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Home -> Community -> Mailing Lists -> Oracle-L -> Re: Bitmap Indexes
Hi,
Here is an article about it bitmap indexes
... I hope this gives you information.
Helen
<BLOCKQUOTE
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
Sent: Friday, February 08, 2002 6:58
AM
Subject: RE: Bitmap Indexes
Kirti,
Just cureous. Normally bitmap indexes ae ae used for low cardinality
columns. I am confused when you state - "Just one table uses it(bitmap index),
with b-tree indexes
>for PKry and one other non-unique index"
If you mean - you use Bitmap index for the non unique index with b-tree
index for PKey I am OK with that.
Shaibal
>From: "Deshpande, Kirti"
>Reply-To: ORACLE-L_at_fatcity.com
>To: Multiple recipients of list ORACLE-L
>Subject: RE: Bitmap Indexes
>Date: Thu, 07 Feb 2002 19:58:20 -0800
>
>Ethan,
> I have been using bitmap and b-tree indexes on the same table
in our data
>mart since 7.3.4. Never had a problem with query performance.
Now we have
>partitioned the tables in 8.1.7.x and most tables that were
using bitmaps do
>not need to use them anymore. Just one table uses it, with
b-tree indexes
>for PKry and one other non-unique index. No problems so far.
> The only issue we had with bitmap indexes was the way it used
up extents,
>if those were not droppped before the dataloads. The problem
still exists
>with 8.1.7.x. Have not tried with 9i yet..
>
>HTH,
>
>- Kirti
>
>
>-----Original Message-----
>Sent: Thursday, February 07, 2002 3:07 PM
>To: Multiple recipients of list ORACLE-L
>
>
>Posted on behalf of a friend...
>
>
>Fellow Oracle data warehouse DBAs (if this isn't you, you can
delete this
>note now):
> I have a question regarding Oracle's bitmap indexes. We have
been DABBLING
>with bitmap indexes with mixed results here. In
>many cases, they are great solutions. In some cases, where we
have a mix of
>bitmap and b-tree indexes on the same table, we occasionally
get into
>trouble --- this has to do with the Oracle optimizer deciding,
on the fly,
>to convert a regular b-tree index into a bitmap. It does this
so that it
>can AND or OR the various bitmap indexes together. Sounds great
on the
>surface but when this occurs, response time goes in the toilet.
>
>In one situation we have, we have a fact table with two bitmap
indexes and a
>few other b-tree indexes. A particular query we run bogs down
(NEVER
>COMPLETES) with this mix of indexes. Based on the access path
that's being
>chosen, we know which b-tree index is being converted on the
fly. If we
>convert that b-tree index into a bitmap (so we now have 3
bitmap indexes and
>Oracle does not need to create the third one on the fly), the
query really
>runs well. If we replace the bitmap indexes with b-tree indexes
(so we only
>have b-tree indexes), we get decent response times. (This is
all on Oracle
>8.1.7.2.0, by the way.)
>
>I'm wondering if the rest of you data warehouse DBAs have gone
"whole hog"
>with bitmap indexes. My testing shows that when Oracle doesn't
have to
>create a bitmap index on the fly, the queries respond
wonderfully. So, I'm
>wondering if our dabbling is actually a bad validation approach
and,
>instead, we should be 'running' with LOTS bitmap indexes
instead of
>'crawling' with only a few of them. In other words, maybe we're
not "taking
>all of our medication", as someone else put it recently.
>
>Any insight would be most appreciated. I'm not looking for
insight on the
>query I have used as an example. I'm looking for a generalized
answer that
>says, "Yes, if you start using bitmap indexes, you should go
TOTALLY to
>bitmap indexes" or, "Hmmm, we're using some bitmap indexes and
some b-tree
>indexes and don't have the problem you have".
>--
>Please see the official ORACLE-L FAQ: http://www.orafaq.com
>--
>Author: Post, Ethan
> INET: Ethan.Post_at_ps.net
>--
>Please see the official ORACLE-L FAQ: http://www.orafaq.com
>--
>Author: Deshpande, Kirti
> INET: kirti.deshpande_at_verizon.com
>
>Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051
>San Diego, California -- Public Internet access / Mailing Lists
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Helen article revised21_bitmap_index.doc Description: MS-Word document
Received on Fri Feb 08 2002 - 12:08:54 CST
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