Block Filter Index

The block filter index is an optional Bitcoin Core database, enabled with -blockfilterindex=basic, that precomputes a compact filter for every block on the chain. These filters follow BIP158, which encodes the scripts and outpoints touched by a block into a probabilistic Golomb-Rice Coded Set (GCS). A light client downloads only the small filter, tests it against its own addresses, and fetches a full block only when the filter signals a possible match. Stored under indexes/blockfilter/basic/, the index is what lets a full node serve BIP157 light clients without exposing which addresses those clients care about.

Why filters beat bloom filters

The older BIP37 bloom-filter approach let a light wallet ask a server to filter blocks for it, which leaked the wallet's addresses to whichever node it connected to. BIP158 inverts the trust model: the full node builds one deterministic filter per block and serves the identical filter to everyone. The wallet does the matching locally, so a spy node never learns the wallet's scripts. A GCS matches every real member with certainty and non-members only with a small false-positive rate, so a wallet may occasionally download an irrelevant block but never misses one of its own transactions.

Cost and use cases

Building the index adds a few gigabytes and some sync time, but it is the backbone of privacy-preserving light wallets, Neutrino-style clients, and self-hosted wallet backends that you want to run against your own node rather than a third party. Sovereign Bitcoiners who serve their phone wallet from a home node typically enable this index for exactly that reason.

For related node tuning, see dbcache and the broader concept of an transaction index (txindex).