Crystal Report / Subreport - Performance Issue

Question

I am having problems with a Crystal Report / Subreport with performance. I am using Crystal Version 11, directly in Crystal Developer.  There are no other programs involved.  I am linking to a SQL Server database using ODBC.
I have narrowed the problem down to this:
The main report has 4 tables.  See diagram using the following link.   http://screencast.com/t/TA9YYlwwl7
The subreport has 1 table, this table has > 2 Million records in it.
The main report links to the subreport with one field:
Main report field SAMPLE.PATIENT = subreport field Z_RESULT_HISTORY.PHIN
When I set the subreport linking within Crystal   it automatically generates the following in the record selection for the subreport:
{Z_RESULT_HISTORY.PHIN} = {?Pm-SAMPLE.PATIENT}
The problem is that the report execution time is dependent on the field that I am using for the record selection in the main report.
Case I   works lightening fast:
There are 16 records returned on the Main Report   each one of these has about 1-5 records returned on the subreport.
{SAMPLE.PATIENT} = "MOUSEMICKEY"  and {SAMPLE.SAMPLE_NUMBER} < 200
Case II   brutally slow   there are 51 records in the main report that qualify   with a few records each in the subreport.  By  brutally slow    I mean a few minutes:
{BATCH.NAME} = "HEP_ARCH-20090420-1"
In this case, I can see in the bottom right of the Crystal Preview window, that it is reading through all 2M records in the Z_RESULT_HISTORY table
Case III   brutally slow - a couple minutes
{BATCH_OBJECTS.OBJECT_ID} = 111
This returns 1 record on the main report and 0 records on the subreport.  Yet I can see it reading through ALL 2 Million records before the report is displayed.
What I can t understand is why the field used for record selection on the MAIN report is affecting the speed of the execution of the subreport.  I need the main report to be selected by BATCH.NAME   yet I can t figure out what I can change to make the report run fast.  When I record select the main report by SAMPLE.PATIENT, I do NOT see the subreport reading all 2M reocrds - the report preview is returned in less than 1 second.
Any help would be much appreciated.

Answer 1

Lindell - your response was very helpful.  I was able to create a SQL Command on the subreport and change the subreport links on the main report to use the paramters in the SQL Command.  The report is now very fast - even when there are lots of detail records on the main report.  It is properly executing the query and not reading all 2M records into memory for each subreport.
I'm still totally confused as to why Crystal was misbehaving so poorly and changing how the subreport queried the database when the only change was the fields used for record selection on the main report.  It really looks like a bug to me, but maybe someone can still enlighten me.
thanks again so much.  We are in Parallel testing for a production rollout - and the users are MUCH happier now!  (which of course makes me much happier!)
Susan
PS I was not aware of the SQL Command - had never used it before.  Took me a little while to figure it out and how to do the linking - but it is very powerful.  Thanks again.