PostgreSQL - Best way to check the existance of reverse valued rows

Question

I know I failed to find a proper title.

For the sake of argument i have this table:

sender|receiver
   a  |   b
   c  |   d
   d  |   e
   b  |   a
   f  |   q
   q  |   f
   t  |   u

I want to calculate the number of rows that have a reverse on the table. For example the row a|b has a reverse on the table as b|a. Similarly f|q has a reverse as q|f on the table. So, for this table, i want "2" as answer.

I calculate this as:

CREATE TABLE #temptab 
(
  sender     VARCHAR,
  receiver   VARCHAR
);

CREATE TABLE #temptab2 
(
  receiver   VARCHAR,
  sender     VARCHAR
);

INSERT INTO #temptab
(
  sender,
  receiver
)
SELECT DISTINCT sender,
       receiver
FROM table

INSERT INTO #temptab2
(
  receiver,
  sender
)
SELECT DISTINCT receiver,
       sender
FROM table

SELECT COUNT(sender)
FROM (SELECT sender,receiver FROM #temptab INTERSECT SELECT receiver,sender FROM #temptab2

Is there a way that I can do this faster?

Answer 1

I would just do:

select count(*)
from #temptab t
where t.sender < t.receiver and
      exists (select 1
              from #temptab tt
              where tt.sender = t.receiver and tt.receiver = t.sender
             );

This should work quite well on Postgres. I'm not sure about the performance on Amazon Redshift.

Another method would use two aggregations:

select count(*)
from (select least(sender, receiver) as x1, greatest(sender_receiver) as x2,
             count(distinct sender) as cnt
      from #temptab
      group by x1, x2
     ) t
where cnt = 2;

However, your version with intersect might be faster.

Answer 2

The fastest way usually is using a join (especially if you have indexes on the two columns):

select count(*)/2
from sr as t1 join sr as t2 on t2.sender=t1.receiver and t2.receiver=t1.sender;

If you have no row with sender=receiver you could also use:

select count(*)
from sr as t1 join sr as t2 on t2.sender=t1.receiver and t2.receiver=t1.sender
where t1.sender < t1.receiver; 

In both cases replace sr by the name of your table.