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In a table with 5 millions rows, a SELECT count(*) FROM table would be instant in MyISAM but would take several seconds in InnoDB.
Why is this that way? Why haven’t they optimise count in InnoDB like MyISAM?
Thanks.
It’s a difference in implementation. InnoDB supports transactions and therefore it has to count the rows based on your transactionally consistent view of the table(s). Since MyISAM doesn’t support ACID properties, if a row is inserted, it’s inserted for everyone and therefore it can just update a count it keeps within the storage engine.