Optimized querying in PostgreSQL - postgresql

Question

Assume you have a table named tracker with following records.
issue_id | ingest_date | verb,status
10 2015-01-24 00:00:00 1,1
10 2015-01-25 00:00:00 2,2
10 2015-01-26 00:00:00 2,3
10 2015-01-27 00:00:00 3,4
11 2015-01-10 00:00:00 1,3
11 2015-01-11 00:00:00 2,4
I need the following results
10 2015-01-26 00:00:00 2,3
11 2015-01-11 00:00:00 2,4
I am trying out this query
select *
from etl_change_fact
where ingest_date = (select max(ingest_date)
from etl_change_fact);
However, this gives me only
10 2015-01-26 00:00:00 2,3
this record.
But, I want all unique records(change_id) with
(a) max(ingest_date) AND
(b) verb columns priority being (2 - First preferred ,1 - Second preferred ,3 - last preferred)
Hence, I need the following results
10 2015-01-26 00:00:00 2,3
11 2015-01-11 00:00:00 2,4
Please help me to efficiently query it.
P.S :
I am not to index ingest_date because I am going to set it as "distribution key" in Distributed Computing setup.
I am newbie to Data Warehouse and querying.
Hence, please help me with optimized way to hit my TB sized DB.

Answer 1

This is a typical "greatest-n-per-group" problem. If you search for this tag here, you'll get plenty of solutions - including MySQL.
For Postgres the quickest way to do it is using distinct on (which is a Postgres proprietary extension to the SQL language)
select distinct on (issue_id) issue_id, ingest_date, verb, status
from etl_change_fact
order by issue_id,
case verb
when 2 then 1
when 1 then 2
else 3
end, ingest_date desc;
You can enhance your original query to use a co-related sub-query to achieve the same thing:
select f1.*
from etl_change_fact f1
where f1.ingest_date = (select max(f2.ingest_date)
from etl_change_fact f2
where f1.issue_id = f2.issue_id);
Edit
For an outdated and unsupported Postgres version, you can probably get away using something like this:
select f1.*
from etl_change_fact f1
where f1.ingest_date = (select f2.ingest_date
from etl_change_fact f2
where f1.issue_id = f2.issue_id
order by case verb
when 2 then 1
when 1 then 2
else 3
end, ingest_date desc
limit 1);
SQLFiddle example: http://sqlfiddle.com/#!15/3bb05/1