1

Consider a collection with an index {x: 1} consisting of the following documents:

  [
    {x : 5},
    {x : 5},
    {x : 5}
  ]

The explain output on the query predicate {x : 5} shows that the total keys examined = total documents examined = documents returned = 3.

This is not what I expected. I expected the total keys examined to equal 1 and total documents examined = total documents returned = 3. The reason for my expectation is because of how I understand how MongoDB treats documents with the same key value in the BTree; if multiple documents have the same key value, the node with that key will have a reference to all documents with the key value. In this case three documents have the same key value x = 5 so MongoDB should scan only this key and return all referenced documents.

Why is the key count = 3 and not 1, is there something am missing?

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  • 2
    I guess you mistake "total keys examined" with "distinct keys examined" Commented Jan 4 at 11:45
  • @WernfriedDomscheit my understanding is that the BTree implementation doesn't allow duplicate keys instead a node with a given key references all documents with that key value. How many keys does MongoDB have to check?, just 1. Commented Jan 4 at 11:56
  • are you saying MongoDB will create 3 duplicate keys in the BTree each referencing one document? Isn't that less efficient both in memory and time. Imagine keeping 1 million duplicate keys in the Btree? Commented Jan 4 at 11:57
  • 1
    MongoDB will create index entries for all three documents. This is why you get 3 keys examined in your explain plan. The query planner identified an index that supports the query shape. The index identified by the query planner has 3 entries - one for each record, and correctly returned 3 for total keys examined. Since you did not supply the actual query I cannot comment on the effectiveness of a covered query. Clearly the explain plan shows documents were examined, so not a covered query. Commented Jan 6 at 22:31
  • 1
    Regarding the idea that one index key would relate to many documents, this is patently wrong. Perhaps you were confusing the topic of Index Prefix Compression? mongodb.com/docs/manual/reference/glossary/… Commented Jan 6 at 22:33
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