Re: How many is too many
- From: Greg Rahn <greg@xxxxxxxxxxxxxxxxxx>
- To: oracle-l@xxxxxxxxxxxxx
- Date: Thu, 11 Aug 2011 22:47:16 -0700
For OLTP, adding indexes is a known quantity (in general). There is a known
and finite number of queries so adding indexes is about limiting the number
of blocks visited.
For DW, adding indexes in this manner (12 indexes on a table, etc.), is the
beginning of the end. Data loads suffer and since query workloads are
frequently unknown (ad-hoc), DBAs often misapply OLTP techniques - adding
indexes for each problem query they see. Seems people forget why databases
have indexes -- as a manner to efficiently access *a small number of rows.*
When indexes are used as a means to reduce IO, it is then being mis-applied
because the platform is not appropriately sized and/or correct
DW technologies are not being applied (e.g. parallel execution + table
scans) for *a large number of rows* problem.
In any case, one has to start asking what good reasoning there is for 12
indexes. That just shouts bad design and/or inappropriate
tuning methodologies. Start asking why 5
times<http://en.wikipedia.org/wiki/5_Whys>
.
On Thu, Aug 11, 2011 at 8:14 PM, Ram Raman <veeeraman@xxxxxxxxx> wrote:
>
> Rule of thumb! Thanks for the response Greg. Does this apply to Data
> warehouse or OLTP applications or both?
>
> On Thu, Aug 11, 2011 at 7:03 PM, Greg Rahn <greg@xxxxxxxxxxxxxxxxxx>wrote:
>
>> Sounds like the "index death spiral" to me.
>> My simple sanity check: when the total number of columns indexed is
>> greater than the number of columns in the table, there is likely a design or
>> tuning problem. Often times, even before then...
>>
>
--
Regards,
Greg Rahn
http://structureddata.org
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