Better cardinality estimation for queries with many booleans (especially those not matched to any indices)

[dyemanov]

• Refactor selectivity estimations
• Apply exponential backoff to the selectivity combined from multiple booleans

In real-world SQL queries, conjuncts (ANDed booleans) are often inter-dependent and simple multiplication of selectivities (that we used so far) results to a very low final selectivity value, thus causing the stream cardinality being under-estimated. To avoid this, apply exponential backoff adjustment:

sel = sel1 * sqrt(sel2) * sqrt(sqrt(sel3)) * ... where sel1 is the least (best) selectivity and selN is the biggest (worst) one

I don't pretend this is the best appoach possible, but it seems working fine for MSSQL, so it's worth trying (actually, it was already tested in production -- with quite good results so far).