On 8/31/2011 9:35 AM, Tore Halvorsen wrote:
Hi,
I'm trying to optimize a query where I have two tables that both have a
timestamp column. I want the result where either of the timestamps is
after a specified time. In a reduced form, like this:
CREATE TABLE a
(
id serial NOT NULL PRIMARY KEY,
time timestamp without time zone NOT NULL DEFAULT now()
);
CREATE INDEX a_time_idx ON a USING btree (time DESC NULLS LAST);
CREATE TABLE b
(
id serial NOT NULL PRIMARY KEY,
time timestamp without time zone NOT NULL DEFAULT now()
);
CREATE INDEX b_time_idx ON b USING btree (time DESC NULLS LAST);
--- generate some data
insert into a(time)
select now() - '10 year'::interval * random() from generate_series(1,
1000000, 1);
insert into b(time)
select now() - '10 year'::interval * random() from generate_series(1,
1000000, 1);
-- Using constraint works as expected, and uses the time index.
select * from a join b using(id)
where a.time >= '2011-08-15';
-- ... both ways...
select * from a join b using(id)
where b.time >= '2011-08-15';
-- However, if I'm trying to do this for both times at once, the time
index is not used at all
select * from a join b using(id)
where a.time >= '2011-08-15' OR b.time >= '2011-08-01'
-- This can be optimized by using CTEs
with am as (
select * from a where time >= '2011-08-15'
)
, bm as (
select * from b where time >= '2011-08-15'
)
select * from am join bm using(id)
-- end
I'm just wondering why the optimizer does things the way it does - and
if the CTE version is the best way to go...
The actual case is slightly more complex and uses more tables - this is
mostly a way to find updated data.
--
Eld på åren og sol på eng gjer mannen fegen og fjåg. [Jøtul]
<demo> 2011 Tore Halvorsen || +052 0553034554
On PG 9, after I ANALYZED the tables, it used indexes:
QUERY PLAN
---------------------------------------------------------------------------------------------------------------
Merge Join (cost=1.59..82778.35 rows=13171 width=20) (actual
time=0.066..1076.616 rows=12966 loops=1)
Merge Cond: (a.id = b.id)
Join Filter: ((a."time" >= '2011-08-15 00:00:00'::timestamp without
time zone) OR (b."time" >= '2011-08-01 0
-> Index Scan using a_pkey on a (cost=0.00..31389.36 rows=1000000
width=12) (actual time=0.007..204.856 ro
-> Index Scan using b_pkey on b (cost=0.00..31389.36 rows=1000000
width=12) (actual time=0.006..224.189 ro
ANALYZE is the magic.
-Andy
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