'Is ORDER BY ordering is guaranteed with READ UNCOMMITTED?

In one of the proprietary RDBMS, I found out that this query:

SELECT LastChecked, BatchId
FROM Logs
ORDER BY LastChecked ASC, BatchId ASC

can output results with invalid ordering:

...
1646605813628          12917
1646605466389          12952
...

I know that LastChecked values are being updated very actively in this table, so my guess is that it is a race condition, due to low transaction isolation level. Unfortunately, this database don't have proper documentation of transaction isolation levels, so it's only my guess (and that's why I won't name the vendor).

Question: is it the same for popular relational databases? Can I somehow see invalid order from query results, with lower transaction isolation level and active writes in the background?

I'm interested in:

  • PostgreSQL
  • MySQL
  • MSSQL
  • optionally others
mysqlsqlsql-serverpostgresqlisolation-level


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