查看“︁MySQL explain”︁的源代码

= Explain =
Explain用来查看执行计划。可用于<span class="article-label">SELECT</span>, <span class="article-label">DELETE</span>, <span class="article-label">INSERT</span>, <span class="article-label">REPLACE</span>, and <span class="article-label">UPDATE</span> statements

== 例子 ==
<pre>
mysql> explain SELECT COUNT(*) FROM vehicle_prod WHERE fin LIKE '%00000' \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: vehicle_prod
   partitions: NULL
         type: index
possible_keys: NULL
          key: idx_vehicle_prod_fin
      key_len: 203
          ref: NULL
         rows: 3182812
     filtered: 11.11
        Extra: Using where; Using index
1 row in set, 1 warning (0.00 sec)
</pre>

其中：
;id:The SELECT identifier
;select_type:The SELECT type
*SIMPLE:Simple SELECT (not using UNION or subqueries)
*PRIMARY:Outermost SELECT
*UNION:Second or later SELECT statement in a UNION
*SUBQUERY:First SELECT in subquery
* ...
;table:The table for the output row
;partitions:The matching partitions
;type:The join type
;possible_keys:The possible indexes to choose
;key:The index actually chosen
;key_len:The length of the chosen key
;ref:The columns compared to the index
;rows:Estimate of rows to be examined
;filtered:Percentage of rows filtered by table condition
;Extra:Additional information

It is possible for key to name an index that is not present in the <span class="article-label">possible_keys</span> value. This can happen if none of the <span class="article-label">possible_keys</span> indexes are suitable for looking up rows, but all the columns selected by the query are columns of some other index. That is, the named index covers the selected columns, so although it is not used to determine which rows to retrieve, an index scan is more efficient than a data row scan.


#https://dev.mysql.com/doc/refman/8.0/en/explain-output.html
[[Category:Database]]