Index scan for PK constraint validation

Hi all,

I was wondering why the index statistics usage were not reflecting the
index usage for primary key constraint validation ?

When we create a table with a primary key, PostgreSQL creates a Btree
index for the validation of this constraint : index is defined unique
(indisunique=t) and primary (indisprimary=t) from the pg_index and
pg_class catalogs.

Inserting data on the above created table and selecting based on the
primary key column will increment the idx_scan from
`pg_stat_user_indexes` view.
But if we insert again in this table with a conflicting id, the primary
key is being validated using the index as stated in our example :

ERROR: 23505: duplicate keyvalueviolates uniqueconstraint"testpkidx_pkey"
DETAIL: Key(id)=(1) already exists.
SCHEMANAME: public
TABLENAME: testpkidx
CONSTRAINTNAME: testpkidx_pkey
LOCATION: _bt_check_unique, nbtinsert.c:664

But checking again the `pg_stat_user_indexes` view, the idx_scan
attribute does not seem to be incremented.

Is the statistics of index usage for constraint validation being
computed elsewhere ?

Are we missing something ?

Thanks in advance.

Regards.
--
Philippe VIEGAS