Re: Retrieve most recent 1 record from joined table

Try to use a "GROUP BY " clause in your select, coupled with an aggregate function (MAX in your case ?). Note that you should not need any subquery, so a single select with inner join should be OK.

2014-08-22 10:05 GMT+02:00 agharta <agharta82@gmail.com>:

Hi all,
This is my first question, don't hate me please if it is the wrong place.

Mi question is relative seimple, but goes me crazy.

First, create some example tables:

----
create table table1(
    t1_ID SERIAL primary key,
    t1_value text
);

create table table2(
    t2_ID SERIAL primary key,
    t2_value text,
    t1_id integer
);


create table table3(
    t3_ID SERIAL primary key,
    t3_value text,
    t3_date timestamp,
    t2_id integer
);

ALTER TABLE table2 ADD FOREIGN KEY (t1_id) REFERENCES table1 (t1_ID) ON DELETE CASCADE;
ALTER TABLE table3 ADD FOREIGN KEY (t2_id) REFERENCES table2 (t2_ID) ON DELETE CASCADE;

CREATE INDEX IDX_TABLE1_T1_value ON table1 (t1_value);
CREATE INDEX IDX_TABLE2_T2_value ON table2 (t2_value);
CREATE INDEX IDX_TABLE3_T3_value ON table3 (t3_value);
CREATE INDEX IDX_TABLE3_T3_DATE ON table3 (t3_date);
-----

As you can see, table3 is referenced to table2 and table2 is referenced to table1

Next, insert some data on tables:

--one millon records into table1
insert into table1(t1_value) select md5(random()::text) from (select generate_series(1,1000000) as a) as c;

--100.000 records on table2 where t1_id is a random number between 1 and 10.001. This guarantees many records pointing to same table1 t1_id
insert into table2(t2_value, t1_id) select md5(random()::text), trunc((random()+1)*10000)::integer from (select generate_series(1,100000) as a) as c;

--again, 1.022.401 records on table3 where t2_id is a random number between 1 and 10.001. This guarantee many records pointing to same table2 t2_id.
-- random numbers (1.022.401) are generated by generated_series function with timestamp (1 minute) interval
insert into table3(t3_value, t2_id, t3_date) select md5(random()::text), trunc((random()+1)*10000)::integer, c.date_val from (select generate_series(timestamp '2013-01-01',timestamp '2014-12-12', interval '1 minute') as date_val) as c;


So, now we should have table3 with many rows per table2(t2_id) and many rows in table2 with same t1_id.


Now, the question:


Joining the tables, how to get ONLY most recent record per table3(t3_date)??

Query example:

select * from table1 as t1
inner join table2 t2 on (t1.t1_id = t2.t1_id and t2.t2_value like('%ab%') )
inner join table3 t3 on (t2.t2_id = t3.t2_id and t3.t3_date <= timestamp '2014-08-20')
order by t3.t2_id, t3.t3_date desc

produces to me this dataset (extract)


t1_id       t1_value      t2_id      t2_value           t1_id      t3_id         t3_value                             t3_date                            t2_id
17098    74127bc80ca759678892c957b6a34fc7    10020 9182c1f48cd008e31d781abc45723a10    17098    857683 0fb6cb380522dd83b6ac4beba2c6e98f    2014-08-19 14:42:00.0    10020
17098    74127bc80ca759678892c957b6a34fc7    10020 9182c1f48cd008e31d781abc45723a10    17098    857088 9c3481bfc0bdea51e62b338a1777cde6    2014-08-19 04:47:00.0    10020
17098    74127bc80ca759678892c957b6a34fc7    10020 9182c1f48cd008e31d781abc45723a10    17098    853208 253dc2083e70dd9e276867f43889973f    2014-08-16 12:07:00.0    10020
17098    74127bc80ca759678892c957b6a34fc7    10020 9182c1f48cd008e31d781abc45723a10    17098    851237 970acf901c4232c178b5dbeda4e44ac8    2014-08-15 03:16:00.0    10020
17098    74127bc80ca759678892c957b6a34fc7    10020 9182c1f48cd008e31d781abc45723a10    17098    847436 cb8d52255eaa850f9d7f203092a2ce13    2014-08-12 11:55:00.0    10020
17098    74127bc80ca759678892c957b6a34fc7    10020 9182c1f48cd008e31d781abc45723a10    17098    819339 d9e36ad622b1db499b3f623cdd46a811    2014-07-23 23:38:00.0    10020
17098    74127bc80ca759678892c957b6a34fc7    10020 9182c1f48cd008e31d781abc45723a10    17098    818022 b9d13239f522e3a22f20d36ea6dab8ad    2014-07-23 01:41:00.0    10020
17098    74127bc80ca759678892c957b6a34fc7    10020 9182c1f48cd008e31d781abc45723a10    17098    803046 c36164f3077894a1986c4922dfb632ec    2014-07-12 16:05:00.0    10020
17098    74127bc80ca759678892c957b6a34fc7    10020 9182c1f48cd008e31d781abc45723a10    17098    788129 32c966feab2212a29f86bebbaa6dfec9    2014-07-02 07:28:00.0    10020


As you can see, there are many t3_id  per single t2_id.

I need the same dataset, but i want only the most recent (one) record per table3, and, of course, directly in the join rule (in production i have a long and complex query).

My personal solution (very slow, i can't use it):

select * from table1 as t1
inner join table2 t2 on (t1.t1_id = t2.t1_id and t2.t2_value like('%ab%') )
inner join table3 t3 on (
t3.t3_id = (select t3_id from table3 where t2_id = t2.t2_id and t3_date <= timestamp '2014-08-20' order by t3_date desc fetch first 1 rows only)
and t3.t2_id = t2.t2_id
)
order by t3.t2_id, t3.t3_date desc


it gives me the right result, but performances are poor........

Same bad performance result with a function that performs the table3 query.

Suggestions?

Thanks to anyone who can answer to me!!!!

Best regards,

Agharta











--
Sent via pgsql-sql mailing list (pgsql-sql@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-sql