MongoDB Indexing (securityIndex)
For example, I have an assembly of documents like this:
{"tag_cloud":[{"value":"games", "count":10}, {"value":"girls", "count":500}]}
{"tag_cloud":[{"value":"games", "count":5}, {"value":"stack", "count":100}]}
{"tag_cloud":[{"value":"mongodb", "count":1000}, {"value":"thread", "count":15}]}
what's the difference between the two types of indexes:
- ensureIndex ({"tag_cloud"})
- ensureIndex ({"tag_cloud.value"}); ensureIndex ({"tag_cloud.count"})
in the context of the request:
db.foo.find({"tag_cloud.value":"games"}).sort({"tag_cloud.count":-1});
Thank!!!
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1 answer
MongoDB can only use one index at a time per query, so your suggestion to create two indexes won't work. Instead, I would create a composite index:
> db.foo.ensureIndex({"tag_cloud.value": 1, "tag_cloud.count": -1})
This index can be used to both filter for a specific item or items you want to view, and then also return in descending order:
> db.foo.find({"tag_cloud.value":"games"}).sort({"tag_cloud.count":-1}).explain()
{
"cursor" : "BtreeCursor tag_cloud.value_1_tag_cloud.count_-1",
"isMultiKey" : true,
"n" : NumberLong(2),
"nscannedObjects" : NumberLong(2),
"nscanned" : NumberLong(2),
...
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