[Sqlalchemy-tickets] Issue #4160: Add postgresl.regclass type for casting tablenames to oids (zzzeek/sqlalchemy)

[Sebastian B. <iss...@bi...>]

New issue 4160: Add postgresl.regclass type for casting tablenames to oids
https://bitbucket.org/zzzeek/sqlalchemy/issues/4160/add-postgreslregclass-type-for-casting

Sebastian Bank:

Postgresql provides the `regclass` type to [simplify system catalog queries](https://www.postgresql.org/docs/10/static/datatype-oid.html) (reflection):

> The regclass input converter handles the table lookup according to the schema path setting, and so it does the “right thing” automatically. Similarly, casting a table's OID to regclass is handy for symbolic display of a numeric OID.

 I think it would be nice (and not too hard) to provide this in `sqlalchemy`. Maybe like this:

```python
In [1]: import sqlalchemy as sa
   ...: 
   ...: engine = sa.create_engine('postgresql://postgres@')
   ...: 
   ...: class regclass(sa.types.UserDefinedType):
   ...:     def get_col_spec(self):
   ...:         return 'regclass'
   ...: 
   ...: pga = sa.table('pg_attribute',
   ...:                *map(sa.column, ['attrelid', 'attname', 'attnotnull']))
   ...: 
   ...: query = pga.select()\
   ...:    .where(pga.c.attrelid == sa.cast('pg_class', regclass))\
   ...:    .where(~pga.c.attnotnull)
   ...: 
   ...: engine.execute(query).fetchall()
Out[1]: [(1259, 'relacl', False), (1259, 'reloptions', False)]

In [2]: engine.scalar(sa.select([sa.cast(1259, regclass)]))
Out[2]: 'pg_class'
```