Multiple indexes with hint

[Rhys Campbell]

Hello All,

I've been looking at a problem Mongo Query and was attempting to specify multiple indexes using the hint(). This lead me onto the following...

https://groups.google.com/forum/#!searchin/mongodb-user/hint$20mutliple/mongodb-user/FgIN9c7siDI/ztrk4q-FRw8J

https://jira.mongodb.org/browse/SERVER-1649

So I guess this still isn't possible but I'd like a clarification is anyone know...

I have a query with a couple of or clauses that would ideally use a different, specific index each. One of them sometimes doesn't, if I specify the index using hint, i.e.

db.collection.find( { "$or": [ { "foo": 1}, { "bar": 2 } ]  }).hint( "index_on_bar");

Would this restrict the first clause to that index only? In my case that index is not at all useful for the first or clause so I think it's causing a collection scan.

I have attempted something like this..

db.collection.find( { "$or": [ { "foo": 1}, { "bar": 2 } ]  }).hint( "index_on_foo", "index_on_bar");

The syntax is accepted but the query doesn;t complete.

Cheers,

Rhys

[Asya Kamsky]

There is no way to hint this, but you can use index filters feature in this case.

See https://docs.mongodb.org/manual/core/query-plans/#index-filters

Asya

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[Rhys Campbell]

Thanks. I'll take a look at this.

Rhys

[Asya Kamsky]

Note that it won't help you prevent a collection scan - if your first "$or" clause has no useful index, then the only way to find those documents is via collection scan, right?

An "$or" is a union of the two sets - when optimizing $or think of the two clauses as two separate queries - your full query will be in the ballpark of the slower of the two of them.

Asya

On Mon, Dec 14, 2015 at 11:58 PM, Rhys Campbell <rhys.jame...@gmail.com> wrote:

Thanks. I'll take a look at this.

Rhys

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