Re: Couple of question on functions

"Vyacheslav Kalinin" <vka(at)mgcp(dot)com> writes:
> Reading the manual recently I came across this: (
> http://www.postgresql.org/docs/8.3/interactive/xfunc-volatility.html )
>> Because of the snapshotting behavior of MVCC (see Chapter 13<http://www.postgresql.org/docs/8.3/interactive/mvcc.html>)
> a function containing only SELECT commands can safely be marked
>> STABLE, even if it selects from tables that might be undergoing
> modifications by concurrent queries. PostgreSQL will execute a STABLE
>> function using the snapshot established for the calling query, and so it
> will see a fixed view of the database throughout that query.

> It stroke me that it might be not all that safe to mark SELECTing only
> function STABLE vs VOLATILE (or vice versa).

What it says is that you *can* mark such a function stable, without
violating the rules for a stable function. It doesn't say that this
choice doesn't affect the results. Feel free to propose better wording...

> Another thing I've recently discover is that SQL function seem to be
> unexpectedly slow to call. Example:
> ...
> perform f2sql(trunc(1000000*random())::int) +
> ...

> Now f4() reports 4.2 sec of runtime on average while f3() - 10.3 sec, that
> is a notable difference especially considering that SQL function is likely
> to be inlined. Do i miss something?

You might think it's "likely", but in fact no inlining will happen
because you have a volatile argument that's used more than once inside
the function body. Inlining would change the results.

regards, tom lane