query plan question, nested loop vs hash join

db_new=# \d activities_example

  Table "public.activities_example"

     Column     |  Type   | Modifiers

----------------+---------+-----------

 id             | integer |

 order_chain_id | integer |

Indexes:

    "activities_example_idx" btree (order_chain_id)

db_new=# \d orders_example

Table "public.orders_example"

 Column |  Type   | Modifiers

--------+---------+-----------

 id     | integer |

db_new=# select count(*) from activities_example ;

  count

---------

 3059965

db_new=# select count(*) from orders_example ;

 count

-------

 19038

db_new=# select count(*) from activities_example where order_chain_id in (select id from orders_example);

 count

-------

 91426

(1 row)

db_new=# explain analyze select * from activities_example where order_chain_id in (select id from orders_example);

                                                             QUERY PLAN

------------------------------------------------------------------------------------------------------------------------------------

 Hash Semi Join  (cost=513.36..57547.59 rows=89551 width=8) (actual time=18.340..966.367 rows=91426 loops=1)

   Hash Cond: (activities_example.order_chain_id = orders_example.id)

   ->  Seq Scan on activities_example  (cost=0.00..44139.65 rows=3059965 width=8) (actual time=0.018..294.216 rows=3059965 loops=1)

   ->  Hash  (cost=275.38..275.38 rows=19038 width=4) (actual time=5.458..5.458 rows=19038 loops=1)

         Buckets: 2048  Batches: 1  Memory Usage: 670kB

         ->  Seq Scan on orders_example  (cost=0.00..275.38 rows=19038 width=4) (actual time=0.015..2.308 rows=19038 loops=1)

 Total runtime: 970.234 ms

(7 rows)

db_new=# set enable_hashjoin = off;

SET

db_new=# explain analyze select * from activities_example where order_chain_id in (select id from orders_example);

                                                                     QUERY PLAN

-----------------------------------------------------------------------------------------------------------------------------------------------------

 Nested Loop  (cost=1629.09..166451.01 rows=89551 width=8) (actual time=16.091..116.476 rows=91426 loops=1)

   ->  Unique  (cost=1628.66..1723.85 rows=19038 width=4) (actual time=15.929..23.156 rows=19038 loops=1)

         ->  Sort  (cost=1628.66..1676.25 rows=19038 width=4) (actual time=15.892..19.884 rows=19038 loops=1)

               Sort Key: orders_example.id

               Sort Method: external sort  Disk: 264kB

               ->  Seq Scan on orders_example  (cost=0.00..275.38 rows=19038 width=4) (actual time=0.015..2.747 rows=19038 loops=1)

   ->  Index Scan using activities_example_idx on activities_example  (cost=0.43..8.60 rows=5 width=8) (actual time=0.002..0.004 rows=5 loops=19038)

         Index Cond: (order_chain_id = orders_example.id)

 Total runtime: 121.366 ms

(9 rows)

-- 

Regards, Andrey Lizenko