Re: BUG #15717: Index not used when ordering by left joined tablecolumn

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От	Alex
Тема	Re: BUG #15717: Index not used when ordering by left joined tablecolumn
Дата	28 марта 2019 г. 08:40:56
Msg-id	765434487.12847357.1553762456299@mail.yahoo.com обсуждение
Ответ на	Re: BUG #15717: Index not used when ordering by left joined table column (Tom Lane <tgl@sss.pgh.pa.us>)
Список	pgsql-bugs

Indeed, with an inner join the index is used.

Thanks for the assistance.

On Thursday, March 28, 2019, 3:19:30 AM GMT+2, Tom Lane <tgl@sss.pgh.pa.us> wrote:

PG Bug reporting form <noreply@postgresql.org> writes:

> The following query runs slow, as it seq scans tables, without using any

> index:

> select

> wp.id as id

> from

> web_page wp

> left join web_url_path url on wp.id=url.page

> order by

> url.priority asc

> limit 1

> I was expecting the "priority" column index would be used to retrieve first

> url then do a reverse join to get the corresponding page, and continue if no

> page is found.

It would probably do that if you used a plain join, or a right join.

But a nestloop plan cannot be driven from the inside of an outer join,

and that's the case this query presents.

regards, tom lane

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