Crystal Reports How does sub report query works??

Message

     Hi again,

I would like to know the working of sub report queries.

main report query:
select *,recid from myTable where recid={?parameterRecID}

Sub report query:

select *,recIdChild from myChildTable 

sub report is linked as
{command.recIDChild} = {?Pm-command.recID}

now the question will 
select *,recIdChild from myChildTable 
query run first and will return all the records from the myChildTable and then will filter the records based on the condition used for linking?
or
will it fetch only the matching records from the SQL for the condition used for linking.

because if it will fetch all the records from the SQL and then will filter the records based on the report link condition then there will be performance issue because i am fetching all the records and then showing only some of those which is definitely not a right way.

Please if someone know then explain it.

TIA,
Surinder Singh