Add new wal_level, logical, sufficient for logical decoding.
When wal_level=logical, we'll log columns from the old tuple as
configured by the REPLICA IDENTITY facility added in commit
07cacba983ef79be4a84fcd0e0ca3b5fcb85dd65. This makes it possible
a properly-configured logical replication solution to correctly
follow table updates even if they change the chosen key columns,
or, with REPLICA IDENTITY FULL, even if the table has no key at
all. Note that updates which do not modify the replica identity
column won't log anything extra, making the choice of a good key
(i.e. one that will rarely be changed) important to performance
when wal_level=logical is configured.
Each insert, update, or delete to a catalog table will also log
the CMIN and/or CMAX values of stamped by the current transaction.
This is necessary because logical decoding will require access to
historical snapshots of the catalog in order to decode some data
types, and the CMIN/CMAX values that we may need in order to judge
row visibility may have been overwritten by the time we need them.
Andres Freund, reviewed in various versions by myself, Heikki
Linnakangas, KONDO Mitsumasa, and many others. REPLICA IDENTITY {DEFAULT | USING INDEX index_name | FULL | NOTHING}
REPLICA IDENTITY
This form changes the information which is written to the write-ahead log to identify rows which are updated or deleted. This option has no effect except when logical replication is in use. DEFAULT records the old values of the columns of the primary key, if any. USING INDEX records the old values of the columns covered by the named index, which must be unique, not partial, not deferrable, and include only columns marked NOT NULL. FULL records the old values of all columns in the row. NOTHING records no information about the old row. In all cases, no old values are logged unless at least one of the columns that would be logged differs between the old and new versions of the row.wal_level determines how much information is written to the WAL. The default value is minimal, which writes only the information needed to recover from a crash or immediate shutdown. archive adds logging required for WAL archiving; hot_standby further adds information required to run read-only queries on a standby server; and, 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, WAL-logging of some bulk operations can be safely skipped, which can make those operations much faster (see Section 14.4.7). Operations in which this optimization can be applied include:
| CREATE TABLE AS |
| CREATE INDEX |
| CLUSTER |
| COPY into tables that were created or truncated in the same transaction |
In hot_standby level, the same information is logged as with archive, plus information needed to reconstruct the status of running transactions from the WAL. To enable read-only queries on a standby server, wal_level must be set to hot_standby or higher on the primary, and hot_standby must be enabled in the standby. It is thought that there is little measurable difference in performance between using hot_standby and archive levels, so feedback is welcome if any production impacts are noticeable.
In logical level, the same information is logged as with hot_standby, plus information needed to allow extracting logical changesets 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.
| relreplident | char | Columns used to form "replica identity" for rows: d = default (primary key, if any), n = nothing, f = all columns i = index with indisreplident set, or default |
