Re: Any justification for sequence table vs. native sequences?

Doug Gorley <doug(dot)gorley(at)gmail(dot)com> wrote:
>
> I just stumbled across this table in a database
> developed by a collegue:
>
>
> field_name | next_value | lock
> ------------+-------------+--------
> id_alert | 500010 | FREE
> id_page | 500087 | FREE
> id_group | 500021 | FREE
>
>
> These "id_" fields correspond to the primary keys
> on their respective tables. Instead of making
> them of type serial, they are of bigints with a
> NOT NULL constraint, and the sequence numbers are
> being managed by the application (not the database.)
>
> I googled around a bit trying to find an argument
> either in favour of or against this approach, but
> didn't find much. I can't see the advantage to
> this approach over using native PostgreSQL sequences,
> and it seems that there are plenty of disadvantages
> (extra database queries to find the next sequence
> number for one, and a locking mechanism that doesn't
> play well with multiuser updates for two.)
>
> Can anyone comment on this? Has anyone ever had to
> apply a pattern like this when native sequences
> weren't sufficient? If so, what was the justification?

The only reason I can think to add that much complexity is to ensure gap-free
sequences, which Postgres' internal sequences do _not_ guarantee.

And yes, it's pretty much guaranteed to be slower than built in sequences, with
blocking when multiple threads want a sequence all at the same time.

I'm rather concerned by the third column, as I'm not sure what his implementation
approach is, and I'm concerned that he's using a home-brewed locking mechanism
instead of using table locks.

--
Bill Moran
http://www.potentialtech.com