Csaba Nagy wrote:
Can we frame a set of guidelines, or may be some test procedure, which
can declare a certain function as deterministic?

You mean postgres should check your function if it is really immutable ?
I can't imagine any way to do it correctly in reasonable time :-)
Imagine a function of 10 parameters which returns the sum of the
parameters all the time except for parameters all 1 it will randomly
return a value _once in a thousand executions_... please find a generic
algorithm which spots this function as not immutable in reasonable
execution time ;-)
So this example is a bit extreme, but don't underestimate the user ;-)

I think you're overly pessimistic here ;-) This classification can be done quite efficiently as long as your language is "static enough". The trick is not to execute the function, but to scan the code to find all other functions and SQL statements a given function may possibly call. If your function calls no SQL statements, and only other functions already marked IMMUTABLE, then it must be IMMUTABLE itself.

It does seem that only pl/pgsql is "static enough" for this to work, though,
making this idea rather unappealing.

I am just saying from the top of my mind. Even otherwise, if we can
even restrict this indexing to only Built-in deterministic functions.,
don't you think it would help the cause of a majority? I have just
made the proposal to create the index with snapshot a optional one.

Restrictions like this are always confusing for the end user (i.e. why
can I use built-ins here and not my own ?). I leave to the actual coders
to say anything about code maintenance concerns...
Yes, and some built-ins have gotten that classification wrong too in the past IIRC. Which probably is a good reason not to trust our users to get it right ;-)

greetings, Florian Pflug

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