Comment by gen220

And another nontrivial one, in compound indices it’s really important that you order your columns in descending selectivity order. I.e. the column with the most unique values should go first.

In degenerate table/index situations, this could lead to index scans that are as slow as table scans, or not using an index at all! Especially common in SaaS schemas where you’re dealing with a tenancy key in many of the indicies: that tenancy key should almost always suffix the composite key not prefix it!!