wal_level

wal_level determines how much information is written to the WAL. The default value is replica, which writes enough data to support WAL archiving and replication, including running read-only queries on a standby server. minimal removes all logging except the information required to recover from a crash or immediate shutdown. Finally, logical adds information necessary to support logical decoding. Each level includes the information logged at all lower levels. This parameter can only be set at server start.

In minimal level, no information is logged for permanent relations for the remainder of a transaction that creates or rewrites them. This can make operations much faster (see populate-pitr). Operations that initiate this optimization include:

• ALTER ... SET TABLESPACE
• CLUSTER
• CREATE TABLE
• REFRESH MATERIALIZED VIEW (without CONCURRENTLY)
• REINDEX
• TRUNCATE

But minimal WAL does not contain enough information to reconstruct the data from a base backup and the WAL logs, so replica or higher must be used to enable WAL archiving (archive_mode) and streaming replication. Note that changing wal_level to minimal makes any base backups taken before unavailable for archive recovery and standby server, which may lead to data loss.

In logical level, the same information is logged as with replica, plus information needed to allow extracting logical change sets from the WAL. Using a level of logical will increase the WAL volume, particularly if many tables are configured for REPLICA IDENTITY FULL and many UPDATE and DELETE statements are executed.

In releases prior to 9.6, this parameter also allowed the values archive and hot_standby. These are still accepted but mapped to replica.