案例：增加JOIN列非空条件

1. SELECT * FROM join_a a JOIN join_b b ON a.b = b.b;

执行计划下：

1. QUERY PLAN
2. ----------------------------------------------------------------------------------------------------------------------
3. Hash Join  (cost=58.35..14677.69 rows=1074607 width=16) (actual time=23.374..23.384 rows=10 loops=1)
4. Hash Cond: (a.b = b.b)
5. ->  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)
6. ->  Hash  (cost=31.49..31.49 rows=2149 width=8) (actual time=0.614..0.614 rows=1000 loops=1)
7. Buckets: 32768  Batches: 1  Memory Usage: 40kB
8. ->  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)
9. Total runtime: 23.716 ms
10. (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。

1. 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：

1. QUERY PLAN 2. --------------------------------------------------------------------------------------------------------------------- 3. Hash Join (cost=58.22..14560.97 rows=1063762 width=16) (actual time=13.237..13.247 rows=10 loops=1) 4. Hash Cond: (a.b = b.b) 5. -> 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) 6. Filter: (b IS NOT NULL) 7. Rows Removed by Filter: 100000 8. -> Hash (cost=31.49..31.49 rows=2138 width=8) (actual time=0.566..0.566 rows=1000 loops=1) 9. Buckets: 32768 Batches: 1 Memory Usage: 40kB 10. -> 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) 11. Filter: (b IS NOT NULL) 12. Total runtime: 13.556 ms 13. (10 rows)