tuning Oracle7 for large queries in PL/SQL, Reportwriter, etc.

I'm running into problems with figuring out how to get reasonable performance from PL/SQL and SQL*Reportwriter (and possibly others).

Effectively, both problems are occuring on the following situation:

Cursor 1: Select store_no, name from stores order by store_no

Cursor 2: Select invoice_no, inv_date, amount from invoices

           where invoices.store_no = stores.store_no
           order by inv_date

Cursor 3:  Select pmt_date, pmt_code, amount from ardetail
           where ardetail.store_no = stores.store_no
           and ardetail.invoice_no = invoices.invoice_no
           order by pmt_date

This runs along very nicely until it gets to the BIG customer (One customer accounts for approximately 16500 out of 34000 invoices.) At this point, things slow down to an absolute CRAWL. (Where before I was getting perhaps 20 transactions/second processed, at this point I'm getting more like 2/second.)

What strategies are others using to improve performance in this sort of situation?

Configuration: NCR 3445 (486/33Mhz, 32MB, Unix SVR4) running 7.0.12.

                SGA is pretty much the default one, approximately 5MB overall,
                3.5MB in the shared memory pool.

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Michael Nolan, Tailored Software Services, Inc.
Lincoln, Nebraska
Sysop for the GEnie DBMS RoundTable
nolan_at_notes.tssi.com, dbms_at_genie.geis.com