Slow query on MongoDB when using $in with multiple values of indexed array field

Question

Slow query on MongoDB when using $in with multiple values of indexed array field.

1) query with 1 element in array is fast ( < 15 ms)

db.collection.count ( { tag : { $in : [ 1 ] } })

I repeated the query for all values of "tag", and all queries are fast.

i = 1; while (i < 1000) { 
db.collection.count( { tag : { $in : [ i ] } }); 
i++; 
}

2) query with 2 or more elements is slow ( > 1m50s)

db.collection.count ( { tag : { $in : [ 1, 2, 3 ] } } )

Some information:

• MongoDB 3.0.7
• Notebook i7 with 8 GB of RAM (2 GB used by mongodb), HDD
• Linux + Docker
• StorageEngine MMAP
• collection with 50M documents (test purpose)
• Index created with: db.collection.createIndex( { tag : 1 } )

Answer 1

MongoDB using COUNT_SCAN for one value in $in array and FETCH for more than 1 value inside $in array. This is the bug in MongoDB for covered count query with $in operator.