Date: Sun, 20 Sep 2015 08:57:17 -0700 (MST)
From: dave <audiotecture@web.de>
To: pgsql-sql@postgresql.org
Message-ID: <1442764637710-5866579.post@n5.nabble.com>
In-Reply-To: <55FEBBE0.8090902@gmail.com>
References: <1442747556700-5866560.post@n5.nabble.com>
 <55FEBBE0.8090902@gmail.com>
Subject: Re: Recursive merging of overlapping arrays in a column
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Sorry, here is the post again in plain text...

i have the following Table:

CREATE TABLE arrays (id SERIAL, arr INT[]);
INSERT INTO arrays (arr) VALUES (ARRAY[1,3,6,9]);
INSERT INTO arrays (arr) VALUES (ARRAY[2,4]);
INSERT INTO arrays (arr) VALUES (ARRAY[3,10,40]);
INSERT INTO arrays (arr) VALUES (ARRAY[3,18,44]);
INSERT INTO arrays (arr) VALUES (ARRAY[63,140,420]);
INSERT INTO arrays (arr) VALUES (ARRAY[42,102,420]);
INSERT INTO arrays (arr) VALUES (ARRAY[2,7]);
INSERT INTO arrays (arr) VALUES (ARRAY[1,3,11]);
INSERT INTO arrays (arr) VALUES (ARRAY[8,12,19]);


I want to merge the arrays which have overlapping elements, so that I get
the result which doesn't contain overlapping arrays anymore:

           arr            
--------------------------
 {1,3,6,9,10,11,18,40,44}
 {2,4,7}
 {8,12,19}
 {42,63,102,140,420}


I am not an expert in SQL and it took me a long time to come up with this
solution:

WITH RECURSIVE clusters AS (
	select DISTINCT uniq(sort_asc(array_cat(a1.arr, a2.arr))) AS arr
	from arrays a1 cross join arrays a2 
	where a1.arr && a2.arr AND
	least(a1.id,a2.id) != greatest(a1.id, a2.id)
	
	UNION
	
	select DISTINCT uniq(sort_asc(array_cat(a1.arr, a2.arr))) AS arr
	from arrays a1 cross join clusters a2 
	where a1.arr && a2.arr AND 
	a1.arr != a2.arr
)

SELECT arr FROM (
	SELECT * FROM (
		SELECT DISTINCT ON (arr[1]) arr 
		FROM clusters
		ORDER BY arr[1], array_length(arr, 1) DESC
	) AS c

	UNION

	SELECT arr FROM arrays WHERE id NOT IN (
		SELECT DISTINCT a1.id
		FROM arrays a1 CROSS JOIN arrays a2 
		WHERE a1.arr && a2.arr AND
		least(a1.id,a2.id) != greatest(a1.id, a2.id)
	)
) AS clustertable
ORDER BY arr;


Which gives me the result:

           arr            
--------------------------
 {1,3,6,9,10,11,18,40,44}
 {2,4,7}
 {3,10,18,40,44}
 {8,12,19}
 {42,63,102,140,420}
(5 rows)


Result number 3 is contained in number one and shouldn't be in the output
anymore, because I only want non overlapping arrays in the result.

Another problem I encountered is that the performance of this query seems to
be very bad. I tried running it on a larger table (~400000 arrays) and it is
still running after ~10h.

I would appreciate any input on this problems, so it would be nice if anyone
could give me a hint how to get only the merged arrays without overlaps in
the resultset and maybe how to build a more elegant and efficient query.


Thanks in advance,

Dave 


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