Peter wrote:
> Given an arbitrary function fn(x) returning numeric.
>
> Question: how often is the function executed?
> [...]
> C.
> select v.v,v.v from (select fn('const') as v) as v;
>
> Answer:
> Once if declared VOLATILE.
> Twice if declared STABLE.
>
> Now this IS a surprize. It is clear that the system is not allowed to
> execute the function twice when declared VOLATILE. It IS ALLOWED to
> execute it twice when STABLE - but to what point, except prolonging
> execution time?
>
> Over all, VOLATILE performs better than STABLE.
The reason is that the subquery with the VOLATILE function can be
flattened; see the EXPLAIN (VERBOSE) output.
There is not guarantee that less volatility means better performance.
Yours,
Laurenz Albe
--
Cybertec | https://www.cybertec-postgresql.com
