The cost estimates for internally created temporary tables stored in the MEMORY  storage engine. Increasing these values increases the cost estimate of using internal temporary tables and makes the optimizer prefer query plans with less use of them. For information about such tables, see Section 8.4.4, “Internal Temporary Table Use in MySQL” . 

The smaller default values for these memory parameters compared to the default values for the corresponding disk parameters ( disk_temptable_create_cost , disk_temptable_row_cost ) reflects the lesser cost of processing memory-based tables. 

• row_evaluate_cost 

The cost of evaluating record conditions. Increasing this value causes a query plan that examines many rows to become more expensive compared to a query plan that examines fewer rows. For example, a table scan becomes relatively more expensive compared to a range scan that reads fewer rows. 

The engine_cost  table contains these columns: 

• engine_name 

The name of the storage engine to which this cost estimate applies. The name is not case-sensitive. If the value is default , it applies to all storage engines that have no named entry of their own. If the server does not recognize the engine name when it reads this table, it writes a warning to the error log. 

• device_type 

The device type to which this cost estimate applies. The column is intended for specifying different cost estimates for different storage device types, such as hard disk drives versus solid state drives. Currently, this information is not used and 0 is the only permitted value. 

• cost_name 

Same as in the server_cost  table. 

• cost_value 

Same as in the server_cost  table. 

• last_update 

Same as in the server_cost  table. 

• comment 

Same as in the server_cost  table. 

• default_value 

The default (compiled-in) value for the cost estimate. This column is a read-only generated column that retains its value even if the associated cost estimate is changed. For rows added to the table at runtime, the value of this column is NULL , with the exception that if the row has the same cost_name  value as one of the original rows, the default_value  column has the same value as that row. 

The primary key for the engine_cost  table is a tuple comprising the ( cost_name , engine_name , device_type ) columns, so it is not possible to create multiple entries for any combination of values in those columns. 

The server recognizes these cost_name  values for the engine_cost  table: 

