Re: MVCC overheads - Mailing list pgsql-hackers

From Tom Lane
Subject Re: MVCC overheads
Date
Msg-id 20865.1468002519@sss.pgh.pa.us
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In response to Re: MVCC overheads  (Alvaro Herrera <alvherre@2ndquadrant.com>)
Responses Re: MVCC overheads
List pgsql-hackers
Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> Pete Stevenson wrote:
>> Maybe I could figure out the lines of code that add versions into a
>> table and then those that collect old versions (they do get collected,
>> right?). Anyway, thought being I could profile while running TPC-C or
>> similar. I was hoping that someone might be able to jump on this with
>> a response that they already did something similar.

> Old tuple versions are "collected" (removed) by either vacuum (see
> vacuumlazy.c) and heap_page_prune.  The latter is one thing that could
> perhaps somehow be offloaded, as it's quite independent from the other
> stuff.  You can prune removable tuples at no additional cost from an
> unlocked dirty page, which is a useful optimization because then
> client-connected backends don't need to prune them later.

VACUUM in itself is an offloading optimization; the whole point of it
is to do maintenance in a background process not foreground queries.
AFAIR, heap_page_prune is just a small subset of VACUUM work that
we decided we could afford to do in foreground.
        regards, tom lane



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