How do I calculate percent of whole within a Postgres JSON array?

Question

I've got several million rows in a PostgreSQL 9.3 table, and I'm struggling to calculate aggregate statistics.

Here are the columns:

object_id | date | column_with_json_data_in_key-value_format | bunch_of_irrelevant_columns

There is one row for every object_id/date combo. There are approximately 70 keys in the JSON column, but any given row only contains ~5-20 keys.

Across all object_id's, for each key, what is the daily average fractional value?

1. Within each row, calculate the value of a particular key divided by the sum of all values for all keys for that particular row
2. If a key doesn't appear in a row, treat it as having a value of 0 for that row
3. Group the results by date
4. Calculate the average value for each key

So my desired output is a table with the following columns:

date | key1_name | key1_fractional_average | keyN_name | keyN_fractional_average

I tried this query for a single row, but it's not working correctly:

SELECT value2, total, (value2/total * 100) AS percent FROM
    (SELECT SUM(value) AS total FROM
        (SELECT CAST(CAST(value AS text) AS integer) FROM json_each((SELECT json_column
            FROM original_table
            WHERE primary_key = 1)::json)) as table1) as total_table,
    (SELECT CAST(CAST(value AS text) AS integer) AS value2 FROM json_each((SELECT json_column
            FROM original_table
            WHERE primary_key = 1)::json)) as value_table

For this particular page_id/insight_date row, the query returns the following:

value2 | total | percent
------------------------
1;          4;         0
2;          4;         0
1;          4;         0

The columns value2 and total are correct, but the percent column incorrectly shows 0's instead of the correct 25%, 50%, 25%.

Unfortunately, because JSON is a fairly new type in Postgres, googling hasn't turned up many examples. I'm thoroughly confused here.

Answer 1

You are dividing integers, which gives you integer division and discards the remainder. You need to cast them to a fractional type.

value2::double precision/total::double precision * 100

You may want to do the cast farther down in the query.