On 2002.05.25 17:38 Rachel Carmichael wrote:
> low cardinality -- good candidate for bitmap. little or no updates to
> the column being indexed -- good candidate for bitmap
>
> "low" cardinality is relative -- 10,000 distinct values in a 300m row
> table is low, 100 distinct values in a 10K row table might be high
>
> but the main criteria I have found is whether or not the column is
> updated because when you update the values in a bitmap index, it's not
> just the one row in the index that is updated but the entire segment
> of
> the bitmap. And THAT can lock the index on you.
>
> Rachel
>
Well, it doesn't exactly lock the whole index, it locks only the rows
that
have the same bitmap value as the row that is being updated. For a low
cardinality
column, that may mean locking down a significant portion of the index
(and the underlying
table, of course). That makes bitmap indexes (and indices) completely
unsuitable
for the OLTP environment.
--
Mladen Gogala
--
Please see the official ORACLE-L FAQ: http://www.orafaq.com
--
Author: Mladen Gogala
INET: mgogala_at_adelphia.net
Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051
San Diego, California -- Public Internet access / Mailing Lists
--------------------------------------------------------------------
To REMOVE yourself from this mailing list, send an E-Mail message
to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in
the message BODY, include a line containing: UNSUB ORACLE-L
(or the name of mailing list you want to be removed from). You may
also send the HELP command for other information (like subscribing).
Received on Sat May 25 2002 - 17:28:21 CDT
