Jonathan

When interbase runs a query it caches the read pages in memory, so
subsequent queries make use of this faster access method instead of reading
from the hard drive again. This means that frequently accessed data gets
delivered faster, which is what you are seeing.
45000 rows in a table is nothing at all, so I suspect you dont have one or
all of the following
1. An index on the field date_recep for the table recep
ie. CREATE ASC INDEX IDX_RECEP_DATE ON RECEP( DATE);
2. An index on the field chauff for the table recep
ie. CREATE ASC INDEX IDX_RECEP_CHAUFF ON RECEP( CHAUFF);
3. An index on the field code for the table employes
ie. CREATE ASC INDEX IDX_EMPLOYES_CODE ON EMPLOYES( CODE);
If you dont have any of the above, create them - I think you will be
pleasantly suprised

Regards
Gary