If a certain entry in the database tables needs to be identified uniquely, e.g. for the purpose of finding or hiding, a primary key is needed. This is the case for the tool qc1_hide.

The primary key should exist for each record and be unique. There is no single key qualified as primary key but a combination of mjd-obs, instrumental keys, and the visibility flag.

MJD-OBS is a fundamental keyword which is always present. Therefore, it should have highest-possible precision, as stored in the FITS headers.

Those instrument keywords which are flagged by 'F' (to show up with filter option in the qc1_plotter interface) also contribute to the primary key.

The visibility flag (defined and used in the context of qc1_hide) is finally needed to identify a record uniquely.

From this follows that the instrument keywords flagged should be selected carefully. It is desirable to keep the set as small as possible to avoid redundancy.

A good guideline for selecting the proper keywords is their usage for calibration product renaming. To make these product names unique is a problem very similar to find the proper primary key.