QUERY PLAN                                             
---------------------------------------------------------------------------------------------------
 Index Scan using pg_constraint_conrelid_index on pg_constraint  (cost=0.00..8.27 rows=1 width=64)
   Index Cond: (conrelid = 54538::oid)
(2 rows)

SELECT conname FROM pg_constraint WHERE conrelid = 'clstr_tst'::regclass;
    conname     
----------------
 clstr_tst_con
 clstr_tst_pkey
(2 rows)

SET enable_indexscan = 0;
EXPLAIN SELECT conname FROM pg_constraint WHERE conrelid = 'clstr_tst'::regclass;
                                        QUERY PLAN                                         
-------------------------------------------------------------------------------------------
 Bitmap Heap Scan on pg_constraint  (cost=4.26..8.27 rows=1 width=64)
   Recheck Cond: (conrelid = 54538::oid)
   ->  Bitmap Index Scan on pg_constraint_conrelid_index  (cost=0.00..4.26 rows=1 width=0)
         Index Cond: (conrelid = 54538::oid)
(4 rows)

SELECT conname FROM pg_constraint WHERE conrelid = 'clstr_tst'::regclass;
    conname     
----------------
 clstr_tst_pkey
 clstr_tst_con
(2 rows)

SET enable_bitmapscan = 0;
EXPLAIN SELECT conname FROM pg_constraint WHERE conrelid = 'clstr_tst'::regclass;
                          QUERY PLAN                           
---------------------------------------------------------------
 Seq Scan on pg_constraint  (cost=0.00..27.15 rows=1 width=64)
   Filter: (conrelid = 54538::oid)
(2 rows)

SELECT conname FROM pg_constraint WHERE conrelid = 'clstr_tst'::regclass;
    conname     
----------------
 clstr_tst_pkey
 clstr_tst_con
(2 rows)

SET enable_indexscan = 1;
SET enable_bitmapscan = 1;
SELECT count(*) FROM pg_constraint;
 count 
-------
    16
(1 row)