Wow many thanks!
I had thought about the order by and limit because it is the natural way to express the problem,.
Here is the test env with realistic number, your solution is very fast, I have to raise my hat (4 sec!)
-------------------------------------------------------
--usefull function to fill with random text
CREATE OR REPLACE FUNCTION rc_random_string(INTEGER )
RETURNS text AS $$
SELECT array_to_string(
ARRAY(
SELECT
substring('ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789' FROM (random()*36)::int + 1 FOR 1)
FROM generate_series(1,$1)
)
,'')
$$ LANGUAGE sql;
--creating tables
DROP TABLE IF EXISTS a;
DROP TABLE IF EXISTS b;
create table a(t float, data text);
create table b(t float, data text);
CREATE INDEX ON a (t);
CREATE INDEX ON b (t);
--filling tables
WITH the_serie AS (
SELECT s+random()/2 AS s, rc_random_string(100) aS data
FROM generate_series(1,100000) AS s
)
insert into a SELECT s, data
FROM the_serie;
WITH the_serie AS (
SELECT s+(random()-0.5)*2 AS s, rc_random_string(100) aS data
FROM generate_series(1,100000) AS s
)
insert into b SELECT s, data
FROM the_serie;
ANALYZE a;
ANALYZE b;
--computing result
DROP TABLE IF EXISTS t;
CREATE TABLE t AS
select a.t As a_t, b.t as b_t
from (
select t, least( least(t, mint), least(t, maxt)) as t2 from (
select t,
(select t from a where a.t >= b.t order by a.t limit 1) as mint,
(select t from a where a.t < b.t order by a.t desc limit 1) as maxt
from b
) as tmp
) as tmp2
inner join a on (tmp2.t2 = a.t)
inner join b on (tmp2.t = b.t)
--------------------------------------------------------
2014-04-11 19:16 GMT+02:00 Andy Colson <andy@xxxxxxxxxxxxxxx>:
On 4/11/2014 7:50 AM, Rémi Cura wrote:
Ok, here is a just sql way. No ranges. No idea if its right. A first pass, so to speak.Hey dear List,
I'm looking for some advice about the best way to perform a "fuzzy"
join, that is joining two table based on approximate matching.
It is about temporal matching
given a table A with rows containing data and a control_time (for
instance 1 ; 5; 6; .. sec, not necessarly rounded of evenly-spaced)
given another table B with lines on no precise timing (eg control_time =
2.3 ; 5.8 ; 6.2 for example)
How to join every row of B to A based on
min(@(A.control_time-B.control_time))
(that is, for every row of B, get the row of A that is temporaly the
closest),
in an efficient way?
(to be explicit, 2.3 would match to 1, 5.8 to 6, 6.2 to 6)
Optionnaly, how to get interpolation efficiently (meaning one has to get
the previous time and next time for 1 st order interpolation, 2 before
and 2 after for 2nd order interpolation, and so on)?
(to be explicit 5.8 would match to 5 and 6, the weight being 0.2 and 0.8
respectively)
Currently my data is spatial so I use Postgis function to interpolate a
point on a line, but is is far from efficient or general, and I don't
have control on interpolation (only the spatial values are interpolated).
Cheers,
Rémi-C
create table a(t float, data text);
create table b(t float, data text);
insert into a values (1), (5), (6);
insert into b values (2.3), (5.8), (6.2);
select a.t, b.t
from (
select t, least( least(t, mint), least(t, maxt)) as t2 from (
select t,
(select t from a where a.t >= b.t order by a.t limit 1) as mint,
(select t from a where a.t < b.t order by a.t desc limit 1) as maxt
from b
) as tmp
) as tmp2
inner join a on (tmp2.t2 = a.t)
inner join b on (tmp2.t = b.t)
The middle part is the magic:
select t,
(select t from a where a.t >= b.t order by a.t limit 1) as mint,
(select t from a where a.t < b.t order by a.t desc limit 1) as maxt
from b
The rest is just to make it usable. If t is indexed, it'll probably be fast too.
-Andy
