Count rows with max per group and additional condition

Question

I have the following table:

 horse_main_id | race_id |    horse_name     | rating100 | race_result 
---------------+---------+-------------------+-----------+-------------
            23 |     159 | Angelology        |       126 |           1
            24 |     159 | Arwoc             |       118 |            
            25 |     159 | Marlo Man         |       118 |            
            26 |     159 | Hucknall          |       113 |            
            27 |     159 | It's Electrifying |       110 |           2
            28 |     159 | Monte Carlo       |       107 |            
            29 |     159 | Cross Constance   |       103 |            
            30 |     160 | Auld Burns        |       119 |            
            31 |     160 | Diamond Jim       |       117 |            
            33 |     160 | Livery            |       115 |            
            34 |     160 | Paraggi           |       115 |            
            35 |     160 | I'll'ava'alf      |       110 |           2
            36 |     160 | Jacks 'n' Kings   |       108 |            
            32 |     160 | Fastnet Isle      |       115 |           1

I would like to operate on groups of race_id, find all groups that have the max(rating100) for their group and race_result = 1, and count all the times this happens.

So this example should return only 1, as for race_id 159 group the max rating100 of 126 also has race_result = 1, but for race_id 160 group, the max rating100 is 119, but does not have race_result = 1.

This is the closest I've got (was told I should use window functions):

SELECT * FROM (
    SELECT horse_name, race_id, race_result
         , max(rating100) OVER (partition by race_id) AS max_rating
    FROM horse_main) t
WHERE race_result = 1;

Answer 1

Since you need a subquery in either case, I would use a plain aggregate in the subquery (may be cheaper):

SELECT count(*)
FROM  (SELECT race_id, max(rating100) AS rating100
       FROM   horse_main
       GROUP  BY 1) x
JOIN   horse_main h USING (race_id, rating100)
WHERE  h.race_result = 1;

If there are many rows per race_id, it will be faster to get group-wise maxima with one of these techniques:

• Optimize GROUP BY query to retrieve latest record per user

---

Or use the window function rank() - that's what you count effectively: all greatest rating100 per race_id with race_result = 1:

SELECT count(*)
FROM  (SELECT race_result, rank() OVER (PARTITION BY race_id ORDER BY rating100 DESC) AS rnk
       FROM   horse_main) x
WHERE  rnk = 1
AND    race_result = 1;

An index on (race_id, rating100 DESC, race_result DESC) should help performance with big tables.

Assuming rating100 is defined NOT NULL, else you need to add NULLS LAST in query and index.

Answer 2

Your query is quite close. In addition to the max rating100 per group, the derived table should also return the individual rating100 values, so that you can check if the row's rating matches its group's maximum.

SELECT
  COUNT(*)
FROM
  (
    SELECT
      horse_name,
      race_id,
      race_result,
      rating100,
      max(rating100) OVER (partition by race_id) AS max_rating
    FROM
      horse_main
  ) AS t
WHERE
  race_result = 1
  AND rating100 = max_rating
;