Postgres 10, select from json object containing json array of objects - json

Question

In the database I have text field contains json with the structure:
"Limits":{
"fields":[
{
"key":"DAILY_LIMIT",
"value":"1559",
"lastModified":1543857829148,
},
{
"key":"MONTHLY_LIMIT",
"value":"25590",
"lastModified":1543857829148,
}
]
}
I need to check if daily_limit exists. It's easy to do with LIKE %DAILY_LIMIT% but performance is not so good and also I won't have access to value (right now I don't need it but maybe in future, it'll be needed). There is an option to check if this key exists without killing the db? I tried with 'Limits'->'fields'-> but I don't know what should be next... And it must be done by query, I cant pass object to backend and then check it

Answer 1

demo: db<>fiddle
If you want to do it the JSON way this could be a solution:
WITH data AS (
SELECT 'somedata' as somedata, '{"Limits":{"fields":[{"key":"DAILY_LIMITS","value":"1559","lastModified":1543857829148},{"key":"MONTHLY_LIMIT","value":"25590","lastModified":1543857829148}]}}'::jsonb as data
)
SELECT
d.*
FROM data d, jsonb_array_elements(data -> 'Limits' -> 'fields')
WHERE value ->> 'key' = 'DAILY_LIMITS'
jsonb_array_elements expands the array into one row each element. In the next step you are able to check the key's value.
But the demo shows, that a simple LIKE would be much faster as #404 mentioned correctly (have a look at the costs of both examples.)