Bitcoin Core optional transaction index redesign reduces size by 40 GB after rebuild

Bitcoin Core has merged a redesign of its optional -txindex database that can shrink the index by roughly 40 GB. The change was merged into the master branch on August 15 and was demonstrated in a contributor’s mainnet test.

What changed

The previous index stored each 32‑byte transaction ID as a database key together with the transaction’s disk position. The new format stores a five‑byte prefix derived from a salted SipHash and a six‑byte suffix that encodes the block sequence and transaction offset. The full transaction ID is still verified before a match is returned, preventing false positives from hash collisions.

Measured impact

• Rebuilt mainnet -txindex size dropped from ~66 GB to ~26 GB, a 61 % reduction.
• Rebuild time improved from 1 hour 50 minutes to 1 hour 19 minutes in the author’s test.
• Lookup latency remained stable at about 0.2 ms.

Operational considerations

Existing nodes that upgrade retain their current index, which continues to occupy the larger footprint. To capture the space savings, operators must delete and recreate the index after the change is released in a stable binary. If a node later downgrades to a version prior to the change, the index must be rebuilt again in the legacy format.

Release roadmap

The code is merged upstream, but stable binaries will be released later. Release notes will detail the migration steps required to rebuild the index and the procedure for handling potential downgrades.