Обсуждение: BUG #16377: select stuck when use order by and limit 1 if order-by-field has one index and result has no records

The following bug has been logged on the website:

Bug reference:      16377
Logged by:          Yunfeng Wang
Email address:      kingyzf@outlook.com
PostgreSQL version: 11.6
Operating system:   Centos 7.6
Description:

select stuck when use order by and limit 1 if order-by-field has one
index:
and possibility is high if result has no records, otherwise possibility is
low;
if no "limit 1" or no index on column pay_date, no problem;

sql:
select * from
(select ( case when pre_pay is null then 0 else pre_pay end ) as "PRE_PAY"

from inpatientprepay
where cancel='0'
and ehrid=4
order by pay_date desc
) as t
limit 1
;
other info: the  inpatientprepay table has nine million records, I think
it's stuck because if no limit 1 ,speed is ok;

Hi,

On Sat, Apr 18, 2020 at 10:04:05AM +0000, PG Bug reporting form wrote:
>The following bug has been logged on the website:
>
>Bug reference:      16377
>Logged by:          Yunfeng Wang
>Email address:      kingyzf@outlook.com
>PostgreSQL version: 11.6
>Operating system:   Centos 7.6
>Description:
>
>select stuck when use order by and limit 1 if order-by-field has one
>index:
>and possibility is high if result has no records, otherwise possibility is
>low;
>if no "limit 1" or no index on column pay_date, no problem;
>
>sql:
>select * from
>(select ( case when pre_pay is null then 0 else pre_pay end ) as "PRE_PAY"
>
>from inpatientprepay
>where cancel='0'
>and ehrid=4
>order by pay_date desc
>) as t
>limit 1
>;
>other info: the  inpatientprepay table has nine million records, I think
>it's stuck because if no limit 1 ,speed is ok;
>

I think we need to see execution plans for both queries.

Also, when you say "stuck" what does that mean? Does it mean it's
waiting on a lock, or is it doing something but not producing results?
Or what?

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services