Index ignored on pkid = curval('some_seq'), used with pkid = (selectcurval(''some_seq') )

On Postgresql 10 and 11

begin;
insert into itemshist(id,repdate,systemdate,done,username) VALUES(1454349,current_date,current_timestamp,'t','foo');

dynacom=# explain analyze update itemshist set reason='{foo,bar}' where pkid =
currval(('public.itemshist_pkid_seq'::text)::regclass);
                                                        QUERY PLAN

------------------------------------------------------------------------------------------------------------------------
  Update on itemshist  (cost=0.00..412082.16 rows=1 width=167) (actual time=15833.238..15833.238 rows=0 loops=1)
    ->  Seq Scan on itemshist  (cost=0.00..412082.16 rows=1 width=167) (actual time=14444.143..15833.210 rows=1
loops=1)
          Filter: (pkid = currval(('public.itemshist_pkid_seq'::text)::regclass))
          Rows Removed by Filter: 14470046
  Planning Time: 0.095 ms
  Trigger for constraint $1: time=0.240 calls=1
  Trigger itemshist_dbmirror_trig: time=3.101 calls=1
  Execution Time: 15836.636 ms
(8 rows)

-- but if I compare against select currval it uses the index:

dynacom=# explain analyze update itemshist set reason='{foo,bar}' where pkid = ( SELECT
currval(('public.itemshist_pkid_seq'::text)::regclass));
                                                             QUERY PLAN

----------------------------------------------------------------------------------------------------------------------------------
  Update on itemshist  (cost=0.45..8.47 rows=1 width=167) (actual time=0.048..0.048 rows=0 loops=1)
    InitPlan 1 (returns $0)
      ->  Result  (cost=0.00..0.01 rows=1 width=8) (actual time=0.006..0.006 rows=1 loops=1)
    ->  Index Scan using itemshist_pkey on itemshist (cost=0.43..8.45 rows=1 width=167) (actual time=0.028..0.033
rows=1loops=1)
 
          Index Cond: (pkid = $0)
  Planning Time: 0.095 ms
  Trigger for constraint $1: time=0.145 calls=1
  Trigger itemshist_dbmirror_trig: time=3.026 calls=1
  Execution Time: 3.273 ms
(9 rows)


-- 
Achilleas Mantzios
IT DEV Lead
IT DEPT
Dynacom Tankers Mgmt