使用Explain判断SQL是否可以使用列存索引

分析示例

在PolarDB MySQL版集群中，列存执行计划以横向树的形式输出，该格式与行存执行计划的输出格式存在明显区别。您可以通过使用Explain查看SQL的执行计划，来判断某条SQL语句是否可以使用列存索引加速功能。

SQL示例：

EXPLAIN SELECT l_orderkey, SUM(l_extendedprice * (1 - l_discount)) AS revenue, o_orderdate, o_shippriority
FROM customer, orders, lineitem
WHERE c_mktsegment = 'BUILDING'
AND c_custkey = o_custkey
AND l_orderkey = o_orderkey
AND o_orderdate < date '1995-03-24'
AND l_shipdate > date '1995-03-24'
GROUP BY l_orderkey, o_orderdate, o_shippriority
ORDER BY revenue DESC, o_orderdate;

行存执行计划显示结果：

+----+-------------+----------+------------+------+--------------------+------------+---------+-----------------------------+--------+----------+----------------------------------------------+
| id | select_type | table    | partitions | type | possible_keys      | key        | key_len | ref                         | rows   | filtered | Extra                                        |
+----+-------------+----------+------------+------+--------------------+------------+---------+-----------------------------+--------+----------+----------------------------------------------+
|  1 | SIMPLE      | customer | NULL       | ALL  | PRIMARY            | NULL       | NULL    | NULL                        | 147630 |    10.00 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE      | orders   | NULL       | ref  | PRIMARY,ORDERS_FK1 | ORDERS_FK1 | 4       | tpch100g.customer.C_CUSTKEY |     14 |    33.33 | Using where                                  |
|  1 | SIMPLE      | lineitem | NULL       | ref  | PRIMARY            | PRIMARY    | 4       | tpch100g.orders.O_ORDERKEY  |      4 |    33.33 | Using where                                  |
+----+-------------+----------+------------+------+--------------------+------------+---------+-----------------------------+--------+----------+----------------------------------------------+
3 rows in set, 1 warning (0.00 sec)

列存执行计划显示结果（横向树形式）：

+----+----------------------------+----------+--------+--------+-----------------------------------------------------------------------------+
| ID | Operator                   | Name     | E-Rows | E-Cost | Extra Info                                                                  |
+----+----------------------------+----------+--------+--------+-----------------------------------------------------------------------------+
|  1 | Select Statement           |          |        |        | IMCI Execution Plan (max_dop = 4, max_query_mem = 858993459)                |
|  2 | └─Sort                     |          |        |        | Sort Key: revenue DESC,o_orderdate ASC                                      |
|  3 |   └─Hash Groupby           |          |        |        | Group Key: (lineitem.L_ORDERKEY, orders.O_ORDERDATE, orders.O_SHIPPRIORITY) |
|  4 |     └─Hash Join            |          |        |        | Join Cond: orders.O_ORDERKEY = lineitem.L_ORDERKEY                          |
|  5 |       ├─Hash Join          |          |        |        | Join Cond: customer.C_CUSTKEY = orders.O_CUSTKEY                            |
|  6 |       │ ├─Table Scan       | customer |        |        | Cond: (C_MKTSEGMENT = "BUILDING")                                           |
|  7 |       │ └─Table Scan       | orders   |        |        | Cond: (O_ORDERDATE < 03/24/1995)                                            |
|  8 |       └─Table Scan         | lineitem |        |        | Cond: (L_SHIPDATE > 03/24/1995)                                             |
+----+----------------------------+----------+--------+--------+-----------------------------------------------------------------------------+
8 rows in set (0.01 sec)

相关文档

列存索引只能访问被其所覆盖的列。因此，当您的SQL语句中所需的表或列未被列存索引完全覆盖时，列存索引将无法生效。

• 检查SQL语句中的表是否已创建列索引：存储过程dbms_imci.check_columnar_index()能够解析输入的SQL语句，提取该语句中使用的所有列，并验证这些列是否被列索引所覆盖。

• 获取创建列存索引的DDL语句：存储过程dbms_imci.columnar_advise()可以针对输入的SQL语句，生成相应的用于添加列存索引的DDL语句。执行后，可以确保该SQL语句中涉及的所有列均被列索引覆盖。

常见问题