As currently implemented, FRUEC assumes that log files will contain exclusively events with timestamps within the 15-minute period corresponding to the timestamp in the filename. However, there can be overlap in timestamps for events from multiple files.
This means that for the aggregate data in the bannerimpressions table, it is not possible to always link each aggregate cells to a single file. Removing that link would break FRUEC's system for purging events from specific files.
It should be possible to fix this by changing the DB schema, creating link tables between files and aggregate bannerimpressions cells, and storing in the link tables the total impression counts aggregated from a given file for each cell.
Spun out from T234352.