question: public CostEstimate costIndexScan ( Index index , long nrows ) { return new CostEstimate ( nrows , model . indexScan ( schema . indexRowType ( index ) , ( int ) nrows ) ) ; }
target:  Estimate cost of scanning given number of rows from this index.  One random access to get there, then nrows-1 sequential accesses following, Plus a surcharge for copying something as wide as the index.