MySQL 执行计划中代价估算的输出信息

来源： http://dev.mysql.com/worklog/task/?id=6510

EXPLAIN JSON should print cost for following:
*) "query_cost" - total cost of a query block, no matter top query or a subquery.
*) "sort_cost" - cost of the first sorting operation (GROUP BY or ORDER BY) where 
and if filesort is used. Cost for second sorting operation isn't printed as 
optimizer doesn't calculate it.
*) "read_cost" - cost of reading for each table used in the query block.
*) "eval_cost" - cost of condition evaluation for each table in the query block.
*) "prefix_cost" - cost of executing prefix join in the query block.  
。
/* *) "data_read_per_join" - estimated amount of data from the table processed
during single query block execution.
*) "rows_produced_per_join"/"rows_examined_per_scan" estimated number of records
from the table (per each table from the query block) produced/examined per single 
query block execution.
*) "used_columns" - list of columns of the table (per each table in the query 
block) used for either read or write in the query.

 另外，还有解释如下：
1) "query_cost" total [sub]query cost per one execution. Wrapped in the
  "cost_info" node, child of the "query_block" node. For the top-level
  select should be equal to the 'last_query_cost' status variable.
2) "sort_cost" cost of filesort, independently of algorithm. Wrapped in the
"cost_info" node, child of the "ordering_operation" node.
3) "read_cost" cost of retrieving data from table, i.e access method cost.
4) "eval_cost" cost of .