row_concurrency.md

Row-level concurrency in OrioleDB

Row-level concurrency in OrioleDB is as close as possible to regular PostgreSQL tables. However, there are still some differencies because tables in OrioleDB are index-organized.

Update of the primary key

Update of primary key is typically rare and unusual situation in index-organized table. Unlike regular update, update of primary key is implemented as a sequence of delete and insert. Concurrent update and delete can't follow this update and may result in error. See the example below. Session 2 gets an error due to concurrent primary key update in session 1.

CREATE TABLE tbl
(
	id int4 primary key,
	value numeric NOT NULL
) USING orioledb;
INSERT INTO tbl VALUES (1, 0.0);

(Session 1)
> BEGIN;
> UPDATE tbl SET id = 2 WHERE id = 1;
UPDATE 1
				(Session 2)
				> BEGIN;
				> UPDATE tbl SET value = value + 1 WHERE id = 1;
				(waiting)
> COMMIT;
COMMIT
				ERROR:  tuple to be locked has its primary key changed due to concurrent update
				> ROLLBACK;
				ROLLBACK

Following the update chain

If some row was deleted and then new row with same primary key value is immediately inserted, then concurrent update or delete may consider the new row as a new version of the old row. See the example below. Session 1 deletes row and then inserts row with same primary key value. Session 2 were intended to update initial row, but finally updates the newly inserted row.

CREATE TABLE tbl
(
	id int4 primary key,
	value numeric NOT NULL
) USING orioledb;
INSERT INTO tbl VALUES (1, 0.0);

(Session 1)
> BEGIN;
> DELETE FROM tbl WHERE id = 1;
DELETE 1
> INSERT INTO tbl VALUES (1, 0.0);
INSERT 0 1
				(Session 2)
				> BEGIN;
				> UPDATE tbl SET value = value + 1 WHERE id = 1;
				(waiting)
> COMMIT;
COMMIT
				UPDATE 1
				> COMMIT;
				COMMIT