Optimistic phase: In this first phase, tables that are unlikely to be modified during the backup process (referred to as the “inactive tables” below, identified by the user with the optimistic-time option or, by exclusion, with the optimistic-busy-tables option) are backed up without locking the MySQL instance. And because those tables are not expected to be changed before the backup is finished, redo logs, undo logs, and system table spaces are not backed up by mysqlbackup in this phase.

https://dev.mysql.com/doc/refman/8.0/en/innodb-persistent-stats.html

Index statistics are used by the optimizer to estimate the cost of different query execution plans and choose the most efficient one. InnoDB supports two types of index statistics: persistent and transient. Persistent statistics are stored on disk in the mysql.innodb_table_stats and mysql.innodb_index_stats tables, and are updated either automatically or manually.  Transient statistics are stored in memory and are recalculated on server restart or table access 1 .

Updating index statistics involves sampling a number of pages from the index and estimating the cardinality and distribution of the values in the indexed columns. This process requires reading pages from disk, which can be I/O intensive, especially for large indexes or tables with many indexes.  The number of pages sampled can be controlled by the innodb_stats_persistent_sample_pages variable, which defaults to 20 2 . Increasing this value can improve the accuracy of the statistics, but also increase the I/O cost.