Consider this explain output for an indexed query. Why does the presence of a FETCH stage mean the query is NOT covered, even though an IXSCAN supplied all the index keys?
{
stage: 'PROJECTION_SIMPLE',
inputStage: {
stage: 'FETCH',
inputStage: { stage: 'IXSCAN', keyPattern: { a: 1, b: 1 } }
}
}