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 slave 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).
Beginning in MariaDB 5.3, the slave will try to choose a good index among any available:
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 will not be available. In the absence of index cardinality, the first unique index will be chosen, if any, else the first non-unique index.
Prior to MariaDB 5.3, the slave would always choose the first index without considering cardinality. The slave could even choose an unusable index (like FULLTEXT) if no other index was available (MySQL Bug #58997), causing row-based replication to break in this case; this is also fixed in MariaDB 5.3.
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