Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> An obvious problem is that, if the abort rate is significantly
>> different from zero, and especially if the aborts are randomly
>> mixed in with commits rather than clustered together in small
>> portions of the XID space, the CLOG rollup data would become
>> useless.
>
> Yeah, I'm afraid that with N large enough to provide useful
> acceleration, the cases where you'd actually get a win would be
> too thin on the ground to make it worth the trouble.
Just to get a real-life data point, I check the pg_clog directory
for Milwaukee County Circuit Courts. They have about 300 OLTP
users, plus replication feeds to the central servers. Looking at
the now-present files, there are 19,104 blocks of 256 bytes (which
should support N of 1024, per Robert's example). Of those, 12,644
(just over 66%) contain 256 bytes of hex 55.
"Last modified" dates on the files go back to the 4th of October, so
this represents roughly three months worth of real-life
transactions.
-Kevin