What are the limitations of partial indexes?

Question

The latest version of MongoDB (v3.2) adds support for partial (filtered) indexes. You supply a filter when you create the index and that filter determines which documents will be referenced in the index and which will not.

Can I use any filter expression (as long as it's a valid filter)? Or are there limitations to the filter being used? If so, what are those limitations?

Syconium

Answer 1

Can I use any filter expression?

No, partial indexes support only a subset of the operators in the filter used. The only supported operators are: $AND (only at the top level), $EQ, $LT, $LTE, $GT, $GTE, $EXISTS and the TYPE_OPERATOR.

That leaves out, for example $NOT, $REGEX, $OR, etc.

You can see that in the source for MongoDB here.

What are those limitations?

There are also some general limitations on partial indexes:

• _id indexes can't be partial indexes.
• Sparse indexes can't be partial indexes.
• Shard key indexes can't be partial indexes.
• Partial indexes aren't supported in versions earlier than 3.2.

Syconium

It doesn't appear that this can be accomplished from the C# driver. At least I don't see PartialIndexFilter as a property of the CreateIndexOptions class – Adventist

@RalphShillington How about PartialFilterExpression? – Syconium

@i3amon You're absolutely right! My error was in doing var options = new CreateIndexOptions(); without setting the generic type. – Adventist

Is it possible to make a second _id index that would be partial (with a different name) ? (I'm trying to make some sort of default filter view which would automatically exclude blacklisted content) – Veneration

It's also worth mentioning that only { $exists: true } is supported, because { $exists: false } is translated internally to an expression which uses the $not operator, which itself is not supported. – Housefly

As a workaround, instead of { $exists: false } I think one can use { $eq: null } which evaluates true when the field has the value null or if the field does not exist. Not identical behaviour, but close enough to build a partial index correctly. – Housefly