# 案例：增加JOIN列非空条件
```
SELECT * FROM join_a a JOIN join_b b ON a.b = b.b;
```
执行计划如下：
```
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------
 Hash Join  (cost=58.35..14677.69 rows=1074607 width=16) (actual time=23.374..23.384 rows=10 loops=1)
   Hash Cond: (a.b = b.b)
   ->  Seq Scan on join_a a  (cost=0.00..2248.10 rows=100010 width=8) (actual time=0.495..12.551 rows=100010 loops=1)
   ->  Hash  (cost=31.49..31.49 rows=2149 width=8) (actual time=0.614..0.614 rows=1000 loops=1)
          Buckets: 32768  Batches: 1  Memory Usage: 40kB
         ->  Seq Scan on join_b b  (cost=0.00..31.49 rows=2149 width=8) (actual time=0.009..0.183 rows=1000 loops=1)
 Total runtime: 23.716 ms
(7 rows)
```
#### 优化分析
1. 分析执行计划可知，在顺序扫描阶段耗时较多。
2. 建议在语句中手动添加JOIN列的非空判断，修改后的语句如下所示。
   ```
   SELECT
    * 
   SELECT * FROM join_a a JOIN join_b b ON a.b = b.b where a.b IS NOT NULL;
   ```
   执行计划如下：
   ```
   QUERY PLAN
   ---------------------------------------------------------------------------------------------------------------------
    Hash Join  (cost=58.22..14560.97 rows=1063762 width=16) (actual time=13.237..13.247 rows=10 loops=1)
      Hash Cond: (a.b = b.b)
      ->  Seq Scan on join_a a  (cost=0.00..2248.10 rows=99510 width=8) (actual time=12.417..12.422 rows=10 loops=1)
            Filter: (b IS NOT NULL)
            Rows Removed by Filter: 100000
      ->  Hash  (cost=31.49..31.49 rows=2138 width=8) (actual time=0.566..0.566 rows=1000 loops=1)
             Buckets: 32768  Batches: 1  Memory Usage: 40kB
            ->  Seq Scan on join_b b  (cost=0.00..31.49 rows=2138 width=8) (actual time=0.011..0.229 rows=1000 loops=1)
                  Filter: (b IS NOT NULL)
    Total runtime: 13.556 ms
   (10 rows)
   ```