What does `target` mean in `jsonb_set` of postgresql? - postgresql

Question

I am using jsonb type in a column in postgresql11. And I'd like to update one field in the json data and I see there is a function jsonb_set which can be used. (https://www.postgresql.org/docs/current/functions-json.html).
However, based on the document,
jsonb_set ( target jsonb, path text[], new_value jsonb [, create_if_missing boolean ] ) → jsonb
Returns target with the item designated by path replaced by new_value, or with new_value added
if create_if_missing is true (which is the default) and the item designated by path does not
exist. All earlier steps in the path must exist, or the target is returned unchanged. As with
the path oriented operators, negative integers that appear in the path count from the end of
JSON arrays. If the last path step is an array index that is out of range, and create_if_missing
is true, the new value is added at the beginning of the array if the index is negative, or at
the end of the array if it is positive.
The first argument is target. What does target mean here? Do I need to do a query to get existing value and put it as target?
I have tried below update statement:
my current data is:
# select "taxes" from "Sites" where "id" = '6daa9b5d-d5b2-4b0d-a8ee-5ad2cb141594';
taxes
--------------------------------------------------------------------------------------------------------------
{"feePercent": 0, "percent": 0}
And I tried below update:
# update "Sites" set "feePercent" = jsonb_set('{"feePercent": 0, "percent": 0}', '{feePercent}', 1) where "siteUuid"='6daa9b5d-d5b2-4b0d-a8ee-5ad2cb141594';
but I got below error:
ERROR: function jsonb_set(unknown, unknown, integer) does not exist
LINE 1: update "Sites" set "feePercent" = jsonb_set('{"feePerce...

Answer 1

jsonb_set() modifies a specific JSON object. So, your target is the JSON object (or JSON column) which you want to modify.
jsonb_set(my_jsonb_to_be_modified, ...)
So, if you had this JSON object;
{"my":"old", "json":"object"}
With the function you can turn it into:
{"my":"new", "json":"object"}
The code is:
demo:db<>fiddle
SELECT jsonb_set('{"my":"old", "json":"object"}', '{my}', '"new"')
The target is the original JSON object, the path points to the element you want to modify, and new_value is the new value for the element you specified in the path.
In that case my had the value old, which turns into new now.

Answer 2

From PostgreSQL v14 on, you can use subscripts to make this UPDATE statement look natural:
UPDATE "Sites"
SET taxes['feePercent'] = to_jsonb(1)
WHERE id = '6daa9b5d-d5b2-4b0d-a8ee-5ad2cb141594';
For earlier versions, you will have to use jsonb_set like this:
UPDATE "Sites"
SET taxes = jsonb_set(taxes, ARRAY['feePercent'], to_jsonb(1))
WHERE id = '6daa9b5d-d5b2-4b0d-a8ee-5ad2cb141594';
The effect is the same: the whole JSON is read, a new JSON is created and stored in a new version of the row.
All this becomes much simpler if you don't use JSON, but regular table columns.