On Wed, 20 Jun 2007, PFC wrote:
> Except planner hints (argh) I see no way to give this information to the
> machine... since it's mostly in the mind of the DBA.
And the mind of the DBA has a funny way of being completely wrong some
days about what's really happening under the hood.
> Maybe a per-table "cache temperature" param (hot, warm, cold), but what
> about the log table, the end of which is cached, but not the old records
> ? It's messy.
One of the things that was surprising to me when I started looking at the
organization of the PostgreSQL buffer cache is how little gross
information about its contents is available. I kept expecting to find a
summary section where you could answer questions like "how much of the
cache currently has information about index/table X?" used as an input to
the optimizer. I understand that the design model expects much of this is
unknowable due to the interaction with the OS cache, and in earlier
versions you couldn't make shared_buffers big enough for its contents to
be all that interesting, so until recently this wasn't worth collecting.
But in the current era, where it's feasible to have multi-GB caches
efficiently managed by PG and one can expect processor time is relatively
cheap, it seems to me one way to give a major boost to the optimizer is to
add some overhead to buffer cache management so it collects such
information. When I was trying to do a complete overhaul on the
background writer, the #1 problem was that I had to assemble my own
statistics on what was inside the buffer cache as it was scanned, because
a direct inspection of every buffer is the only way to know things like
what percentage of the cache is currently dirty.
I can't figure out if I'm relieved or really worried to discover that Tom
isn't completely sure what to do with effective_cache_size either.
--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD