Sorting and indexes in Mongo are a complex topic. Mongo also has a special error that prevents you from doing a sort without an index if you have too many items. So it's good that you're asking about indexes, because an un-indexed sort will eventually start failing.
There is a bug in JIRA that seems to cover your issue, however there are some extra details to consider.
The first thing to note are your last queries:
db.tasks.find({title:/^japan/}).sort({total:-1})
db.tasks.find({keywords:/^japan/}).sort({total:-1})
These queries will fail eventually because you are only indexing on title not on title/total. Here's a script that will demonstrate the problem.
> db.foo.ensureIndex({title:1})
> for(var i = 0; i < 100; i++) { db.foo.insert({title: 'japan', total: i}); }
> db.foo.count()
100
> db.foo.find({title: 'japan'}).sort({total:-1}).explain()
... uses BTreeCursor title_1
> // Now try with one million items
> for(var i = 0; i < 1000000; i++) { db.foo.insert({title: 'japan', total: i}); }
> db.foo.find({title: 'japan'}).sort({total:-1}).explain()
Sat Mar 31 05:57:41 uncaught exception: error: {
"$err" : "too much data for sort() with no index. add an index or specify a smaller limit",
"code" : 10128
}
So if you plan to query & sort on title and total, then you need an index on both, in that order:
> db.foo.ensureIndex({title:1,total:1})
> db.foo.find({title: 'japan'}).sort({total:-1}).explain()
{
"cursor" : "BtreeCursor title_1_total_1 reverse",
...
The JIRA bug I listed above is for something like the following:
> db.foo.find({$or: [title:/^japan/, title:/^korea/]}).sort({total:-1})
Yours is slightly different, but it will encounter the same problem. Even if you have both indexes on title/total and keyword/total MongoDB will not be able to use indexes optimally.
