We are interested in the operation get_storage_at(block_id, contract_address, storage_key) -> value here.
Bonsai-trie does not have an history of the key-values, it only has a flat storage for the latest state.
We may want to remove that flat storage from bonsai-trie as it's not used and we don't plan on using it.
Instead, we have implemented our own optimized lookup, which is implemented with a column that looks like this:
The trick here is to rely on the fact that rocksdb columns are sorted trees.
The rocksdb get operation does not allow getting the value of (contract_1, key_1) at block 15 here, because
the key has not been modified at that block.
Instead, we use rocksdb iterators.
This allows us to get the most recent value for a (contract, key) at that block, by using a single rocksdb lookup.
In addition, we also set up a rocksdb prefix extractor for
that column, so that rocksdb can know
to treat the <contract> || <key> part differently from the block_n part, which allows optimisations.
We also use this exact technique to store contract class hash history and contract nonce history - so that we're able to quickly get the nonce/class hash of a contract at a specific block.