Bitcoin Core merged a redesign of its optional transaction index, reducing the database size by roughly 40 GB in a mainnet test. Operators using the -txindex flag retain their existing indexes after upgrading; achieving the full savings requires recreating the index.

The change, merged as pull request #35531 on August 15, shrank a rebuilt mainnet txindex from about 66 GB to 26 GB, a reduction of approximately 61%. The redesign replaces the previous key format, which stored each 32‑byte transaction ID with its disk position, with a five‑byte salted SipHash prefix followed by a six‑byte suffix that encodes block sequence and transaction offset. A full‑ID verification step remains to prevent collisions.

Lookup performance stayed steady, averaging about 0.2 milliseconds, and the rebuild completed in 1 hour 19 minutes, compared to 1 hour 50 minutes with the prior format. The reduction applies only to the optional index; the blockchain and other node data are unaffected.

Full savings are realized only after a planned re‑creation of the database. The project notes that previous releases cannot read entries written in the compact format, and downgrading to an older version would trigger another rebuild in the legacy format.

The update will be included in a forthcoming stable release, and operators should follow the migration guidance to apply the changes.