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How to perform subtraction in a text field of the same column and show the result as a database increment?

Prashant

My question is above. How can I calculate the growth of the database between running the script times if I have text fields as a total_size?

I've made a query but it doesn't show the data:

SELECT
    b.snap_id, b.dttm - a.dttm, 
    (SELECT NULLIF(regexp_replace(snapshots.snap_pg_size_pretty.total_size,'','MB'), '')::numeric as t_db_growth 
     FROM snapshots.snap_pg_size_pretty)
FROM 
    snapshots.snap_pg_size_pretty a, 
    snapshots.snap_pg_size_pretty b
WHERE 
    a.snap_id = $1 
    AND b.snap_id = $2 
ORDER BY 
    snap_id DESC;