> Dr. Viktor Leis in the paper has shown the importance of Cardinality > estimates in producing good query plans relative to cost models and > enumeration space. Even before isolating the impact of cardinalities on > query plan (by injecting true cardinalities, to be taken as part of this > project itself), I speculate that cardinality estimation has a lot of scope > for improvement in Derby. >
I share your suspicion, though I'm eager to see the benchmark numbers before we come to any definite conclusions. > > I am proposing the introduction of optional table sampling in order to > improve the cardinality estimation, the cardinality estimates can then > obtained reliably in presence of table samples specially when we are > filtering on set of attributes that are mutually co-related which Derby > currently ignores by taking in account assumption of uniformity and > independence between attributes of the same table. > > I think this would be a wonderful direction to explore! It sounds like quite a lot of work, but I'm sure it can be broken down into smaller pieces of infrastructure which can serve as milestones along the way to improvement. One of Derby's goals, over the years, has been to require as little administration as possible. In keeping with that vision, it would be valuable to me to understand how features such as the ones you describe can be incorporated without requiring a lot of attention from a Database Administrator to use properly. thanks, bryan
