What happend first in mysql: join or where

Question

Let's say I have two tables A and B and the following query:

select *
from A
inner join B on A.id = B.id
Where A.id = 5

Does mysql first performs the join or the where?

Edit:

Cause if for example A contains 1000 rows, after the where condition it'll contain only 1 row. Performing join on a 1 row table is much more efficient so it seems like performing the where first and only then the join is more efficient.

Answer 1

The join happens before the where, however...

The where clause is a filter for all rows returned by the join, but the optimizer will recognise that if an index exists on A.id, it will be used to retrieve rows from A that match, then the join will happen, then theoretically the where clause will filter the results, but again the optimizer will recognise that the condition will already be met so it will skip it as a filter.

All that said, the optimizer will always return the same result as would be returned without the optimizer.