Long running INSERT+SELECT query

Hi,

We're trying to populate a table with aggregated data from other tables. 
For that we're running a huge INSERT+SELECT query which joins several 
tables, aggregates values, and then inserts the results into another 
table. The problem we're facing is that while the query is running , 
some records in tables referenced by the results table may be deleted, 
which causes the following error:

ERROR:  insert or update on table "..." violates foreign key constraint 
"..."
DETAIL:  Key (...)=(...) is not present in table "...".

Who do we make sure that such aggregating query would not fail?

Regards,
Vitaliy