I've come to believe that designing systems like SQL to have queries that work transparently whether or not an index exists is wrong. There may be people that don't care how their query is actually executed, but I doubt it's the common case.

So many times I've fought or watched other developers fight with getting a DBMS to use an index. Or been surprised to find out that production doesn't have that index but staging does. Or found out that someone added a lower() to a query that made the index stop working. Or found out that index turns out to be more expensive than scanning the table directly in a circumstance that is different between instances of the same software.

Contrast that to an API like the Cassandra Thrift interface. You know if you're doing something expensive because the API only exposes the cheap things. It doesn't have an implicit sequential scan. You know you're doing one because you had to type out `while(next=...)` right in your code.

Something as simple as

    SELECT mycolumn FROM mytable WHERE INDEXED b=5

vs

    SELECT mycolumn FROM mytable WHERE SCAN b=5

and refusing to run if the assertion isn't met would go a long way to making the API predictable