How to optimize selection of pairs from one column of the table? - sql

Question

I'm using PostgreSQL 9.5.19, DBeaver 6.3.4
I have a table where one row is - user's name, place he attended, time when he was there
I need to select all pairs of places where any user was (if user was at place a and place b i need row like this: user, place a, place b, time at place a, time at place b)
The ponds table:
CREATE TABLE example.example (
tm timestamp NOT NULL,
place_name varchar NOT NULL,
user_name varchar NOT NULL
);
Some sample data:
INSERT INTO example.example (tm, place_name, user_name)
values
('2020-02-25 00:00:19.000', 'place_1', 'user_1'),
('2020-03-25 00:00:19.000', 'place_2', 'user_1'),
('2020-02-25 00:00:19.000', 'place_1', 'user_2'),
('2020-03-25 00:00:19.000', 'place_1', 'user_3'),
('2020-02-25 00:00:19.000', 'place_2', 'user_3');
I'm trying this script:
select
t.user_name
,t.place_name as r1_place
,max(t.tm) as r1_tm
,t2.place_name as r2_place
,min(t2.tm) as r2_tm
from example.example as t
join example.example as t2 on t.user_name = t2.user_name
and t.tm < t2.tm
and t.place_name <> t2.place_name
where t.tm between '2020-02-25 00:00:00' and '2020-03-25 15:00:00'
and t2.tm between '2020-02-25 00:00:00' and '2020-03-25 15:00:00'
group by t.user_name
, t.place_name
, t2.place_name
Seems like it gives me the right result, but it works really slow.
Can I optimize it somehow?

Answer 1

I would suggest trying indexes. For this query:
select t.user_name, t.place_name as r1_place, max(t.tm) as r1_tm,
t2.place_name as r2_place, min(t2.tm) as r2_tm
from schema.table t join
schema.table t2
on t.user_name = t2.user_name and
t.tm < t2.tm and
t.place_name <> t2.place_name
where t.tm between '2020-03-25 00:00:00' and '2020-03-25 15:00:00' and
t2.tm between '2020-03-25 00:00:00' and '2020-03-25 15:00:00'
group by t.user_name, t.place_name, t2.place_name
I would suggest an index on (tm, user_name, place_name) and on (user_name, tm, place_name) -- yes, both, one for each reference.

Answer 2

Colleague helped me to create window function:
select
subq.*
,EXTRACT(EPOCH FROM (subq.next_tm - subq.tm)) as seconds_diff
from (
select
t1.user_name,
t1.place_name,
t1.tm,
lead(t1.place_name) over w as next_place_name,
lead(t1.tm) over w as next_tm
from example.example as t1
window w as (partition by t1.user_name order by tm asc)
)subq
where
next_place_name is not null
and next_tm is not null
and place_name <> next_place_name
;