How to avoid multiple call to function:

Question

In our datawarehouse we have a huge receipt row table where all metrics ar stored in the local currency. On top over that we have views which calculate metrics to the desired currency.
So basically all views looks like this
select geo_region,
product_group,
customer_group,
metric1 * (select get_exchange_rate(currency_id) from dual) metric1,
metric1 * (select get_exchange_rate(currency_id) from dual) metric2,
metric1 * (select get_exchange_rate(currency_id) from dual) metricx,
group by..
As we have about 20 metrics we notices that the function is called 20 times per row.
Is there really anyway to avoid that? Shouldn't it be it's just the exact same call with the same in-parameters over and over again.
We've tried with local sys_context and the performance is better but the call to the context is still performed 20 times. Any Ideas?

Answer 1

Can you avoid multiple function calls? Maybe, if as in your example all the function calls values are computing the same result. If they operate on different columns then you'll have to perform the function call anyway.
Either way you should be able to eliminate the (near as I can tell) pointless subquery from dual
You might be able to avoid the repeated function calls if the values are always the same. If every computation you could save the function call (and subquery!) by doing it once and then using assignments after the initial query using variables after the initial query, perhaps using NULL in the query as placeholders to select into a record - something like
select inital_region,
         product_group,
         customer_group,
         metric1 * exchange_rate(currency_id) metric1,
         null metric2,
v_metric2 := metric1;
...Message was edited by (fixed typo):
riedelme