Re: efficient way to do "fuzzy" join

[Rémi Cura <remi.cura@xxxxxxxxx>]

(please note that this random string function is NOT the good way to do it,

i should random int then use it as index to an array containing all the letter)

Thanks a lot for this new version!

It seems to be slower than your first solution (no index use I guess, I gave up after 5 minutes vs 5 sec for the previous). Morevover, I canno't make assumption about a fixed interval (2 sec in your example). But I think I see where you are going.

After some test, the fastest is using BETWEEN and range.

(it is way faster than using the <@, strangely)

Here is the code :
-------------------------------------------------------

--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(gid int, t numeric, r numrange, data text);
    create table b(gid int, t numeric, data text);
    CREATE INDEX ON a (t);
    CREATE INDEX ON b (t);
    --CREATE INDEX ON a USING spgist (r);
    CREATE INDEX ON a (r);

--filling tables
    WITH the_serie AS (
        SELECT s AS gid,  s+random()/2-0.5 AS s, rc_random_string(100) aS data
        FROM generate_series(1,100000) AS s
      
    )
    insert into a (gid, t,r, data) SELECT gid, s, numrange((lag(s,1) over(order by gid ASC))::numeric  ,s::numeric) , data
    FROM the_serie;
   

    WITH the_serie AS (
        SELECT  s as gid, s+(random()-0.5)*2 AS s, rc_random_string(100) aS data
        FROM generate_series(1,100000) AS s
    )
    insert into b (gid, t, data) SELECT gid,s, data
    FROM the_serie;

ANALYZE a;
ANALYZE b;

--computing join with range

    --slow : 80 sec
    DROP TABLE IF EXISTS t;
    CREATE TABLE t AS
        SELECT b.*
        FROM b LEFT JOIN a ON (b.t <@ a.r)
        ORDER BY gid ASC
        LIMIT 30

    --slow: 80 sec
    DROP TABLE IF EXISTS t;
    CREATE TABLE t AS
        SELECT b.*
        FROM a,b
        WHERE b.t <@a.r

    --fast : 3sec
    DROP TABLE IF EXISTS t;
    CREATE TABLE t AS
        SELECT b.* , a.data as d2
        FROM a,b
        WHERE b.t BETWEEN lower(a.r) AND upper(a.r)

    --fast : 8 sec
    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)
-------------------------------------------------------

Thanks again,
Rémi-C

2014-04-11 20:18 GMT+02:00 Rémi Cura <remi.cura@xxxxxxxxx>:

Wow many thanks!

I had thought about the order by and limit because it is the natural way to express the problem,

but I had discarded it for fear of suchbad complexity

(theoretically, for each row of B , compute the distance to every other row of A!)
.

And it's okay if 2 row from B share the same join to row from A, because when interpolating it will be different.

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:

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

Ok, here is a just sql way.  No ranges.  No idea if its right.  A first pass, so to speak.



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