Row-based Replication With No Primary Key

Understand the performance implications and best practices for replicating tables without primary keys when using row-based logging, including how to avoid full table scans.

The terms master and slave have historically been used in replication, and MariaDB has begun the process of adding primary and replica synonyms. The old terms will continue to be used to maintain backward compatibility - see MDEV-18777 to follow progress on this effort.

MariaDB improves on row-based replication (see binary log formats) of tables which have no primary key but do have some other index. This is based in part on the original Percona patch row_based_replication_without_primary_key.patch, with some additional fixes and enhancements.

When row-based replication is used with UPDATE or DELETE, the replica needs to locate each replicated row based on the value in columns. If the table contains at least one index; an index lookup will be used (otherwise a table scan is needed for each row, which is extremely inefficient for all but the smallest table and generally to be avoided).

In MariaDB, the slave will try to choose a good index among any available:

• The primary key is used, if there is one.

• Else, the first unique index without NULL-able columns is used, if there is one.

• Else, a choice is made among any normal indexes on the table (e.g. a FULLTEXT index is not considered).

The choice of which of several non-unique indexes to use is based on the cardinality of indexes; the one that is most selective (has the smallest average number of rows per distinct tuple of column values) is preferred. Note that for this choice to be effective, for most storage engines (like MyISAM, InnoDB) it is necessary to make sure ANALYZE TABLE has been run on the slave, otherwise statistics about index cardinality is not available. In the absence of index cardinality, the first unique index is chosen, if any, else the first non-unique index.

See Also

_{This page is licensed: CC BY-SA / Gnu FDL}