> Given an arbitrary function fn(x) returning numeric.
> Question: how often is the function executed?
> select v.v,v.v from (select fn('const') as v) as v;
> 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.
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