| From: | Peter Geoghegan <pg(at)heroku(dot)com> |
|---|---|
| To: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
| Cc: | Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>, Robert Haas <robertmhaas(at)gmail(dot)com> |
| Subject: | Re: Abbreviated keys for Numeric |
| Date: | 2015-02-21 22:16:47 |
| Message-ID: | CAM3SWZRre+ryKWDDCuxHyTC-5ZSVqfvbCa1V8GUr=7N1kT2v-w@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Sat, Feb 21, 2015 at 2:09 PM, Peter Geoghegan <pg(at)heroku(dot)com> wrote:
> I can recreate the problem you see with text sort regressions.
> Abbreviation is aborted for the case in question, unsurprisingly, and
> fairly far in. With that many tuples, the idea of taking abbreviated
> cardinality as a proxy for full cardinality becomes less important,
> because either way you have to do at least 10 comparisons per item on
> average.
Actually, it's closer to 20 comparisons past 1 million, on average.
See my earlier 0002-* patch comments [1].
[1] http://www.postgresql.org/message-id/attachment/35861/0002-Estimate-total-number-of-rows-to-be-sorted.patch
(search for "20")
--
Peter Geoghegan
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