Re: reducing random_page_cost from 4 to 2 to force index scan

2011/5/15 Josh Berkus <josh@agliodbs.com>:
> Stuart,
>
>> I think random_page_cost causes problems because I need to combine
>> disk random access time, which I can measure, with a guesstimate of
>> the disk cache hit rate.
>
> See, that's wrong. Disk cache hit rate is what effective_cache_size
> (ECS) is for.
>
> Really, there's several factors which should be going into the planner's
> estimates to determine a probability of a table being cached:
>
> * ratio between total database size and ECS
> * ratio between table size and ECS
> * ratio between index size and ECS
> * whether the table is "hot" or not
> * whether the index is "hot" or not
>
> The last two statistics are critically important for good estimation,
> and they are not things we currently collect.  By "hot" I mean: is this
> a relation which is accessed several times per minute/hour and is thus
> likely to be in the cache when we need it?  Currently, we have no way of
> knowing that.
>
> Without "hot" statistics, we're left with guessing based on size, which
> results in bad plans for small tables in large databases which are
> accessed infrequently.
>
> Mind you, for large tables it would be even better to go beyond that and
> actually have some knowledge of which

*which* ?
 do you mean 'area' of the tables ?

> disk pages might be in cache.
> However, I think that's beyond feasibility for current software/OSes.

maybe not :) mincore is available in many OSes, and windows have
options to get those stats too.

>
> --
> Josh Berkus
> PostgreSQL Experts Inc.
> http://pgexperts.com
>
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Cédric Villemain               2ndQuadrant
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