possible to speed up this query? - sql

Question

I have the following query which takes a little too long to execute. I have posted the EXPLAIN ANALYZE for the query. Anything I can do to improve its speed?
EXPLAIN analyze SELECT c.*, match.user_json FROM match INNER JOIN conversation c
ON match.match_id = c.match_id WHERE c.from_id <> 142822281 AND c.to_id =
142822281 AND c.unix_timestamp = (SELECT max( unix_timestamp ) FROM conversation
WHERE match_id = c.match_id GROUP BY match_id)
EXPLAIN ANALYZE results
Nested Loop (cost=0.00..16183710.79 rows=2 width=805) (actual time=2455.133..2502.781 rows=34 loops=1)
Join Filter: (match.match_id = c.match_id)
Rows Removed by Join Filter: 71502
-> Seq Scan on match (cost=0.00..268.51 rows=2151 width=723) (actual time=0.006..4.973 rows=2104 loops=1)
-> Materialize (cost=0.00..16183377.75 rows=2 width=90) (actual time=0.034..1.168 rows=34 loops=2104)
-> Seq Scan on conversation c (cost=0.00..16183377.74 rows=2 width=90) (actual time=70.972..2421.949 rows=34 loops=1)
Filter: ((from_id <> 142822281) AND (to_id = 142822281) AND (unix_timestamp = (SubPlan 1)))
Rows Removed by Filter: 22010
SubPlan 1
-> GroupAggregate (cost=0.00..739.64 rows=10 width=16) (actual time=5.358..5.358 rows=1 loops=450)
Group Key: conversation.match_id
-> Seq Scan on conversation (cost=0.00..739.49 rows=10 width=16) (actual time=3.355..5.320 rows=17 loops=450)
Filter: (match_id = c.match_id)
Rows Removed by Filter: 22027
Planning Time: 1.132 ms
Execution Time: 2502.926 ms

Answer 1

This is your query:
SELECT c.*, m.user_json
FROM match m INNER JOIN
conversation c
ON m.match_id = c.match_id
WHERE c.from_id <> 142822281 AND
c.to_id = 142822281 AND
c.unix_timestamp = (SELECT max( c2.unix_timestamp )
FROM conversation c2
WHERE c2.match_id = c.match_id
GROUP BY c2.match_id
);
I would suggest writing it as:
SELECT DISTINCT ON (c.match_id) c.*, m.user_json
FROM match m INNER JOIN
conversation c
ON m.match_id = c.match_id
WHERE c.from_id <> 142822281 AND
c.to_id = 142822281 AND
ORDER BY c.match_id, c.unix_timestamp DESC;
Then try an index on: conversation(to_id, from_id, match_id). I assume you have an index on match(match_id).