It makes sense with factorial function to do an error check on the
domain. Calculate beforehand, and figure out what the largest sensible
domain value is.
For instance, in Maple, I get this:
> y:=92838278!;
Error, object too large
>
The error message returns instantly.
For reasonably large values, it might make sense to pre-compute
factorials and store them in an array. It should also be possible to
store 1/2 of Pascal's triangle in memory and demand load that memory
segment the first time someone asks for factorials or combinations or
permutations.
Just a thought.
> -----Original Message-----
> From: pgsql-hackers-owner@postgresql.org [mailto:pgsql-hackers-
> owner@postgresql.org] On Behalf Of Jim C. Nasby
> Sent: Friday, June 08, 2007 6:45 PM
> To: pgsql-hackers@postgresql.org
> Subject: [HACKERS] Issues with factorial operator
>
> I'm working with a customer that recently discovered that some code
had
> generated the following nice query...
>
> SELECT ... WHERE table_id = 92838278! AND ...
>
> So their production server now has several processes that are trying
to
> compute some absurdly large factorial. There's two issues here:
>
> 1) the computation doesn't check for signals. This means both a plain
> kill and pg_cancel_backend() are useless.
>
> 2) Even though the answer is going to be an obscene number of digits,
> and that's supposed to be fed into a numeric, there's no overflow or
> bounds checking occurring. This is true even if I store into a field
> defined as numeric:
>
> decibel=# create table n(n numeric);
> CREATE TABLE
> decibel=# insert into n select 3333!;
> INSERT 0 1
> decibel=# select char_length(trim(n, '0')) from n;
> char_length
> -------------
> 9466
> (1 row)
>
> So at the very least the documentation is confusing:
>
> The type numeric can store numbers with up to 1000 digits of precision
> and perform calculations exactly.
> ...
> Specifying
>
> NUMERIC
>
> without any precision or scale creates a column in which numeric
values
> of any precision and scale can be stored, up to the implementation
limit
> on precision.
>
> Yet here we have a numeric that's storing nearly 10,000 digits of
> precision.
> --
> Jim Nasby decibel@decibel.org
> EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)
