Postgres 14

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create table the_table (id int, jsonb_t jsonb);

CREATE TABLE

insert into the_table
VALUES
  (1, '[ {"x" : 1 , "y": 2} , {"x" : 2 , "y": 3} , {"x": 3, "y" : 4} ]'),
  (2, '[ {"x" : 1 , "y": 3} , {"x" : 3 , "y": 3} , {"x": 8, "y" : 2} ]'),
  (3, '[ {"x" : 1 , "y": 4} , {"x" : 4 , "y": 3} , {"x": 5, "y" : 9} ]');

INSERT 0 3

select *
from the_table
where id = 3;

id
jsonb_t

3
[{"x": 1, "y": 4}, {"x": 4, "y": 3}, {"x": 5, "y": 9}]

SELECT 1

update the_table
  set jsonb_t = jsonb_set(jsonb_t, 
                    array[jsonb_array_length(jsonb_t)-1]::text[], 
                    jsonb_t -> jsonb_array_length(jsonb_t)-1 || 
                        '{"y":8}' || 
                         jsonb_build_object('x', (jsonb_t -> jsonb_array_length(jsonb_t)-1 ->> 'x')::int + 1)
                 )
where id = 3;

UPDATE 1

select *
from the_table
where id = 3;

id
jsonb_t

3
[{"x": 1, "y": 4}, {"x": 4, "y": 3}, {"x": 6, "y": 8}]

SELECT 1