Re: [SQL] Self-referencing table question

[Edmund Bacon]

Sometimes using a temp table is a better idea:
e.g.
-- start by creating a temp table 'tids' that hold the to_ids that
-- we are interested in.
 SELECT to_id
   INTO TEMP TABLE tids
   FROM correlation
  WHERE from_id  = 1234
  ORDER BY val DESC limit 100;
-- The following temp table makes use of the primary key on
-- the correlation table, and the stated goal from the original
-- question that:
--         from_id > to_id
--     and from_id in (tids.to_id)
--    and to_id in (tids.to_id)
 SELECT t1.to_id AS from_id, t2.to_id
   INTO TEMP TABLE from_to
   FROM tids t1, tids t2
  WHERE t1.to_id > t2.to_id;
-- Now we can use the from_to table as an index into the correlation
-- table.
 SELECT c.from_id, c.to_id, c.val
   FROM from_to
   JOIN correlation c USING(from_id, to_id)
  WHERE val > 0.5;
The explain analyze for the final select works out to:
Nested Loop  (cost=0.00..50692.00 rows=8488 width=16) (actual 
time=0.171..150.095 rows=2427 loops=1)
  ->  Seq Scan on from_to  (cost=0.00..79.38 rows=5238 width=8) (actual 
time=0.006..7.660 rows=4950 loops=1)
  ->  Index Scan using correlation_pkey on correlation c  
(cost=0.00..9.63 rows=2 width=16) (actual time=0.024..0.025 rows=0 
loops=4950)
        Index Cond: (("outer".from_id = c.from_id) AND ("outer".to_id = 
c.to_id))
        Filter: (val > 0.5::double precision)
Total runtime: 152.261 ms

Richard Huxton wrote:
Sean Davis wrote:
I answer my own question, if only for my own records.  The following  
query is about 5-6 times faster than the original.  Of course, if  
anyone else has other ideas, I'd be happy to hear them.

Sean
explain analyze select from_id,to_id,val from exprsdb.correlation 
where  from_id in (select to_id from exprsdb.correlation where 
from_id=2424  order by val desc limit 100) and to_id in (select to_id 
from  exprsdb.correlation where from_id=2424 order by val desc limit 
100) and  val>0.6 and to_id<from_id;

Might not be any faster, but you can do this as a self-join with 
subquery:

SELECT c1.from_id, c1.to_id, c1.val
FROM
  correlation c1,
  (
    SELECT to_id FROM correlation WHERE from_id=2424
    ORDER BY val DESC LIMIT 100
  ) AS c2
  (
    SELECT to_id FROM correlation WHERE from_id=2424
    ORDER BY val DESC LIMIT 100
  ) AS c3
WHERE
  c1.from_id = c2.to_id
  AND c1.to_id = c3.to_id
  AND c1.val > 0.5
  AND c1.to_id < from_id
;
I think PG should be smart enough nowadays to figure out these two 
queries are basically the same.

--
Edmund Bacon <[EMAIL PROTECTED]>
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