No - I agree - Analysis cache hit rate as a single indicator is
dangerous. You can easily increase cache hit rate by de-optimizing a
good query so it uses more CPU cylces, and therefore has a higher
cache hit rate. All information has to be taken as a whole when
performing optimization on a system. Cache hit rate is just one
factor. For data warehousing, it's obviously that you are going to
have a lower cache hit rate because you are often performing scans
across large data sets that will never fit in memory. But for most
system, not necesarily just OLTP, a high cache hit ratio is
acheivable. Cache hit ratio is just one small indication of
performance.
Relating to that - How to extract this kind of information from
postgresql? Is there a way to get the cache hti ratio, or determine
the worst 10 queries in a database?
Alex Turner
NetEconomist
On 12 Jan 2005 12:25:23 -0500, Greg Stark <gsstark@mit.edu> wrote:
>
> Alex Turner <armtuk@gmail.com> writes:
>
> > Infact the cache hit ratio that Oracle suggests is the minimum good
> > value is 95%. Anything below that is bad news.
>
> Well that seems very workload dependent. No amount of cache is going to be
> able to achieve that for a DSS system chugging sequentially through terabytes
> of data. Whereas for OLTP systems I would wouldn't be surprised to see upwards
> of 99% hit rate.
>
> Note that a high cache hit rate can also be a sign of a problem. After all, it
> means the same data is being accessed repeatedly which implicitly means
> something is being done inefficiently. For an SQL database it could mean the
> query plans are suboptimal.
>
> On several occasions we found Oracle behaving poorly despite excellent cache
> hit rates because it was doing a sequential scan of a moderately sized table
> instead of an index lookup. The table was small enough to fit in RAM but large
> enough to consume a significant amount of cpu, especially with the query being
> run thousands of times per minute.
>
> --
> greg
>
>
