MYSQL optimizer: Handling Subqueires

[Rakshit Trivedi]

Hi,

I wanted to know the handling of subquery conditions in the WHERE clause. The order of rows in EXPLAIN statement shows the plan for the main select first and then that of the sub-selects.

I had two questions regarding this:

1.) Whenevr a subquery is used, does it affect the table access plan and the cardinality estimation for the table in the outer select on which this subquery condition applies.

For example, in the following query

select colum1
from
t1,t2
where
t1.attribname=t2.attribname
and
t1.attribname<= (select t1.attribname from t1 where t1.attribname< constant value)

does the subquery condition for t1 affect its cardinality estimation and/or table access plan? As such, seeing the EXPLAIN output for such queries, I feel it's not affecting.

2.) If not, how the subqueries are handled by optimizer? Are they planned independently from the outer query? And if this istrue, does it mean that the outer query planning does not take subquery conditions into consideration for optimization?