I try to generate unique identifiers for the ticket table's records. A ticket belongs to a file record, which also have an identifier. If the file's identifier is 'SOMEPREFIX/F01' then its tickets should be 'SOMEPREFIX/F01/PT001', 'SOMEPREFIX/F01/PT002', 'SOMEPREFIX/F01/PT003' ...
The architecture: JS Client, a node.js API with some ORM, but I am also able to run raw SQL statements. DB: PG 9.6
The problem: If the client send (almost) the same time a bunch of ticket create requests, the DB generates sometimes the same identifier.
Here is an sql query I generate dynamically in the API after the ticket is already inserted:
BEGIN; UPDATE ticket SET ticket_number_index = (COALESCE((SELECT MAX(ticket_number_index) FROM ticket WHERE file_id = 'D530'), 0) + 1), ticket_number = CONCAT('SOMEPREFIX\F717\PT', TO_CHAR((COALESCE((SELECT MAX(ticket_number_index) FROM ticket WHERE file_id = 'D530'), 0) + 1), 'fm00000')) WHERE id = 'D3571'; SELECT * FROM ticket WHERE id = 'D3571'; COMMIT;
I would assume that BEGIN and COMMIT makes the code atomic, and after the first UPDATE the next one can't produce the same identifier. Although I have just saved saved two tickets (two POST queries started almost the same time by the client), and I ended up with 'SOMEPREFIX\F717\PT1' and 'SOMEPREFIX\F717\PT1'