EXPLAIN

EXPLAIN 语句仅用于显示查询的执行计划，而不执行查询。EXPLAIN ANALYZE 可执行查询，补充 EXPLAIN 语句。如果 EXPLAIN 的输出与预期结果不匹配，可考虑在查询的每个表上执行 ANALYZE TABLE。

语句 DESC 和 DESCRIBE 是 EXPLAIN 的别名。EXPLAIN <tableName> 的替代用法记录在 SHOW [FULL] COLUMNS FROM 下。

语法图

ExplainSym:

ExplainSym

ExplainStmt:

ExplainStmt

ExplainableStmt:

ExplainableStmt

示例

  1. mysql> EXPLAIN SELECT 1;
  2. +-+-+++
  3. | id                | count | task | operator info |
  4. +-+-+++
  5. | Projection_3      | 1.00  | root | 1             |
  6. | └─TableDual_4     | 1.00  | root | rows:1        |
  7. +-+-+++
  8. 2 rows in set (0.00 sec)
  10. mysql> CREATE TABLE t1 (id INT NOT NULL PRIMARY KEY auto_increment, c1 INT NOT NULL);
  11. Query OK, 0 rows affected (0.10 sec)
  13. mysql> INSERT INTO t1 (c1) VALUES (1), (2), (3);
  14. Query OK, 3 rows affected (0.02 sec)
  15. Records: 3  Duplicates: 0  Warnings: 0
  17. mysql> EXPLAIN SELECT * FROM t1 WHERE id = 1;
  18. +-+-++--+
  19. | id          | count | task | operator info      |
  20. +-+-++--+
  21. | Point_Get_1 | 1.00  | root | table:t1, handle:1 |
  22. +-+-++--+
  23. 1 row in set (0.00 sec)
  25. mysql> DESC SELECT * FROM t1 WHERE id = 1;
  26. +-+-++--+
  27. | id          | count | task | operator info      |
  28. +-+-++--+
  29. | Point_Get_1 | 1.00  | root | table:t1, handle:1 |
  30. +-+-++--+
  31. 1 row in set (0.00 sec)
  33. mysql> DESCRIBE SELECT * FROM t1 WHERE id = 1;
  34. +-+-++--+
  35. | id          | count | task | operator info      |
  36. +-+-++--+
  37. | Point_Get_1 | 1.00  | root | table:t1, handle:1 |
  38. +-+-++--+
  39. 1 row in set (0.00 sec)
  41. mysql> EXPLAIN INSERT INTO t1 (c1) VALUES (4);
  42. ERROR 1105 (HY000): Unsupported type *core.Insert
  44. mysql> EXPLAIN UPDATE t1 SET c1=5 WHERE c1=3;
  45. ++-++-+
  46. | id                  | count    | task | operator info                                               |
  47. ++-++-+
  48. | TableReader_6       | 10.00    | root | data:Selection_5                                            |
  49. | └─Selection_5       | 10.00    | cop  | eq(test.t1.c1, 3)                                           |
  50. |   └─TableScan_4     | 10000.00 | cop  | table:t1, range:[-inf,+inf], keep order:false, stats:pseudo |
  51. ++-++-+
  52. 3 rows in set (0.00 sec)
  54. mysql> EXPLAIN DELETE FROM t1 WHERE c1=3;
  55. ++-++-+
  56. | id                  | count    | task | operator info                                               |
  57. ++-++-+
  58. | TableReader_6       | 10.00    | root | data:Selection_5                                            |
  59. | └─Selection_5       | 10.00    | cop  | eq(test.t1.c1, 3)                                           |
  60. |   └─TableScan_4     | 10000.00 | cop  | table:t1, range:[-inf,+inf], keep order:false, stats:pseudo |
  61. ++-++-+
  62. 3 rows in set (0.00 sec)

如果未指定 FORMAT，或未指定 FORMAT ="row"，那么 EXPLAIN 语句将以表格格式输出结果。更多信息，可参阅 Understand the Query Execution Plan。

除 MySQL 标准结果格式外，TiDB 还支持 DotGraph。需按照下列所示指定 FORMAT ="dot"：

  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)

如果你的计算机上安装了 dot 程序（在 graphviz 包中），可使用以下方法生成 PNG 文件：

  1. dot xx.dot -T png -O
  3. The xx.dot is the result returned by the above statement.

如果你的计算机上未安装 dot 程序，可将结果复制到 本网站 以获取树形图：

Explain Dot

MySQL 兼容性

  • EXPLAIN 的格式和 TiDB 中潜在的执行计划都与 MySQL 有很大不同。
  • TiDB 不像 MySQL 那样支持 EXPLAIN FORMAT = JSON。
  • TiDB 目前不支持插入语句的 EXPLAIN。

另请参阅