senvey/Database-Management-System — comment on commit 361e8dd9a8a83f4048bbed5bbc9c93c0affaad1a
File: qe/qe.cc line 473
By senvey:

This might be NULL for the first time if you set it as NULL in the constructor, but it would not be so afterward. When left scan reaches its end, this left_data could still hold data from the last tuple, since you have not set it to NULL at the end of this method.