$or and sort () query performance analysis in MongoDB

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Mongos> Db.find ({"User": "Jhon"}). Sort ({"Name": 1}). Limit. Explain () { "Cursor": "Btreecursor user_1", "nscanned": 10100, "Nscannedobjects": 10100, "N": 100, "Scanandorder": true, "Millis": 116, "Nyields": 0, "Nchunkskips": 0, "Ismultikey": false, "Indexonly": false, "Indexbounds": { "User": [ [ "Jhon", "Jhon" ] ] } } Second, I do $or query with sort (): Mongos> Db.find ({"$or": [{"User": "Jhon"}, {"Owner": "Jhon"}]}). Sort ({"Name": 1}). Limit. Explain () { "Cursor": "Btreecursor name_1", "nscanned": 1010090, "Nscannedobjects": 1010090, "N": 100, "Millis": 3800, "Nyields": 0, "Nchunkskips": 0, "Ismultikey": false, "Indexonly": false, "Indexbounds": { ' Name ': [ [ { "$minElement": 1 }, { "$maxElement": 1 } ] ] } } Last, I do $or query without sort (): Mongos> Db.find ({"$or": [{"User": "Jhon"}, {"Owner": "Jhon"}]}). Limit. Explain () { "Cursor": "Btreecursor user_1", "nscanned": 100, "Nscannedobjects": 100, "N": 100, "Millis": 0, "Nyields": 0, "Nchunkskips": 0, "Ismultikey": false, "Indexonly": false, "Indexbounds": { "User": [ [ "Jhon", "Jhon" ] ] } }

You can see:

In the first query, it was a good performance when you used the sort () alone.

In the second query, when the union uses $or and sort (), performance is poor.

In the third query, using $or alone, the performance is very good.