Query plan when using currval

Hi,

Can someone please help me understand why the first query doesn't use the
primary key index whereas the second query does use it:

bcaas=> explain analyze select id from log_statement where id =
currval('log_statement_id_seq');
QUERY PLAN

--------------------------------------------------------------------------------------------------------
Seq Scan on log_statement (cost=0.00..14.05 rows=1 width=8) (actual
time=0.027..0.027 rows=0 loops=1)
Filter: (id = currval('log_statement_id_seq'::regclass))
Rows Removed by Filter: 9
Planning time: 0.081 ms
Execution time: 0.048 ms
(5 rows)

bcaas=> explain analyze select id from log_statement where id = (select
currval('log_statement_id_seq'));
QUERY PLAN

---------------------------------------------------------------------------------------------------------------------------------------
Index Only Scan using log_statement_pkey on log_statement
(cost=0.16..8.18 rows=1 width=8) (actual time=0.017..0.017 rows=0 loops=1)
Index Cond: (id = $0)
Heap Fetches: 0
InitPlan 1 (returns $0)
-> Result (cost=0.00..0.01 rows=1 width=8) (actual time=0.003..0.004
rows=1 loops=1)
Planning time: 0.105 ms
Execution time: 0.045 ms
(7 rows)

(This is with pg 10.3)

Thanks,

Steve