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* Why is n_distinct always -1 for range types?
@ 2013-09-19 20:47 Josh Berkus <josh@agliodbs.com>
2013-09-25 17:29 ` Re: Why is n_distinct always -1 for range types? Josh Berkus <josh@agliodbs.com>
2013-09-25 17:53 ` Re: Why is n_distinct always -1 for range types? Peter Geoghegan <peter.geoghegan86@gmail.com>
0 siblings, 2 replies; 3+ messages in thread
From: Josh Berkus @ 2013-09-19 20:47 UTC (permalink / raw)
To: pgsql-performance
Test:
1. create a table with a range type column.
2. insert 1000 identical values into that column.
3. analyze
4. n-distinct will still be listed as -1 (unique) for the column.
Why?
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Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com
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^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: Why is n_distinct always -1 for range types?
2013-09-19 20:47 Why is n_distinct always -1 for range types? Josh Berkus <josh@agliodbs.com>
@ 2013-09-25 17:29 ` Josh Berkus <josh@agliodbs.com>
1 sibling, 0 replies; 3+ messages in thread
From: Josh Berkus @ 2013-09-25 17:29 UTC (permalink / raw)
To: pgsql-performance
On 09/19/2013 01:47 PM, Josh Berkus wrote:
> Test:
>
> 1. create a table with a range type column.
> 2. insert 1000 identical values into that column.
> 3. analyze
> 4. n-distinct will still be listed as -1 (unique) for the column.
>
> Why?
>
Anyone?
--
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com
--
Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
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^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: Why is n_distinct always -1 for range types?
2013-09-19 20:47 Why is n_distinct always -1 for range types? Josh Berkus <josh@agliodbs.com>
@ 2013-09-25 17:53 ` Peter Geoghegan <peter.geoghegan86@gmail.com>
1 sibling, 0 replies; 3+ messages in thread
From: Peter Geoghegan @ 2013-09-25 17:53 UTC (permalink / raw)
To: Josh Berkus <josh@agliodbs.com>; +Cc: pgsql-performance
On Thu, Sep 19, 2013 at 1:47 PM, Josh Berkus <josh@agliodbs.com> wrote:
> 4. n-distinct will still be listed as -1 (unique) for the column.
>
> Why?
Because of this:
https://github.com/postgres/postgres/blob/master/src/backend/utils/adt/rangetypes_typanalyze.c#L205
We only collect and use histograms of lower and upper bounds for range
types, and the fraction of empty ranges.
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Regards,
Peter Geoghegan
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2013-09-19 20:47 Why is n_distinct always -1 for range types? Josh Berkus <josh@agliodbs.com>
2013-09-25 17:29 ` Josh Berkus <josh@agliodbs.com>
2013-09-25 17:53 ` Peter Geoghegan <peter.geoghegan86@gmail.com>
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