Sometimes, data is selected from the database, then some processing happens in the application, and finally, some changes are made back on the database side. This is a classic example of SELECT FOR UPDATE.
Here is an example that shows the way SELECT is often executed in the wrong way:
BEGIN;SELECT * FROM invoice WHERE processed = false; ** application magic will happen here ** UPDATE invoice SET processed = true ... COMMIT;
The problem here is that two people might select the same unprocessed data. Changes made to these processed rows will then be overwritten. In short, a race condition will occur.
To solve this problem, developers can make use of SELECT FOR UPDATE. Here is how it can be used. The ...