Query plan not available

Hi, due to the performance issues we are facing, we try to investigate the query plan.

We found some description how to generate it in admin guide, however it is not sufficient in a sense that it only refers how to generate but not interpretation which information means what.

What is worse, we now cannot generate it on rev. 52 which used to work on rev. 47.

Please someone kindly advise how to resolve the issues above.