Server Status Variables The number of unused blocks in the  MyISAM  key cache. You can use this value to determine how much of the key cache is in use; see the discussion of  key_buffer_size  in Section 5.1.8, “Server System Variables” . The number of used blocks in the  MyISAM  key cache. This value is a high-water mark that indicates the maximum number of blocks that have ever been in use at one time. The number of requests to read a key block from the  MyISAM  key cache. The number of physical reads of a key block from disk into the  MyISAM  key cache. If  Key_reads  is large, then your  key_buffer_size  value is probably too small. The cache miss rate can be calculated as  Key_reads / Key_read_requests . The number of requests to write a key block to the  MyISAM  key cache. The number of physical writes of a key block from the  MyISAM  key cache to disk. The total cost of the last compiled query as computed by the query optimizer. This is useful for comparing the cost of different query plans for the same query. The default value of 0 means that no query has been compiled yet. The default value is 0.  Last_query_cost  has session scope. In MySQL 8.0.16 and later, this variable shows the cost of queries that have multiple query blocks, summing the cost estimates of each query block, estimating how many times non-cacheable subqueries are executed, and multiplying the cost of those query blocks by the number of subquery executions. (Bug #92766, Bug #28786951) Prior to MySQL 8.0.16,  Last_query_cost  was computed accurately only for simple, “flat”  queries, but not for complex queries such as those containing subqueries or  UNION . (For the latter, the value was set to 0.) The number of iterations the query optimizer made in execution plan construction for the previous query. Last_query_cost  has session scope. The number of attempts to connect to locked user accounts. For information about account locking and unlocking, see Section 6.2.19, “Account Locking” . The number of  SELECT  statements for which the execution timeout was exceeded. The number of  SELECT  statements for which a nonzero execution timeout was set. This includes statements that include a nonzero  MAX_EXECUTION_TIME  optimizer hint, and statements that include • Max_execution_time_set • Max_execution_time_exceeded • Locked_connects • Last_query_partial_plans • Last_query_cost • Key_writes • Key_write_requests • Key_reads • Key_read_requests • Key_blocks_used 961