Sparse indexes are considered incompatible with equality predicates that compare to null. This prevents the query {a: null} from selecting a sparse index {a: 1}, as documents that do not contain the field "a" (such as the empty document) would incorrectly be withheld from this query were it to use this index.

However, this compatibility check is not performed on assignments created from the plan cache. This can lead to missing query results.

To illustrate, see the following code snippet. The assertion on the last line fails.

db.foo.drop();

db.foo.insert({});
db.foo.insert({a: 1, b: 1});
db.foo.insert({a: 1, b: 2});

db.foo.ensureIndex({a: 1});
db.foo.ensureIndex({a: 1, b: 1}, {sparse: true});

assert.eq(1, db.foo.find({a: null, b: null}).itcount()); // Picks/caches "a_1" ("a_1_b_1" is incompatible).
db.foo.getPlanCache().clear();
assert.eq(1, db.foo.find({a: 1, b: 2}).itcount()); // Picks/caches "a_1_b_1".
assert.eq(1, db.foo.find({a: null, b: null}).itcount()); // Uses cached plan, assertion fails.