Postgres No Index Only Scan On Delete? - postgresql

Question

I have a query running in Postgres 9.3.9 where I want to delete some records from a temp table based on using an EXISTS clause that matches a specific partial index condition I created. The following related query uses an Index Only Scan on this partial index (abbreviated as 'conditions' below):
EXPLAIN
SELECT l.id
FROM temp_table l
WHERE NOT EXISTS
(SELECT 1
FROM customers cx
WHERE cx.id = l.customer_id
AND ( conditions ));
QUERY PLAN
----------------------------------------------------------------------------------------------
Nested Loop Anti Join (cost=0.42..252440.38 rows=43549 width=4)
-> Seq Scan on temp_table l (cost=0.00..1277.98 rows=87098 width=8)
-> Index Only Scan using customers__bad on customers cx (cost=0.42..3.35 rows=1 width=4)
Index Cond: (id = l.customer_id)
(4 rows)
Here is the actual delete query SQL. This doesn't but I am convinced should use the same Index Only Scan as above, and I wonder if it's a bug in Postgres? Notice the higher cost:
DELETE
FROM temp_table l
WHERE EXISTS(SELECT 1
FROM cnu.customers cx
WHERE cx.id = l.customer_id
AND ( conditions ));
QUERY PLAN
------------------------------------------------------------------------------------------------
Delete on temp_table l (cost=0.42..495426.94 rows=43549 width=12)
-> Nested Loop Semi Join (cost=0.42..495426.94 rows=43549 width=12)
-> Seq Scan on temp_table l (cost=0.00..1277.98 rows=87098 width=10)
-> Index Scan using customers__bad on customers cx (cost=0.42..6.67 rows=1 width=10)
Index Cond: (id = l.customer_id)
(5 rows)
To show that it should be possible on delete to get the same plan, I had to do this, and it gave me the plan I wanted, and was twice as fast as the query above that uses an Index Scan instead of Index Only Scan:
WITH the_right_records AS
(SELECT l.id
FROM temp_table l
WHERE NOT EXISTS
(SELECT 1
FROM cnu.customers cx
WHERE cx.id = l.customer_id
AND ( conditions ))
DELETE FROM temp_table t
WHERE NOT EXISTS (SELECT 1
FROM the_right_records x
WHERE x.id = t.id);
QUERY PLAN
------------------------------------------------------------------------------------------------------
Delete on temp_table t (cost=253855.72..256902.88 rows=43549 width=34)
CTE the_right_records
-> Nested Loop Anti Join (cost=0.42..252440.38 rows=43549 width=4)
-> Seq Scan on temp_table l (cost=0.00..1277.98 rows=87098 width=8)
-> Index Only Scan using customers__bad on customers cx (cost=0.42..3.35 rows=1 width=4)
Index Cond: (id = l.customer_id)
-> Hash Anti Join (cost=1415.34..4462.50 rows=43549 width=34)
Hash Cond: (t.id = x.id)
-> Seq Scan on temp_table t (cost=0.00..1277.98 rows=87098 width=10)
-> Hash (cost=870.98..870.98 rows=43549 width=32)
-> CTE Scan on the_right_records x (cost=0.00..870.98 rows=43549 width=32)
(11 rows)
I've noticed this same behavior in other examples. So anyone have any ideas?