EXPLAIN语句用于显示语句的执行计划。您可以使用EXPLAIN语句来确认POLARDB是否从查询的执行计划中剪去分区的。 以下为您演示分区剪枝的效率，首先要创建一个简单表：

CREATE TABLE sales
(
  dept_no     number,   
  part_no     varchar2,
  country     varchar2(20),
  date        date,
  amount      number
)
PARTITION BY LIST(country)
(
  PARTITION europe VALUES('FRANCE', 'ITALY'),
  PARTITION asia VALUES('INDIA', 'PAKISTAN'),
  PARTITION americas VALUES('US', 'CANADA')
);

然后，执行一个包括EXPLAIN语句的约束查询：

EXPLAIN (COSTS OFF) SELECT * FROM sales WHERE country = 'INDIA';

产生的查询计划结果显示了服务器将只扫描表sales_asia，country值为INDIA的记录可能会存储到这个表中：

postgres=# EXPLAIN (COSTS OFF) SELECT * FROM sales WHERE country = 'INDIA';
                    QUERY PLAN                     
---------------------------------------------------

 Append
   ->  Seq Scan on sales
         Filter: ((country)::text = 'INDIA'::text)
   ->  Seq Scan on sales_asia
         Filter: ((country)::text = 'INDIA'::text)
(5 rows)

如果您要执行一个查询来搜索一条与没有包括在分区键中的值匹配的记录：

EXPLAIN (COSTS OFF) SELECT * FROM sales WHERE dept_no = '30';  

产生的查询计划显示了服务器必须查询所有的分区来定位满足查询的记录：

postgres=# EXPLAIN (COSTS OFF) SELECT * FROM sales WHERE dept_no = '30';
               QUERY PLAN               
-----------------------------------------
 Append
   ->  Seq Scan on sales
         Filter: (dept_no = 30::numeric)
   ->  Seq Scan on sales_europe
         Filter: (dept_no = 30::numeric)
   ->  Seq Scan on sales_asia
         Filter: (dept_no = 30::numeric)
   ->  Seq Scan on sales_americas
         Filter: (dept_no = 30::numeric)
(9 rows)

排除约束在查询子分区表时同样适用：

CREATE TABLE sales
(
  dept_no     number,
  part_no     varchar2,
  country     varchar2(20),
  date        date,
  amount      number
)
PARTITION BY RANGE(date) SUBPARTITION BY LIST (country)
(
  PARTITION "2011" VALUES LESS THAN('01-JAN-2012')
  (
    SUBPARTITION europe_2011 VALUES ('ITALY', 'FRANCE'),
    SUBPARTITION asia_2011 VALUES ('PAKISTAN', 'INDIA'),
    SUBPARTITION americas_2011 VALUES ('US', 'CANADA')
  ),
  PARTITION "2012" VALUES LESS THAN('01-JAN-2013')
  (
    SUBPARTITION europe_2012 VALUES ('ITALY', 'FRANCE'),
    SUBPARTITION asia_2012 VALUES ('PAKISTAN', 'INDIA'),
    SUBPARTITION americas_2012 VALUES ('US', 'CANADA')
  ),
  PARTITION "2013" VALUES LESS THAN('01-JAN-2014')
  (
    SUBPARTITION europe_2013 VALUES ('ITALY', 'FRANCE'),
    SUBPARTITION asia_2013 VALUES ('PAKISTAN', 'INDIA'),
    SUBPARTITION americas_2013 VALUES ('US', 'CANADA')
  )
);

当您查询这个表时，查询计划器会从搜索路径中剪去任何可能不包含您想要的结果集的分区或子分区。

postgres=# EXPLAIN (COSTS OFF) SELECT * FROM sales WHERE country = 'US' AND date = 'Dec 12, 2012';
                                 QUERY PLAN                                 
-----------------------------------------------------------------------------
 Append
   ->  Seq Scan on sales
         Filter: (((country)::text = 'US'::text) AND (date = '12-DEC-12 00:00:00'::timestamp without time zone))
   ->  Seq Scan on sales_2012
         Filter: (((country)::text = 'US'::text) AND (date = '12-DEC-12 00:00:00'::timestamp without time zone))
   ->  Seq Scan on sales_americas_2012
         Filter: (((country)::text = 'US'::text) AND (date = '12-DEC-12 00:00:00'::timestamp without time zone))
(7 rows)