How to modify array under specific JSONB key in PostgreSQL? - arrays

Question

We're storing various heterogeneous data in a JSONB column called ext and under some keys we have arrays of values. I know how to replace the whole key (||). If I want to add one or two values I still need to extract the original values (that would be ext->'key2' in the example lower) - in some cases this may be too many.
I realize this is trivial problem in relational world and that PG still needs to overwrite the whole row anyway, but at least I don't need to pull the unchanged part of the data from DB to the application and push them back.
I can construct the final value of the array in the select, but I don't know how to merge this into the final value of ext so it is usable in UPDATE statement:
select ext, -- whole JSONB
ext->'key2', -- JSONB array
ARRAY(select jsonb_array_elements_text(ext->'key2')) || array['asdf'], -- array + concat
ext || '{"key2":["new", "value"]}' -- JSONB with whole "key2" key replaced (not what I want)
from (select '{"key1": "val1", "key2": ["val2-1", "val2-2"]}'::jsonb ext) t
So the question: How to write such a modification into the UPDATE statement?
Example uses jsonb_*_text function, some values are non-textual, e.g. numbers, that would need non _text function, but I know what type it is when I construct the query, no problem here.
We also need to remove the values from the arrays as well, in which case if the array is completely empty we would like to remove the key from the JSONB altogether.
Currently we achieve this with this expression in the UPDATE statement
coalesce(ext, '{}')::jsonb - <array of items to delete> || <jsonb with additions> (<parts> are symbolic here, we use single JDBC parameter for each value). If the final value of the array is empty, the key for that value goes into the first array, otherwise the final value appears int he JSONB after || operator.
To be clear:
I know the path to the JSONB value I want to change - it's actually always a single key on the top level.
I know whether that key stores single value (no problem for those) or array (that's where I don't have satisfying solution yet), because we know the definitions of each key, this is stored separately.
I need to add and/or remove multiple values I provide, but I don't know what is in the array at that moment - that's the whole point, so that application doesn't need to read it.
I may also want to replace the whole array under the key, but this is trivial case and I know how to do this.
Finally, if removal results in an empty array, we'd like to get rid of the key as well.
I could probably write a function doing it all if necessary but I've not committed to that yet.
Obviously, restructuring the data out of that JSONB column is not an option. Eventually I want to make it more flexible and data with these characteristics would go to some other table, but at this moment we're not able to do it with our application.

Answer 1

You can use jsonb_set to modify an array which is placed under some key.
To update a value in an array you should specify a zero-based index within the array in the below example.
To add a new element on a start/end - specify negative/positive index which is greter than array's length.
UPDATE <table>
SET ext = jsonb_set(ext, '{key2, <index>}', '5')
WHERE <condition>