Unnecessary usage of iterator

[b-tarczynski]

When using index query with operator eq or in it would be better to use simple badger.Get to get value of that index rather than using iterator which iterate over every index and decode value of KeyList.

    type TestStruct struct {
        Value string
        Index int `badgerhold:"index"`
    }

    for i := 0; i < 10_000; i++ {
        testStruct := TestStruct{
            Value: strconv.Itoa(i),
            Index: i,
        }
        err := s.storage.database.Badger.Insert(i, &testStruct)
        s.NoError(err)
    }

    results := make([]TestStruct, 0, 1)
    err := s.storage.database.Badger.Find(
        &results,
        bh.Where("Index").Eq(5000).Index("Index"),
    )
    s.NoError(err)
    s.Len(results, 1)
    s.Equal(results[0].Index, 5000)

In above test badgerhold.Store iterates over all records and calls Decode function 10k times. It should be just badger.Get call for badgerhold.KeyList and one get for each value in that struct.

[msieczko]

Hey @timshannon, I would go further and label this issue as a bug. Iterating over all index keys (PrefetchValues enabled), instead of doing a simple index query in this case, seems to kill performance benefits of the index.