Re: pg_depend query

"Sim Zacks" <sim@nospam.com> writes:
> I am trying to write a query on the pg_depend table and it is confusing the
> hell out of me. Hopefully someone could help.
> I would like to know which views depend on another view.

The dependencies are actually from the view's _RETURN rule to the
other view.  So for example:

regression=# create view v1 as select * from int4_tbl;
CREATE VIEW
regression=# create view v2 as select * from v1;
CREATE VIEW

The only dependency v2 has explicitly is one on its namespace:

regression=# select classid::regclass,objid,objsubid,
regression-# refclassid::regclass,refobjid,refobjsubid,deptype from pg_depend
regression-# where classid = 'pg_class'::regclass and objid = 'v2'::regclass;
 classid  | objid  | objsubid |  refclassid  | refobjid | refobjsubid | deptype

----------+--------+----------+--------------+----------+-------------+---------

 pg_class | 855926 |        0 | pg_namespace |     2200 |           0 | n
(1 row)

regression=# select nspname from pg_namespace where oid = 2200;
 nspname
---------
 public
(1 row)

But if we look for what *depends on* v2, we find:

egression=# select classid::regclass,objid,objsubid,
regression-# refclassid::regclass,refobjid,refobjsubid,deptype from pg_depend
regression-# where refclassid = 'pg_class'::regclass and refobjid = 'v2'::regclass;
  classid   | objid  | objsubid | refclassid | refobjid | refobjsubid | deptype
------------+--------+----------+------------+----------+-------------+---------
 pg_rewrite | 855928 |        0 | pg_class   |   855926 |           0 | n
 pg_rewrite | 855928 |        0 | pg_class   |   855926 |           0 | i
 pg_type    | 855927 |        0 | pg_class   |   855926 |           0 | i
(3 rows)

regression=# select rulename from pg_rewrite where oid = 855928;
 rulename
----------
 _RETURN
(1 row)

The second of these is an implicit dependency from the view's _RETURN rule
to the view.  If we now look for everything the _RETURN rule depends on:

regression=# select classid::regclass,objid,objsubid,
regression-# refclassid::regclass,refobjid,refobjsubid,deptype from pg_depend
regression-# where classid = 'pg_rewrite'::regclass and objid = 855928;
  classid   | objid  | objsubid | refclassid | refobjid | refobjsubid | deptype
------------+--------+----------+------------+----------+-------------+---------
 pg_rewrite | 855928 |        0 | pg_class   |   855926 |           0 | n
 pg_rewrite | 855928 |        0 | pg_class   |   855923 |           1 | n
 pg_rewrite | 855928 |        0 | pg_class   |   855926 |           0 | i
(3 rows)

or more legibly

regression=# select classid::regclass,objid,objsubid,
regression-# refclassid::regclass,refobjid::regclass,refobjsubid,deptype from pg_depend
regression-# where classid = 'pg_rewrite'::regclass and objid = 855928;
  classid   | objid  | objsubid | refclassid | refobjid | refobjsubid | deptype
------------+--------+----------+------------+----------+-------------+---------
 pg_rewrite | 855928 |        0 | pg_class   | v2       |           0 | n
 pg_rewrite | 855928 |        0 | pg_class   | v1       |           1 | n
 pg_rewrite | 855928 |        0 | pg_class   | v2       |           0 | i
(3 rows)

The second of these rows shows that the _RETURN rule depends on v1's
first column, and it's that dependency that you are looking for.

You can probably collapse this into one query with some sub-selects,
but I'll leave that exercise to the reader.

The important point here is that an implicit dependency is sort of a
two-way street, because it indicates that the dependent object (here the
_RETURN rule) is an integral part of the referenced object (here the v2
view), and therefore a DELETE CASCADE will cascade in *both* directions.
Dropping v2 should obviously drop its _RETURN rule (that's the normal
forward direction of cascaded drops).  But dropping something that the
_RETURN rule depends on forces dropping both the _RETURN rule and its
owning view (reverse direction).  You can see this happening if you look
at DROP's notices:

regression=# drop view v1;
NOTICE:  rule _RETURN on view v2 depends on view v1
NOTICE:  view v2 depends on rule _RETURN on view v2
ERROR:  cannot drop view v1 because other objects depend on it
HINT:  Use DROP ... CASCADE to drop the dependent objects too.
regression=#

The cascade went through the _RETURN rule, not directly to v2.

            regards, tom lane