For mysql execution plan can be added before the select Explain to implement, it can tell us how your statement performance.
The following is a detailed description of explain, are also official, a reference in the future.
| id |
SELECT identifier. This is a SELECT query sequence number |
| select_type |
SELECT type, which can be any of the following:
- SIMPLE : Simple SELECT (not using UNION or subqueries)
- PRIMARY : the outermost SELECT
- UNION : The second or rear UNION SELECT statement
- UNION DEPENDENT : UNION in the second SELECT statement or the back, depending on outside of inquiry
- The RESULT UNION : UNION results
- SUBQUERY : the first sub-query SELECT
- SUBQUERY DEPENDENT : the first sub-query SELECT, depending on outside inquiry
- DERIVED : export table SELECT (FROM clause subqueries)
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| table |
Output line table referenced |
| type |
Join type. Various types of coupling are given below, in accordance with the type ordered from best to worst type:
- System : Only one line of table (= system table). This is a special case of the const join type.
- const : Table at most one matching row, which will be read at the start of the query. Because only one row, column values in this row may be considered to optimize the remaining portion is constant. const table quickly because they are read only once!
- eq_ref : For each combination of rows from the front of the table, reads a line from the table. This is probably the best connection type, except const type.
- REF : For each combination of rows from the preceding table, all rows matching the index values read from this table.
- ref_or_null : The connection type as ref, but added the line MySQL can search specifically contain NULL values.
- index_merge : the coupling type indication index combined optimization method.
- unique_subquery : The type of the alternative form of ref IN subqueries of the following: value IN (SELECT primary_key FROM single_table WHERE some_expr) unique_subquery is an index lookup function, can completely replace the subquery, more efficient.
- index_subquery : This type is similar to the coupling unique_subquery. IN subquery can be replaced, but only for the following sub-query in the form of a non-unique index: value IN (SELECT key_column FROM single_table WHERE some_expr)
- Range : only retrieves a row given range, using an index to select the rows.
- index : the coupling with ALL the same type, except that only the index tree is scanned. This is usually faster than ALL, because the index file is usually smaller than the data file.
- ALL : For each combination of rows from the previous table, a full table scan.
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| possible_keys |
It indicates which indexes MySQL could use to find the rows in the table |
| key |
Display key (index) MySQL actually decided to use. If you do not select an index, the key is NULL. |
| key_len |
Display MySQL decided to use key lengths. If the key is NULL, the length is NULL. |
| ref |
Shows which columns or constants are selected together with a key from a table using the row. |
| rows |
Display the number of rows MySQL believes it must examine the query execution. Of data between a plurality of lines can be estimated by multiplying the number of rows to be processed. |
| filtered |
It shows the percentage of the estimated value of the number of rows filtered through conditions. |
| Extra |
This column contains the MySQL query to resolve details
- DISTINCT : MySQL found after a matching line for the current row combination stop searching for more rows.
- EXISTS Not : the MySQL LEFT JOIN able to query optimization, after a match was found LEFT JOIN standard line, no more rows in the check table of the foregoing combinations of rows.
- Record the checked for each Range (Map index: #) : the MySQL found no good index may be used, but if the value found from the front of the column of the table is known, the index portion may be used.
- Filesort the Using : MySQL requires additional one pass to find out how to retrieve the rows in sorted order.
- Index the Using : from the use of only information in the index tree without having to search further reads the actual line to retrieve column information in the table.
- The Temporary the Using : In order to solve the query, MySQL needs to create a temporary table to hold the result.
- WHERE the using : the WHERE clause restricts a row which matches the next table or sent to a client.
- Sort_union the using (...), the using Union (...), the using INTERSECT (...) : illustrate how these functions merge type join index_merge index scan.
- Using index for group-by:类似于访问表的Using index方式,Using index for group-by表示MySQL发现了一个索引,可以用来查 询GROUP BY或DISTINCT查询的所有列,而不要额外搜索硬盘访问实际的表。
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type：这是重要的列，显示连接使用了何种类型。从最好到最差的连接类型为const、eq_reg、ref、range、index和ALL
type显示的是访问类型，是较为重要的一个指标，结果值从好到坏依次是：system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL
一般来说，得保证查询至少达到range级别，最好能达到ref。
我们在进行条件查询时，建议使用索引，否则将引起全表扫描，IO的开销和程序的性能都没法保证！