MySQL中explain可以查看內部執行計劃，他可以解釋的有SELECT, DELETE, INSERT, REPLACE,
和UPDATE語句（在MySQL5.6.3後纔出現這麼多）。另外，這只是查詢執行計劃，並沒有執行。同時，使用explain時也可以使用EXTENDED 或者 PARTITIONS參數。
以下是使用執行計劃出現的格式：
| Column | Meaning |
|---|---|
id |
The SELECT identifier（事件ID） |
select_type |
The SELECT type（查詢類型） |
table |
The table for the output row（表名） |
partitions |
The matching partitions（符合的分區） |
type |
The join type（聯合查詢類型） |
possible_keys |
The possible indexes to choose（可能選取的索引） |
key |
The index actually chosen（真實的索引） |
key_len |
The length of the chosen key（所選索引的字段的長度） |
ref |
The columns compared to the index（） |
rows |
Estimate of rows to be examined（被查詢的預估值） |
filtered |
Percentage of rows filtered by table condition（根據條件過濾的百分比） |
Extra |
Additional information（額外信息） |
1、select_type：
select_typeValue |
Meaning |
|---|---|
SIMPLE |
Simple SELECT (not
using UNION or
subqueries) |
PRIMARY |
Outermost SELECT |
UNION |
Second or later SELECT statement
in a UNION |
DEPENDENT UNION |
Second or later SELECT statement
in a UNION,
dependent on outer query |
UNION RESULT |
Result of a UNION. |
SUBQUERY |
First SELECT in
subquery |
DEPENDENT SUBQUERY |
First SELECT in
subquery, dependent on outer query |
DERIVED |
Derived table SELECT (subquery
in FROM clause) |
MATERIALIZED |
Materialized subquery |
UNCACHEABLE SUBQUERY |
A subquery for which the result cannot be cached and must be re-evaluated for each row of the outer query |
UNCACHEABLE UNION |
The second or later select in a UNION that
belongs to an uncacheable subquery (seeUNCACHEABLE SUBQUERY) |
