Calculating Average Value of JSONB array in Postgres

Question

I have a column called "value" in my "answers" table.

| value |
|---------|
| [1,2] |
| [1] |
| [1,2,3] |

The type of "value" is "jsonb".

I want to get the average value of each array in each row:

SELECT avg(value) AS avg_value
FROM answers

But this doesn't work because avg() is not a jsonb function. I've tried:

SELECT avg(value::integer[]) as avg_value
FROM answers

i.e. tried to cast the jsonb arrays into integer arrays and then taking the avg, but I get the following error: "cannot cast type jsonb to integer[]. null".

Any ideas?

Answer 1

You need to unnest the JSON array with jsonb_array_elements.

You can do this in a correlated subquery:

SELECT
 (SELECT AVG(value::decimal(18,5))
 FROM jsonb_array_elements(value)
 ) AS avg_value
FROM answers;

DB Fiddle

• user236222: Could you please explain what the decimal(18,5) does? Specifically the numbers 18 and 5.
• Charlieface: Precision and scale. I just picked anything, you could convert to decimal(5,2) if you like, or even float
• user236222: Thank you for your help!