|
|
Posted by Alexander Kuznetsov on 04/16/06 22:15
Serge Rielau wrote:
> 1. A thumb rule for using TBSCAN over (index scan & page fetch) is when
> the selectivity is worse than, say 5%. Sure any DBSM will pick it's own
> secret number,
Serge,
I think that threshold selectivity is not a constant - for SQL Server
it also depends on the width of bookmarks. So, if bookmarks are 4 byte
integers, the threshold selectivity is lower than if bookmarks are
50-byte character fields.
Similarly, besides selectivity DB2 also considers clustering factor,
and if an index has a high clustering factor, even low 50% selectivity
might be good enough for an access via an index, correct?
Navigation:
[Reply to this message]
|