Ciro Santilli @cirosantilli 34

nodejs/sequelize/raw/parallel_update_worker_threads.js contains a base example that can be used to test what can happen when queries are being run in parallel. But it is broken due to a sqlite3 Node.js package bug: github.com/mapbox/node-sqlite3/issues/1381...

nodejs/sequelize/raw/parallel_update_async.js is an async version of it. It should be just parallel enough to allow observing the same effects.

This is an example of a transaction where the SQL READ COMMITTED isolation level if sufficient.

These examples run queries of type:

Sample execution:which does:

The fear then is that of a classic read-modify-write failure.

But as www.postgresql.org/docs/14/transaction-iso.html page makes very clear, including with an explicit example of type UPDATE accounts SET balance = balance + 100.00 WHERE acctnum = 12345;, that the default isolation level, SQL READ COMMITTED isolation level, already prevents any problems with this, as the update always re-reads selected rows in case they were previously modified.

Since in PostgreSQL "Read uncommitted" appears to be effectively the same as "Read committed", we won't be able to observe any failures on that database system for this example.

nodejs/sequelize/raw/parallel_create_delete_empty_tag.js contains an example where things can actually blow up in read committed.

Vs SQL SERIALIZABLE isolation level on PostgreSQL: dba.stackexchange.com/questions/284744/postgres-repeatable-read-vs-serializable

nodejs/sequelize/raw/parallel_create_delete_empty_tag.js is an example which experimentally seems to be solved by REAPEATABLE READ, although we are not sure that this is truly the case and why. What is clear is that that example is not solved by the SQL READ COMMITTED isolation level.

In PostgreSQL, this is the first isolation level which can lead to postgreSQL serialization failures, this does not happen to SQL READ COMMITTED isolation level in that DBMS. You then have to retry the transaction.