单表查询

本主题显示单个表的简单查询和复杂查询的样本 SET EXPLAIN 输出。

图 1 显示了简单查询的 SET EXPLAIN 输出。

图: 简单查询的部分 SET EXPLAIN 输出

QUERY:
------
SELECT fname, lname, company FROM customer

Estimated Cost: 2
Estimated # of Rows Returned: 28

    1) virginia.customer: SEQUENTIAL SCAN

图 2 显示了 customer 表上复杂查询的 SET EXPLAIN 输出。

图: 复杂查询的部分 SET EXPLAIN 输出

QUERY:
------
SELECT fname, lname, company FROM customer
WHERE company MATCHES 'Sport*' AND
        customer_num BETWEEN 110 AND 115
ORDER BY lname

    Estimated Cost: 1
Estimated # of Rows Returned: 1
Temporary Files Required For: Order By

    1) virginia.customer: INDEX PATH

        Filters: virginia.customer.company MATCHES 'Sport*'

    (1) Index Keys: customer_num   (Serial, fragments: ALL)
        Lower Index Filter: virginia.customer.customer_num >=
110 
        Upper Index Filter: virginia.customer.customer_num <=
115

图 2 中的以下输出行显示了第二个查询的索引扫描的范围：

Lower Index Filter: virginia.customer.customer_num >= 110
以索引键值 110 开始索引扫描。
Upper Index Filter: virginia.customer.customer_num <= 115
以索引键值 115 停止索引扫描。