Bitcoin Core reduces optional txindex size by 40 GB

Bitcoin Core has merged a redesign of its optional transaction index, reducing the database size by about 40 GB in a contributor's mainnet test. Operators using -txindex can retain existing indexes through an upgrade, but full savings require database recreation. Pull request #35531, merged on Aug. 15, cut the rebuilt mainnet txindex from approximately 66 GB to 26 GB, achieving a 61% reduction. This change affects only the optional index; the blockchain and other node data remain unchanged. The new design uses a shorter lookup key, improving efficiency while maintaining verification against collisions. Performance tests showed lookups at about 0.2 milliseconds, with the mainnet rebuild taking 1 hour 19 minutes compared to 1 hour 50 minutes previously. Existing txindex databases remain readable post-upgrade, but to realize the full 40 GB saving, a recreation of the index is necessary. Downgrading to previous releases will incur additional migration costs, as older versions cannot read the new compact format. Operators must follow release-specific notes for proper database recreation and downgrade procedures once the change is included in a stable Bitcoin Core binary.