SnapbuildSync

Waiting for a serialized historical catalog snapshot to reach durable storage. It is often observed in processes involved in logical replication or decoding (such as walsender backends). It occurs when the server is waiting for a serialized historical catalog snapshot

Whats this?

To perform logical replication, PostgreSQL needs to decode Write-Ahead Logs (WAL) into logical changes (like INSERT or UPDATE). However, the WAL only contains data; it doesn't contain the full table "schema" or catalog at every point in time. The SnapBuild machinery is responsible for:

Scanning the WAL to find the necessary information about running transactions.
Reconstructing what the system catalogs (the "metadata") looked like at a specific point in time so it can correctly interpret the data in the WAL.
Saving this reconstructed snapshot to a file in $PGDATA/pg_replslot/slot_name/snapshots/

The SnapbuildSync wait happens when the database is performing an fsync() on these snapshot files to ensure they are safely written to stable storage.

pgstat_report_wait_start(WAIT_EVENT_SNAPBUILD_SYNC);