Does MySql analyze / optimize queries before executing them?

Question

If you'd write a query like so:

SELECT * FROM `posts` WHERE `views` > 200 OR `views` > 100

Would MySql analyze that query and realize that it's actually equivalent to this?

SELECT * FROM `posts` WHERE `views` > 100

In other words, would MySql optimize the query such that it skips any unnecessary WHERE checks?

I'm asking because I'm working on a piece of code that, for now, generates queries with redundant WHERE clauses. I'm wondering if I should optimize those queries before I send them to MySql, or if that's unnecessary, because MySql would do it anyway.

Answer 1

Yes. MySQL does optimize queries before running them. In fact, what runs has no obvious relationship to the SQL statement itself -- it is a directed acyclic graph .

In the process, MySQL determines what indexes to use for the query, what join algorithms, sorts lists of constants in in lists, and much more. The subject of optimizations is too long for Stack Overflow answer.

The optimizer also does some simplifications of the query. I'm not sure if those simplifications extend to inequalities. However, there is little overhead in making the comparison twice.

Answer 2

EXPLAIN SELECT... Shows how the query was rewritten -- but it still has the OR.

The "Optimizer trace" says the same thing. However, when it gets into discussing the "cost", it gets smart and merges the two comparisons. (This is the case at least as far back as 5.6.)

In many cases, OR should be avoided like covid.