Case: Adding NOT NULL for JOIN Columns

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

The execution plan is as follows:

                                                      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)

Optimization Analysis

As shown in the execution plan, the sequential scan phase is time consuming.

Therefore, you are advised to manually add NOT NULL for JOIN columns in the statement, as shown below:

SELECT
 * 
SELECT * FROM join_a a JOIN join_b b ON a.b = b.b where a.b IS NOT NULL;

The execution plan is as follows:

                                                     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)