实用工具语句
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
{EXPLAIN | DESCRIBE | DESC}
tbl_name [col_name]
{EXPLAIN | DESCRIBE | DESC}
[explain_type]
explainable_stmt
explain_type:
FORMAT = format_name
format_name:
"DOT"
explainable_stmt: {
SELECT statement
| DELETE statement
| INSERT statement
| REPLACE statement
| UPDATE statement
}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：
create table t(a bigint, b bigint);
desc format = "dot" select A.a, B.b from t A join t B on A.a > B.b where A.a < 10;
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;
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| dot contents |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|
digraph HashRightJoin_7 {
subgraph cluster7{
node [style=filled, color=lightgrey]
color=black
label = "root"
"HashRightJoin_7" -> "TableReader_10"
"HashRightJoin_7" -> "TableReader_12"
}
subgraph cluster9{
node [style=filled, color=lightgrey]
color=black
label = "cop"
"Selection_9" -> "TableScan_8"
}
subgraph cluster11{
node [style=filled, color=lightgrey]
color=black
label = "cop"
"TableScan_11"
}
"TableReader_10" -> "Selection_9"
"TableReader_12" -> "TableScan_11"
}
|
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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：
dot xx.dot -T png -O
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：
USE 语句
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。