Re: pg_depend query

Thanks Tom,

It took me the better part of the day but I banged out a fully working
function.

Sim

"Tom Lane" <tgl@sss.pgh.pa.us> wrote in message
news:26493.1108581843@sss.pgh.pa.us...
> "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
>
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