Question

which one is faster

select * from parents p
inner join children c on p.id = c.pid
where p.x = 2

OR

select * from 
(select * from parents where p.x = 2)
p
inner join children c on p.id = c.pid
where p.x = 2

Answer 1

In MySQL, the first one is faster:

SELECT  *
FROM    parents p
INNER JOIN
        children c
ON      c.pid = p.id
WHERE   p.x = 2

, since using an inline view implies generating and passing the records twice.

In other engines, they are usually optimized to use one execution plan.

MySQL is not very good in parallelizing and pipelining the result streams.

Like this query:

SELECT  *
FROM    mytable
LIMIT 1

is instant, while this one (which is semantically identical):

SELECT  *
FROM    (
        SELECT  *
        FROM    mytable
        )
LIMIT 1

will first select all values from mytable, buffer them somewhere and then fetch the first record.

For Oracle, SQL Server and PostgreSQL, the queries above (and both of your queries) will most probably yield the same execution plans.