Subject Re: [firebird-support] Re: Making evaluation of UDFs constant across a query
Author Mike Pomraning
On Sat, Mar 19, 2011 at 7:11 AM, karolbieniaszewski <
liviuslivius@...> wrote:

> --- In, Mike Pomraning <mjp@...> wrote:
> >
> > Is there a technique to make the return value of a UDF constant across
> > the execution of a single query?
> >
> > For example, I'd like getExactTimestamp() -- or, rather, a UDF very
> > much like it -- to return the same result every time it is evaluated,
> > much like the context variable CURRENT_TIMESTAMP does.
> > [[SNIP]]
> An why you need do something so wrong like fb_sleep in query?
> What is your real interest?

I'd like to perform date/time arithmetic in UTC in some queries, without
assuming that the server's time zone is UTC. (fb_sleep() was written merely
to exaggerate the delays to show obviously that the UDF is evaluated once
per row, rather than just once per query.)

If I understand correctly, CURRENT_TIMESTAMP unfortunately cannot be
guaranteed to be in UTC. getExactTimestampUTC(), unfortunately, is called
repeatedly, meaning that something like this:

SELECT getExactTimestampUTC() - record_insertion_timestamp_utc AS
how_recent, ...

will show a marked skew if the query takes too long (e.g., system is heavily
loaded, or there are millions of rows).


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