Re: How many is too many

  • From: Greg Rahn <greg@xxxxxxxxxxxxxxxxxx>
  • To: Kevin Closson <ora_kclosson@xxxxxxxxx>
  • Date: Fri, 12 Aug 2011 10:29:56 -0700

Using PX with an index FFS could be a viable solution for a "skinny" table
scan.  The big problem that I've seen is when its not a multi-block read
FFS, it's index -> table by rowid, which results in single block reads and
that is very inefficient for a large amount of rows/data via direct path
reads from disk.

Sure - I'm still buying beers. [?][?][?][?][?][?]

On Fri, Aug 12, 2011 at 9:48 AM, Kevin Closson <ora_kclosson@xxxxxxxxx>wrote:

> What about parallel execution + index scan? If a query only needs 12 of
> 100 columns then it's sort of columnar in that case.
> I do, of course, agree on the damage to load times but it is conceivable
> that drop, load, index could be possible particularly when one knows
> partitioning.
> We shouldn't forget that Oracle had the market share of DW before Exadata.
> Someone, somewhere worked out how to get indexes to work for DW. I agree
> that most people usually get it wrong though and I agree that working out
> I/O is a smart thing to do.  Most of us still Party Like It's 1999 thinking
> that high-performance, high-bandwidth I/O is some holy grail never to be
> achieved.
> A single 3.5" 15K RPM SAS/FC  drive can produce 200+ MB/s streaming. Start
> there, add drives and scale the plumbing. It's not  rocket science.
> P.S., Are you still buying the beer next time Greg? :-)
> ------------------------------
> *From:* Greg Rahn <greg@xxxxxxxxxxxxxxxxxx>
> *To:* oracle-l@xxxxxxxxxxxxx
> *Sent:* Thursday, August 11, 2011 10:47 PM
> *Subject:* Re: How many is too many
> 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<>
> .
> 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

Greg Rahn

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