[N1QL] n1ql query sometimes selects different indexes

Very similar scenario to MB-31203

Create default bucket

Insert docs like the following:

create 2 indexes:

create index idx1 on default(a);
create index idx2 on default(a,d);

run queries:

explain select * from default where a > 0;

Sometimes idx1 is picked up, sometimes idx2 is picked up, however we should expect it to always pickup idx1 right? Will check if this problem also exists in 6.5.1