Merge join vs merge semi join against primary key

Hello,

I'm seeing some inexplicable (to me) behavior in PostgreSQL 9.2. I checked
the archives, but I still can't explain it. Apologies if I missed something.

1. When I join two tables with "WHERE id IN (...)" versus with an explicit
join, and the join column for the inner table is a primary key, I would
expect
the same behavior in both cases, but the optimizer is choosing a merge join
in
one case and a merge semi join in the other. There's at most one customer
with a given id. Why not do a semi join?

2. Even though the join methods are different, I would expect about the same
performance in either case, but one query takes only a few hundred
milliseconds while the other takes hundreds of seconds. Ouch!

Can anyone help me explain this behavior?

Some details are below. Let me know if it would be helpful to gather others.

Sean

production=> select version();
version
----------------------------------------------------------------------------------------
PostgreSQL 9.2.13 on i686-pc-linux-gnu, compiled by gcc (Debian 4.7.2-5)
4.7.2, 32-bit
(1 row)

production=> \d customers
Table "public.customers"
Column | Type | Modifiers
---------------+----------+-------------------------------------------------------
id | bigint | not null default
nextval('customers_id_seq'::regclass)
group_id | bigint |
...
Indexes:
"customers_pkey" PRIMARY KEY, btree (id)
...

production=> select count(*) from customers;
count
--------
473733
(1 row)

production=> \d balances
Table "public.balances"
Column | Type | Modifiers
-------------------+----------+------------------------------------------------------
id | bigint | not null default
nextval('balances_id_seq'::regclass)
balance | integer | not null default 0
tracking_number | integer | not null
customer_id | bigint | not null
...
Indexes:
"balances_pkey" PRIMARY KEY, btree (id)
"balances_customer_tracking_number_index" UNIQUE, btree (customer_id,
tracking_number)
...

production=> select count(*) from balances;
count
-------
16876
(1 row)

production=> analyze verbose customers;
INFO: analyzing "public.customers"
INFO: "customers": scanned 14280 of 14280 pages, containing 475288 live
rows and 1949 dead rows; 300000 rows in sample, 475288 estimated total rows
ANALYZE

production=> analyze verbose balances;
INFO: analyzing "public.balances"
INFO: "balances": scanned 202 of 202 pages, containing 16876 live rows and
0 dead rows; 16876 rows in sample, 16876 estimated total rows
ANALYZE

production=> explain analyze SELECT * FROM balances where customer_id IN
(SELECT id from customers WHERE group_id = 45);

QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------
Merge Semi Join (cost=2442.14..19958.30 rows=16876 width=80) (actual
time=119.905..145.126 rows=7318 loops=1)
Merge Cond: (balances.customer_id = customers.id)
-> Index Scan using balances_customer_id_index on balances
(cost=0.00..727.79 rows=16876 width=80) (actual time=0.302..9.477
rows=16876 loops=1)
-> Index Scan using customers_pkey on customers (cost=0.00..64192.97
rows=184 width=8) (actual time=103.354..126.459 rows=359 loops=1)
Filter: (group_id = 45)
Rows Removed by Filter: 141684
Total runtime: 146.659 ms
(7 rows)

production=> explain analyze SELECT ac.* FROM balances ac join customers o
ON o.id = ac.customer_id WHERE o.group_id = 45;

QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
Merge Join (cost=2214.50..20216.86 rows=30 width=80) (actual
time=185.615..201991.752 rows=7318 loops=1)
Merge Cond: (ac.customer_id = o.id)
-> Index Scan using balances_customer_tracking_number_index on balances
ac (cost=0.00..1007.49 rows=16876 width=80) (actual time=0.068..25.036
rows=16876 loops=1)
-> Index Scan using customers_pkey on customers o (cost=0.00..63836.61
rows=836 width=8) (actual time=159.840..201915.765 rows=7672 loops=1)
Filter: (group_id = 45)
Rows Removed by Filter: 212699113
Total runtime: 201995.044 ms
(7 rows)