| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Peter Geoghegan <pg(at)bowt(dot)ie> |
| Cc: | PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Anastasia Lubennikova <a(dot)lubennikova(at)postgrespro(dot)ru>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> |
| Subject: | Re: Enabling B-Tree deduplication by default |
| Date: | 2020-01-29 21:02:01 |
| Message-ID: | CA+TgmobudKeiE5_=z4VRHS8DhLhDTn2Mg=kCr33pi4R5-xKNxQ@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Wed, Jan 29, 2020 at 2:50 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> It's tempting to try to reason about the state of an index over time
> like this, but I don't think that it's ever going to work well.
> Imagine a unique index where 50% of all values are NULLs, on an
> append-only table. Actually, let's say it's a non-unique index with
> unique integers, and NULL values for the remaining 50% of rows -- that
> way we don't get the benefit of the incoming-item-is-duplicate
> heuristic.
I mean, if you guess wrong and deduplicate less frequently, you are no
worse off than today.
But it depends, too, on the magnitude. If a gain is both large and
probable and a loss is both unlikely and improbable, then accepting a
bit of slowdown when it happens may be the right call.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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