as I could not find the reason in the source code, can someone tell me why the OID counter jumps by 3 between two create table statements?
postgres=# create table t1 ( a int ); CREATE TABLE postgres=# create table t2 ( a int ); CREATE TABLE postgres=# select oid,relname from pg_class where relname in ('t1','t2'); oid | relname -------+--------- 16453 | t1 16456 | t2 (2 rows)
These seems not to happen with other objects, e.g. namespaces:
postgres=# create schema a; CREATE SCHEMA postgres=# create schema b; CREATE SCHEMA postgres=# select oid,nspname from pg_namespace where nspname in ('a','b'); oid | nspname -------+--------- 16459 | a 16460 | b (2 rows)
... or indexes:
postgres=# select oid,relname from pg_class where relname in ('i1','i2'); oid | relname -------+--------- 16461 | i1 16462 | i2
When you create a table, it also creates two data types: tablename and _tablename. For example, for your table t1, you should have a t1 type and a _t1 type. Both have OIDs. On my cluster, your example gives me:
# select oid,relname from pg_class where relname in ('t1','t2'); ┌───────┬─────────┐ │ oid │ relname │ ├───────┼─────────┤ │ 24635 │ t1 │ │ 24638 │ t2 │ └───────┴─────────┘ (2 rows)
Time: 0.507 ms
# select oid, typname from pg_type where typname like '%t1' or typname like '%t2' and oid>24000 order by oid; ┌───────┬─────────┐ │ oid │ typname │ ├───────┼─────────┤ │ 24636 │ _t1 │ │ 24637 │ t1 │ │ 24639 │ _t2 │ │ 24640 │ t2 │ └───────┴─────────┘ (4 rows)
Time: 1.203 ms
The jump between t1 OID (24635) and t2 OID (24638) is the _t1 data type OID (24636) and the t1 data type OID (24637).