I have documents with the following structure:

{{{_id:"asdf", consumed: true, timestamp: ISODate("2018-12-10T11:57:57.960+0000") }}}

For querying I have two different (non partial) indices:

1. "timestamp_-1" (timestamp ascending)
2. "consumed_1_timesamp_-1 (compound index with consumed ascending and timestamp ascending)

When I create a query like 

find({consumed: false, timestamp: { "$lte" : ISODate("2018-12-10T11:57:57.960+0000")}}

the first index is used, which leads to bad performance.

In what case would be for this query the first index better than the second index? Since consumed is a Boolean, using the second index will always look at equal or lower elements - in my case 17.000 instead of 17.000.000 elements.

Attached is a explain('allPlansExecution') query. I added here a limit(400) query, since the evaluation would take several minutes when the first index is used.

I know that i can hint to the second index, but in this case it should be very trivial for mongodb-server to detect that the second index is better than the first index.