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Performing division with PostgreSQL / json

I have written a simple query against a table in a Postgres database that contains a column “manifest” of type json. Each cell contains a very long value and I am extracting the numerical value for “size”.

I need to create a new column (perhaps call it “size in MB”), and perform division against size. Specifically, I need to take the value of size and divide it by 1024, two times. I have tried every example I can find to no avail.

Here is my current query, and a small snippet of the result:

select customers.name, images.customer_id, images.captured_at, images.name,
 manifest -> 'size' as image_size
from public.images
inner join public.customers on customers.id = images.customer_id
where (captured_at > current_date - interval '12 months')
order BY captured_at desc

Name       customer_id   captured_at   name         image_size
Acme Inc   12345         2022-05-31    Central HMA  628032520

The goal is to take image_size, divide by 1024 two times, and store the result in a new column called “size in MB”. So in this case, the result of the math would be 598.

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Answer

To extract a scalar value (not another json) use the ->> operator (not ->).
That returns type text. So you also need to cast before doing math on the number:

(manifest ->> 'size')::bigint  -- or numeric? see below!

See:

You could do integer division, which truncates, computing 598 like you projected:

SELECT 628032520 / 1024 / 1024  -- 598  -- truncated!

Casting to a floating point number or numeric avoids integer division and shows your sample value to be much closer to 599:

SELECT 628032520.0 / 2^20  -- 598.9384841918945313  -- precise

The numeric constant 123 resolves to type integer, while 123.0 (containing a point .) resolves to numeric.
And 1024 * 1024 = 1048576 = 2^20.

Maybe round? Or keep two fractional digits?

SELECT round(628032520.0 / 2^20)     -- 599    -- rounded
     , round(628032520.0 / 2^20, 2)  -- 599.94

But consider the built-in function pg_size_pretty() instead which is made for this very purpose:

SELECT pg_size_pretty(628032520.0)  -- 599 MB  -- formatted text
     , pg_size_pretty(232520.0)     -- 227 kB

So:

SELECT c.name AS customer, i.customer_id, i.captured_at, i.name AS image
     , pg_size_pretty((c.manifest ->> 'size')::numeric) AS image_size
FROM   public.images i
JOIN   public.customers c ON c.id = i.customer_id
WHERE  captured_at > date_trunc('day', now()) - interval '12 months'
ORDER  BY captured_at DESC;

Should give you:

customer   customer_id   captured_at   image        image_size
Acme Inc   12345         2022-05-31    Central HMA  599 MB

I also fixed a couple other issues with your query.

Assuming captured_at is type timestamptz (like it probably should be), current_date would introduce a dependency on time zone setting of the session, which is a sneaky, unnecessary source of corner-case errors. See:

And “name” is not a good name.

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