Обсуждение: pgsql: Improve generation algorithm for database system identifier.

Improve generation algorithm for database system identifier.

As noted some time ago, the original coding had a typo ("|" for "^")
that made the result less unique than intended.  Even the intended
behavior is obsolete since it was based on wanting to produce a
usable value even if we didn't have int64 arithmetic --- a limitation
we stopped supporting years ago.  Instead, let's redefine the system
identifier as tv_sec in the upper 32 bits (same as before), tv_usec
in the next 20 bits, and the low 12 bits of getpid() in the remaining
bits.  This is still hardly guaranteed-universally-unique, but it's
noticeably better than before.  Per my proposal at
<29019.1374535940@sss.pgh.pa.us>

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/5035701e07e8bd395aa878465a102afd7b74e8c3

Modified Files
--------------
src/backend/access/transam/xlog.c |   13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)

Hi Tom,

On 2014-04-26 19:11:14 +0000, Tom Lane wrote:
> Improve generation algorithm for database system identifier.
>
> As noted some time ago, the original coding had a typo ("|" for "^")
> that made the result less unique than intended.  Even the intended
> behavior is obsolete since it was based on wanting to produce a
> usable value even if we didn't have int64 arithmetic --- a limitation
> we stopped supporting years ago.  Instead, let's redefine the system
> identifier as tv_sec in the upper 32 bits (same as before), tv_usec
> in the next 20 bits, and the low 12 bits of getpid() in the remaining
> bits.  This is still hardly guaranteed-universally-unique, but it's
> noticeably better than before.  Per my proposal at
> <29019.1374535940@sss.pgh.pa.us>

> Modified Files
> --------------
> src/backend/access/transam/xlog.c |   13 +++++++------
> 1 file changed, 7 insertions(+), 6 deletions(-)

I just noticed that pg_resetxlog.c has a copy of the identifer
generation algorithm that's not been adjusted by that commit.

Greetings,

Andres Freund

--
 Andres Freund                       http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

Andres Freund <andres@2ndquadrant.com> writes:
> I just noticed that pg_resetxlog.c has a copy of the identifer
> generation algorithm that's not been adjusted by that commit.

Oh, I'd forgotten about that :-(.  Thanks, will fix.

            regards, tom lane