You might have noticed an error 

failed to get accounts owned by program Tokenkeg...: Tokenkeg... excluded from account secondary indexes; this RPC method unavailable for key

This error is when you send an unexpected payload or not sending the correct data to the Solana validator.

To better explain it, we'll use a real world example

Get tokens with the same update authority

Let's say you want to get all tokens with the same update authority. There's an easy way with an SDK and a hard way by calling the getProgramAccounts and calculating the bytes.

The easy way would be to use the Metaplex SDK and do 

await METAPLEX.nfts().findAllByUpdateAuthority({updateAuthority: AUTHORITY})

The hard way is to check the bytes manually. For this, you need to understand this diagram

We would build thegetProgramAccountsin the following way:

{
  "method": "getProgramAccounts",
  "jsonrpc": "2.0",
  "params": [
    "metaqbxxUerdq28cj1RbAWkYQm3ybzjb6a8bt518x1s",  # Token Metada Account, do not change! do not use Tokenkeg!
    {
      "encoding": "jsonParsed",
      "commitment": "processed",
      "filters": [
        {
          "memcmp": {
            "offset": 1,
            "bytes": "7ys9jKmtNT2ARdutN5Jmx9hpNmUULb4o6H8usfrpvgg9". # updateAuthority to match
          }
        }
      ]
    }
  ],
  "id": "1"
}

• offset: set to 1, as the position of theUpdated Authorityfield within theToken Metadata Programstarts at offset 1
• bytes: set to the public key of the account you want to search for to be theMint Authority

The first result is the Token Metadata Account with address8yQpSuMs9LkGWwsF8M3snkQKHEKNGG8rzEG8C3PLof7b.
Again, this is the Token Metadata Account for a certain NFT, not the Token Program Account, so if you check that account in the Solana Explorer, you will not see anything beneficial.

So, the gross of the work now is to obtain the Mint Account from the Mint field of the Token Metadata Account we just got.

For that, you need to deserialize thedatafield from the response above (I recommend you use our guide to get started on how to deserialize Solana data).
You will need the Field names, Offset, and Sizes of the Token Metadata Account from the Metaplex docs.

Once you decode the data, you can extract the Mint field, which would be the Token Program Account of the NFT itself.