I suppose I should've said "persistent" indexes. There was also only a single type supported: hash table. And it was hard coded to 2 fields. You had to change the code (which was C) to change the database.

This system was rarely restarted, so in practice it didn't matter what it did at startup, as long as it didn't take more than a few minutes, but it did place some limitations on data size. (This was a 32-bit system and everything was memory mapped.)