问题
I have read putting indexes on low cardinality fields is pointless. Would this hold true for a compound index as such:
db.perms.createIndex({"owner": 1, "object_type": 1, "target": 1});
With queries as such:
db.perms.find({"owner": "me", "object_type": "square"});
db.perms.find({"owner": "me", "object_type": "circle", "target": "you"});
The amount of distinct object_type's would grow over time (probably no more than 10 or 20 max) but would only start out with about 2 or 3.
Similarly would a hash index be worth looking into?
UPDATE:
owner and target would grow immensely. Think of this like a file system wherein the owner would "own" a target (i.e. file). But, like unix systems, a file could be a folder, a symlink, or a regular file (hence the type). So although there are only 3 object_type's, a owner and target combination could have thousands of entries with an even distribution of types.
回答1:
I may not be able to answer your question, but giving my cents for index cardinality:
Index cardinality: it refers to the number of index points for each different type of index that MongoDB supports.
- Regular - for every single key that we put in the index, there's certainly going to be an index point. And in addition, if there is no key, then there's going to be an index point under the null entry. We get 1:1 relative to the number of documents in the collection in terms of index cardinality. That makes the index a certain size. It's proportional to the collection size in terms of it's end pointers to documents
- Sparse - when a document is missing a key being indexed, it's not in the index because it's a null and we don't keep nulls in the index for a sparse index. We're going to have index points that could be potentially less than or equal to the number of documents.
- Multikey - this is an index on array values. There'll be multiple index points (for each element of the array) for each document. So, it'll be greater than the number of documents.
Let's say you update a document with a key called tags and that update causes the document to need to get moved on disk. Assume you are using the MMAPv1 storage engine. If the document has 100 tags in it, and if the tags array is indexed with a multikey index, 100 index points need to be updated in the index to accommodate the move?
来源：https://stackoverflow.com/questions/33550084/mongodb-add-fields-of-low-cardinality-to-compound-indexes