The merge join implementation should throw a runtime error in case of unsorted inputs.
The assertion is already present but it is done with Java 'assert' keywork, this makes the assertion not to be evaluated in production.
It happened in production that due to a bug (out of the scope of this issue) a merge join was fed by an input which was not sorted according to the merge sort keys
This is current code in 1.17
This change will enable that assertion even when Java assertions are not enabled.
The impact is not very significant (a few CPU cycles) but prevents invalid results to be returned by the query.