• DESCRIBE 语句
  • EXPLAIN 语句
  • USE 语句

    DESCRIBE 语句

    DESCRIBE 和 EXPLAIN 是同义词，另外还可以缩写为 DESC。请参考 EXPLAIN 语句的用法。

    EXPLAIN 语句

    1. {EXPLAIN | DESCRIBE | DESC}
    2.     tbl_name [col_name]
    4. {EXPLAIN | DESCRIBE | DESC}
    5.     [explain_type]
    6.     explainable_stmt
    8. explain_type:
    9.     FORMAT = format_name
    11. format_name:
    12.     "DOT"
    14. explainable_stmt: {
    15.     SELECT statement
    16.   | DELETE statement
    17.   | INSERT statement
    18.   | REPLACE statement
    19.   | UPDATE statement
    20. }

    EXPLAIN 语句详细信息参考理解 TiDB 执行计划章节。

    除了 MySQL 标准的结果格式之外，TiDB 还支持输出 DotGraph 结果，这时需要指定 FORMAT = "dot"，示例如下：

    1. create table t(a bigint, b bigint);
    2. desc format = "dot" select A.a, B.b from t A join t B on A.a > B.b where A.a < 10;
    4. TiDB > desc format = "dot" select A.a, B.b from t A join t B on A.a > B.b where A.a < 10;desc format = "dot" select A.a, B.b from t A join t B on A.a > B.b where A.a < 10;
    5. +--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    6. | dot contents                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
    7. +--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    8. |
    9. digraph HashRightJoin_7 {
    10. subgraph cluster7{
    11. node [style=filled, color=lightgrey]
    12. color=black
    13. label = "root"
    14. "HashRightJoin_7" -> "TableReader_10"
    15. "HashRightJoin_7" -> "TableReader_12"
    16. }
    17. subgraph cluster9{
    18. node [style=filled, color=lightgrey]
    19. color=black
    20. label = "cop"
    21. "Selection_9" -> "TableScan_8"
    22. }
    23. subgraph cluster11{
    24. node [style=filled, color=lightgrey]
    25. color=black
    26. label = "cop"
    27. "TableScan_11"
    28. }
    29. "TableReader_10" -> "Selection_9"
    30. "TableReader_12" -> "TableScan_11"
    31. }
    32.  |
    33. +--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
    34. 1 row in set (0.00 sec)

    如果电脑上安装了 dot 程序 (包含在 graphviz 软件包中)，可以通过如下方式生成 PNG 文件：

    1. dot xx.dot -T png -O
    3. 这里的 xx.dot 是上面的语句返回结果。

    如果没有安装 dot，可以将结果拷贝到这个网站，可以得到一个树状图：

    Explain Dot

    USE 语句

    1. USE db_name

    切换默认 Database，当 SQL 语句中的表没有显示指定 Database 时，即使用默认 Database。