As a support engineer, I often see situations when the cardinality of a table is not correct. When InnoDB calculates the cardinality of an index, it does not scan the full table by default. Instead it looks at random pages, as determined by options innodb_stats_sample_pages, innodb_stats_transient_sample_pages and innodb_stats_persistent_sample_pages, or …[Read more]
I believe InnoDB storage engine architecture is great for a lot of online workloads, however, there are no silver bullets in technology and all design choices have their trade offs. In this blog post I’m going to talk about one important InnoDB limitation that you should consider.
InnoDB is a multiversion concurrency control (MVCC) storage engine which means many versions of the single row can exist at the same time. In fact there can be a huge amount of such row versions. Depending on the isolation mode you have chosen, InnoDB might have to keep all row versions going back to the earliest active read view, but at the very least it will have to keep all versions going back to the start of SELECT query which is currently running.
In most cases this is not a big deal – if you have many short transactions happening you will have only a few row versions to deal with. If you just use the system for reporting queries but do not …[Read more]