sequences and "addval('myseq', value)"

I have an idea, let me know what you all think.

rather than do:

select sum( field) from largetable

I want (need) to be able to do something like this:

select setval('myseq', select sum(field) from largetable);

and in a trigger

SELECT addval('myseq', field_size);

I have already created this function and am testing it. I basically took
"nextval" and assigned "byval" to a parameter and removed the value
caching code.

Anyway, I'm not quite getting the idea of caching sequence values. I
understand the performance benefits, but it seems problematic across
multiple backends, almost ensuring "holes" in the sequence of numbers.