Low cardinality index

Couchbase Server
1

Why we must have an index on type field? As type is a low cardinality field I think whole scan must be faster

2

Couchbase bucket can have any type of unrelated documents. Normally queries are done on related documents by using type or any others means.
Example: default bucket has
orders 1K
sales 10K
airports 20K

if you want query on orders FULL SCAN does 31K rows, If you do on type you only do 1K

3

if you want query on orders FULL SCAN does 31K rows, If you do on type you only do 1K

In RDBMS worlds , 31K full scan is faster than 1K index scan + 1K finding data , Why it is not true in couchbase?

4

@vsr1 reply please …