Virtuoso v7.5.2 yield duplicata with empty OPTIONAL columns

[mhoangvslev]

Wrap tpi with STR(...) as ... because there is a bug where Virtuoso v7.5.2 yield duplicata with empty OPTIONAL columns

Steps to reproduce bug:

1. Take two query instances of the same template: A: (q02 instance 8) and B: (q02 instance 5)
2. Execute A, wait for completion, then execute B, watt for completion
3. There should be duplicated results for query B

[mhoangvslev]

Temporary solutions:

• [x] Cast each term in SELECT to a specific type, e.g STR(). This solution only works if you export the result to CSV files where types do not matter.
• [x] Upgrade Virtuoso
• [x] Re-execute the query

[mhoangvslev]

Upgrade to 7.2.8 didn't fix the issue

[mhoangvslev]

Reported to Virtuoso, https://github.com/openlink/virtuoso-opensource/issues/568