Bruce Momjian wrote:
> Jan Wieck wrote:
>> Manfred Koizar wrote:
>> > On Thu, 21 Aug 2003 21:10:34 +0530, "Shridhar Daithankar"
>> > <shridhar_daithankar@persistent.co.in> wrote:
>> >>Point I am trying to make is to tune FSM and autovacuum frequency
>> >>such that you catch all the dead tuples in RAM
>> >
>> > You might be able to catch the pages with dead tuples in RAM, but
>> > currently there's no way to keep VACUUM from reading in all the clean
>> > pages, which can be far more ...
>>
>> Which leads us to a zero gravity vacuum, that does the lazy vacuum for
>> pages currently available in the buffer cache only. And another pg_stat
>> column telling the number of tuples vacuumed so that an autovac has a
>> chance to avoid IO consuming vacuum runs for relations where 99% of the
>> dead tuples have been caught in memory.
>
> What would be really interesting is to look for dead tuples when you
> write/discard a buffer page and add them to the FSM --- that is probably
> the latest time you still have access to the page and has the highest
> probability of being recyclable.
>
True, but it's again in the time critical path of a foreground
application because it's done by a backend who has to read another page
on behalf of a waiting client right now. Also, there is only a small
probability that all the pages required to do the index purge for the
tuples reclaimed are in memory too. Plus there is still no direct
connection between a heap tuples ctid and the physical location of it's
index tuples, so purging an index requires a full scan of it, which is
best done in bulk operations.
Jan
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