RE: Creating Histograms
- From: ryan.gaffuri@xxxxxxxxxxx
- To: oracle-l@xxxxxxxxxxxxx
- Date: Wed, 21 Jul 2004 19:25:14 +0000
Cost is irrelavent. Ignore it. Its strictly internal to Oracle. You cannot
compare costs of two different plans. This is all over asktom.
The primary stats to look at are:(though other stats have uses).
consistent gets (logical IO) -- 99.99% of the time the query with the lower
value is better. Every once in a while I found that a 30% increase in logical
IOs when doing a faster full scan results in a 30% or more reduction in
response times. This is on large batch processes only and do not have to stand
up under concurrency. I do not have a repeatable case and no idea why...
Cardinality/Rows -- this one is more important in OLTPs, since you want to
primary reduce 'rows' at each operation of your query. The earlier in the plan
you can weed out unnecessary rows, the better the performance.
Bytes -- this is typically more useful with full table scans, hash joins, and
sort merges, since you are reading all the bytes in the block. Methodology with
this one is the same as Cardinality/Rows. You will use this when doing batch
processes and reporting. Less often with OLTPs.
>
> My point was only that "choosing" lowest cost plan of a given statement is
> not necessarily indicative of the "best" plan.
> --
> .
> David
>
> > Hi David, then how do you get cbo uses the histogram and do a full scan,
> > when this is more than 90% of the data
> > changing the optimizer index parameter?
>
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