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Bitcoin Core’s optional txindex shrinks about 40 GB after rebuild

The space saving comes from a redesigned compact lookup key, but operators must recreate the txindex database to capture the full reduction, with another rebuild needed after any downgrade.

Reported · Written up by our desk · Market Structure

Bitcoin Core has merged a redesign of its optional transaction index, reducing the database footprint by about 40 GB in one contributor’s mainnet test, according to CryptoSlate. The change landed in master on Aug. 15 as pull request #35531 and is aimed at operators running the -txindex option.

In the test, a rebuilt mainnet txindex dropped from about 66 GB to 26 GB, a roughly 61.0% reduction. CryptoSlate reports the saving is confined to this optional index, with the rest of the node’s data directory, including blockchain data, not included in the measurement.

The redesign replaces storage of full 32-byte transaction IDs with a shorter lookup key, consisting of a five-byte prefix derived from a salted SipHash plus a six-byte suffix encoding the block sequence and transaction offset. Bitcoin Core still checks the full transaction ID after locating candidates, to prevent false matches from possible key collisions.

CryptoSlate also notes existing -txindex databases remain readable through an upgrade, so an immediate forced rebuild is avoided. However, realizing the full benchmark reduction requires recreating the index, and a downgrade can trigger a second rebuild because previous releases cannot read entries written in the compact format.

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