Find max value from column that has a json object with key-value pairs - sql

Question

I Have a table that has a column of a JSON string (key-value pairs) of items, I want to return only the key-value pair of the largest value
I can do this by first UNNESTing the JSON object and then taking the largest value by ORDER BY item, value (DESC) and using array_agg to get the largest one. The problem is that this means creating multiple tables and is slow. I am hoping that in one operation, I'll be able to extract the largest key-value pair.
This:
| id | items |
| -- | ---------------------------------- |
| 1 | {Item1=7.3, Item2=1.3, Item3=9.8} |
| 2 | {Item2=4.4, Item3=5.2, Item1=0.1} |
| 3 | {Item5=6.6, Item2=1.4, Item4=1.5} |
| 4 | {Item6=0.9, Item7=11.2, Item4=8.1} |
Should become:
| id | item | value |
| -- | ----- | ----- |
| 1 | Item3 | 9.8 |
| 2 | Item3 | 5.2 |
| 3 | Item5 | 6.6 |
| 4 | Item7 | 11.2 |
I don't actually need the value, so long as the item is the largest from the JSON object, so the following would be fine as well:
| id | item |
| -- | ----- |
| 1 | Item3 |
| 2 | Item3 |
| 3 | Item5 |
| 4 | Item7 |

Answer 1

Presto's UNNEST performance got improved in Presto 316. However, you don't need UNNEST in this case.
You can
convert your JSON to arary of key/value pairs using JSON CAST and map_entries
reduce the array to pick the key for highest value
since key/value pairs are represented as anonymous row elements, it's very convenient to use positional access to row elements with subscript operator, (available since Presto 314)
Use query like
SELECT
id,
reduce(
-- conver JSON to array of key/value pairs
map_entries(CAST(data AS map(varchar, double))),
-- initial state for reduce (must be same type as key/value pairs)
(CAST(NULL AS varchar), -1e0), -- assuming your values cannot be negative
-- reduction function
(state, element) -> if(state[2] > element[2], state, element),
-- reduce output function
state -> state[1]
) AS top
FROM (VALUES
(1, JSON '{"Item1":7.3, "Item2":1.3, "Item3":9.8}'),
(4, JSON '{"Item6":0.9, "Item7":11.2, "Item4":8.1}'),
(5, JSON '{}'),
(6, NULL)
) t(id, data);
Output
id | top
----+-------
1 | Item3
4 | Item7
5 | NULL
6 | NULL
(4 rows)

Answer 2

Store the values one per row in a child table.
CREATE TABLE child (
id INT NOT NULL,
item VARCHAR(6) NOT NULL,
value DECIMAL(9,1),
PRIMARY KEY (id, item)
);
You don't have to do a join to find the largest per group, just use a window function:
WITH cte AS (
SELECT id, item, ROW_NUMBER() OVER (PARTITION BY id ORDER BY value DESC) AS rownum
FROM mytable
)
SELECT * FROM cte WHERE rownum = 1;
Solving this with JSON is a bad idea. It makes your table denormalized, it makes the queries harder to design, and I predict it will make the query performance worse.