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From:Chris Knipe Date:May 7 2010 9:08am
Subject:Re: Where to index - over 15m records and growing
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On Fri, May 7, 2010 at 10:42 AM, Rob Wultsch <wultsch@stripped> wrote:

> Second thing:
> How selective is
> "WHERE IVAOData.TrackerTime >=UNIX_TIMESTAMP('2010-04-01 00:00:00')
>        AND IVAOData.TrackerTime <=UNIX_TIMESTAMP('2010-04-30 23:59:50') "
>
> Test by running
> SELECT COUNT(*)
> FROM  IVAOData
> WHERE IVAOData.TrackerTime >=UNIX_TIMESTAMP('2010-04-01 00:00:00')
>        AND IVAOData.TrackerTime <=UNIX_TIMESTAMP('2010-04-30 23:59:50')
>
> If this is a large proportion of the row count then you are probably
> in store for pain. It sounds like you are matching half the table. Big
> (intermediate) result sets often end in pain.
>

At this stage, you are correct.  We have roughly 2 months worth of data in
the table and are selecting about half (one months worth), thus about 50%.
With 6 months worth of data in the table and selecting one months worth of
data, that's roughly 16% of the data - but it will still be a bulk large
result... Hmmm, something tells me I need to rethink this yes.



> Third thing:
> My (rather sleepy) gut thinks your best bet is a a composite index on
> the table IVAOData on the columns TrackerTime and FlightID. This will
> make all access to the table in this query hit a covering index.
>

Took over 12 hours to create the index on TrackerTime, and you're right - I
should have seen and realised this.  I will drop the index on TrackerTime
and re-create it using both colums as I should have done in the first place.



> Fourth thing:
> What do you intend to ask the database with this query. This query
> smells overly broad


The idea is to get a count of the number of entries from Dep to Des during
the last month.  I.E.  How many records are there where Dep and Des are the
same during the last month.  With some changes to the application that
captures the data in the first place, I should be able to drop the need for
this query completely.  You have made me think a bit here and you're right.
This is not the way to do it.

I'll rethink this a bit more and come up with something better.

PS - Started the query before my first email was even posted, it's still
running... 3948 Seconds the last time I checked...



-- 

Regards,
Chris Knipe

Thread
Where to index - over 15m records and growingChris Knipe7 May
  • Re: Where to index - over 15m records and growingAnirudh Sundar7 May
    • Re: Where to index - over 15m records and growingChris Knipe7 May
      • Re: Where to index - over 15m records and growingRob Wultsch7 May
        • Re: Where to index - over 15m records and growingChris Knipe7 May
          • Re: Where to index - over 15m records and growingPeter Brawley7 May
            • Re: Where to index - over 15m records and growingJohnny Withers7 May
      • Re: Where to index - over 15m records and growingAnirudh Sundar8 May