问题
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?
回答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:
_idindexes 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.
来源：https://stackoverflow.com/questions/34292679/what-are-the-limitations-of-partial-indexes