Re: Index compression vs. table compression

Rick Denoire <100.17706_at_germanynet.de> wrote:
> "Richard Foote" <richard.foote_at_bigpond.nospam.com> wrote:
>
> >...discriminates against them. However blocks that are placed in the
> >KEEP pool can remain cached with some assurance, *if* the size of the
> >KEEP pool is larger than the sum of the size of all KEEP objects. The
> >danger with the KEEP pool though is that you "keep" and allocate
> >resources to an object that could be better utilised elsewhere.
>
> Your last sentence is worth a couple of comments.
>
> When I was assisting Oracle classes sometime ago and was learning
> about Oracle cache types, I was concerned by the way all other
> participants accepted the straight explanation and simple guidelines
> given for the KEEP cache unquestioned, because I couldn't. If Oracle
> implemented such an elaborate method to hold popular blocks as long as
> possible in the default cache, and if that method works, why should I
> try to circumvent the underlying logics and artificially force my own
> rules? Why should I decide by myself which tables/indexes should go
> into this special KEEP cache if the hot objects (or better hot blocks)
> would end up being kept in the default cache automatically anyway? The
> instructor (it was at Oracle's own training location) could not
> mention the *special* reasons I needed.

I had an object which was used infrequently and sparsely most of the time, but every week or so we would run queries that beat the snot out of it. Those queries tended to happen when a lot of high-level executives and conslutants were looking over my shoulder impatiently waiting for the answers. So I figured that that object was a good candidate for the keep pool, because how often you use any given block is not always a good estimate of how important it is to access that block quickly. (But instead of putting it in the keep pool, which I didn't have authority to do, I just wrote a script to preload the object into the default pool by repeatedly making scattered reads an hour before the meetings).

Xho

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