Re: Problem with the sequence

I wrote a very basic entry about this at improveyourweb some weeks ago
(http://www.improveyourweb.com/?q=node/2) Hope it helps.

In the source code, sequence.c, you can read something like:

/*
* Decide whether we should emit a WAL log record. If so, force up
* the fetch count to grab SEQ_LOG_VALS more values than we actually
* need to cache. (These will then be usable without logging.)
*
* If this is the first nextval after a checkpoint, we must force a new
* WAL record to be written anyway, else replay starting from the
* checkpoint would fail to advance the sequence past the logged
* values. In this case we may as well fetch extra values.
*/
if (log < fetch)...

log is log_cnt (or log_cnt-1 if is_called equals false)
fetch is cache_value (or cache_value-1 if is_called equals false)

If someone more can help, it would interesting for all understand this better.

On Thu, 03 Feb 2005 09:22:55 +0000, Richard Huxton <dev(at)archonet(dot)com> wrote:
> sid tow wrote:
> > HI
> >
> > I have a problem locating the documentation for "sequence". I want to get the detailed information about the columns present in a sequence table ie when I do
> >
> > psql=# select * from foo_seq;
> > sequence_name | last_value | increment_by | max_value | min_value | cache_value | log_cnt | is_cycled | is_called
> > ---------------------------+------------+--------------+---------------------+-----------+-------------+---------+-----------+-----------
> > foo_seq | 11 | 1 | 9223372036854775807 | 1 | 1 | 0 | f | t
> > (1 row)
> >
> > I need to know where do i find documentation to know what the columns specify and I have already checked the man pages of create_sequence where I did not find much.
> > Can somebody give me ref to a link where I get such information.
>
> From psql:
> \d foo_seq
> Sequence "public.foo_seq"
> Column | Type
> ---------------+---------
> sequence_name | name
> last_value | bigint
> increment_by | bigint
> max_value | bigint
> min_value | bigint
> cache_value | bigint
> log_cnt | bigint
> is_cycled | boolean
> is_called | boolean
>
> From the manuals:
> CREATE [ TEMPORARY | TEMP ] SEQUENCE name [ INCREMENT [ BY ] increment ]
> [ MINVALUE minvalue | NO MINVALUE ] [ MAXVALUE maxvalue | NO MAXVALUE ]
> [ START [ WITH ] start ] [ CACHE cache ] [ [ NO ] CYCLE ]
>
> For the ones not mentioned in the manual:
> last_value - is the last value given out by the sequence
> is_called - says whether nextval() has been called in this session
> (and so whether it is safe to call currval())
> log_cnt - don't know, googling suggests it's to do with WAL logging.
>
> Does that help?
> --
> Richard Huxton
> Archonet Ltd
>
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