| From: | Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru> |
|---|---|
| To: | Fornaroli Christophe <cfornaro(at)gmail(dot)com> |
| Cc: | pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Patch: pg_trgm: gin index scan performance for similarity search |
| Date: | 2015-12-24 18:06:09 |
| Message-ID: | CAPpHfduvmuQRzmKUWG-i0EgAw=NhDH=3PfDQ6jdnpsxcSx0GvA@mail.gmail.com |
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| Lists: | pgsql-hackers |
Hi, Christophe!
On Thu, Dec 24, 2015 at 6:28 PM, Fornaroli Christophe <cfornaro(at)gmail(dot)com>
wrote:
> This code uses this upper bound for the similarity: ntrue / (nkeys -
> ntrue). But if there is ntrue trigrams in common, we know that the indexed
> string is at least ntrue trigrams long. We can then use a more aggressive
> upper bound: ntrue / (ntrue + nkeys - ntrue) or ntrue / nkeys. Attached is
> a patch that changes this.
>
Good catch, thank you! The estimate in pg_trgm was not optimal.
I think it would be good to add comment which would explicitly state why do
we use this upper bound.
------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company
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