query like:

SELECT * FROM t WHERE a = 1 HAVING b = 2

it is converted to:

SELECT * FROM t WHERE a = 1 AND b = 2

If you use EXPLAIN FORMAT=JSON, you can see the condition that will be attached to the table:

"attached_condition": "((`test`.`t`.`a` = 1) and (`test`.`t`.`b` = 2))"

Øystein Grøvlen,
Senior Principal Software Engineer,
MySQL Group, Oracle,
Trondheim, Norway