jbove@vetstar.com (Joseph Bove) writes:
> I do a rather simple query: select count (*) from large-table where
> column = some value;
>
> About 80% of the time, the response time is sub-second. However, at
> 10% of the time, the response time is 5 - 10 seconds.
Does it seem data-dependent?
That is, does the time vary for different values of "some value?"
If a particular value is particularly common, the system might well
revert to a sequential scan, making the assumption that it is quicker
to look at every page in the table rather than to walk through
Enormous Numbers of records.
I had a case very similar to this where a table had _incredible_
skewing of this sort where there were a small number of column values
that occurred hundreds of thousands of times, and other column values
only occurred a handful of times.
I was able to get Excellent Performance back by setting up two partial
indices:
- One for WHERE THIS_COLUMN > VITAL_VALUE;
- One for WHERE THIS_COLUMN < VITAL_VALUE;
The REALLY COMMON values were in the range < VITAL_VALUE.
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Christopher Browne
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