Обсуждение: CREATE SYNONYM suggestions
I have a need for relation synonyms in PostgreSQL. I don't see it in
7.2.1 but the catalog seems to be able to support it more or less.
Here's what I intend to do:
1) Create a duplicate record in pg_class for the base table information
but with the relname set to the synonym name.
2) Duplicate the attribute information in pg_attribute for the base
table but with the attrelid set to the synonym oid.
(see test SQL below)
Is there anything fundamentally wrong with this approach? In particular,
could this concievably break anything. I do understand that it's not a
perfect approach since the attributes are not dynamic in so far as any
changes made to the base table. However, it does appear to provide a
superior solution than using a view with a full set of rules. That said,
is there a safe way of creating a "true" duplicate record in pg_class
(including the oid) so that a "true" synonym could be created?
Here's the testing I did:
insert into pg_class
select 'syn_test', reltype, relowner, relam, relfilenode, relpages,
reltuples, reltoastrelid, reltoastidxid,
relhasindex, relisshared, relkind, relnatts, relchecks, reltriggers,
relukeys, relfkeys,
relrefs, relhasoids, relhaspkey, relhasrules, relhassubclass, relacl
from pg_class where lower(relname) = lower('tbl_test')
;
insert into pg_attribute
select c2.oid, attname, atttypid, attstattarget, attlen, attnum,
attndims, attcacheoff, atttypmod, attbyval,
attstorage, attisset, attalign, attnotnull, atthasdef
from pg_class c1, pg_class c2, pg_attribute a1
where attrelid = c1.oid
and lower(c1.relname) = lower('tbl_test')
and lower(c2.relname) = lower('syn_test')
;
select * from tbl_test; (no problems)
select * from syn_test; (no problems)
delete from pg_attribute
where attrelid = (select oid from pg_class where lower(relname) =
lower('syn_test'))
;
delete from pg_class
where lower(relname) = lower('syn_test')
;
Thanks!
Marc L.
On Wed, 2002-07-24 at 02:22, Marc Lavergne wrote: > I have a need for relation synonyms in PostgreSQL. I don't see it in > 7.2.1 but the catalog seems to be able to support it more or less. > > Here's what I intend to do: > > 1) Create a duplicate record in pg_class for the base table information > but with the relname set to the synonym name. This will eventually cause a problem when the file oid changes and the old one gets removed. Cluster is one of those commands that will do that. Other than that, any table changes won't be propagated -- but you already mentioned that.
Marc Lavergne <mlavergne-pub@richlava.com> writes:
> I have a need for relation synonyms in PostgreSQL. I don't see it in
> 7.2.1 but the catalog seems to be able to support it more or less.
> Here's what I intend to do:
> 1) Create a duplicate record in pg_class for the base table information
> but with the relname set to the synonym name.
> 2) Duplicate the attribute information in pg_attribute for the base
> table but with the attrelid set to the synonym oid.
> Is there anything fundamentally wrong with this approach?
YES. You just broke relation locking (a lock by OID will only lock
one access path to the table). Any sort of ALTER seems quite
problematical as well; how will it know to update both sets of catalog
entries?
A view seems like a better idea, especially since you can do it without
any backend changes.
regards, tom lane
I thought that it might involve more than met the eye. I'm resisting the "view" approach since, like my bad kludge, it locks down the table definition and as a result doesn't provide a very effective synonym mechanism. I'm looking into the commands/view.c as a basis for introducing the concept of synonyms. Based on what I see, it looks like implementing it should be too terrible. Sadly, it looks a lot like this would require introducing a new relation type. I'll have to investigate and possibly submit the patch(es) later. The question is, since CREATE SYNONYM appears to be a SQL extension, is this something the group would want to incorporate? Tom Lane wrote: > Marc Lavergne <mlavergne-pub@richlava.com> writes: > >>I have a need for relation synonyms in PostgreSQL. I don't see it in >>7.2.1 but the catalog seems to be able to support it more or less. > > >>Here's what I intend to do: > > >>1) Create a duplicate record in pg_class for the base table information >>but with the relname set to the synonym name. > > >>2) Duplicate the attribute information in pg_attribute for the base >>table but with the attrelid set to the synonym oid. > > >>Is there anything fundamentally wrong with this approach? > > > YES. You just broke relation locking (a lock by OID will only lock > one access path to the table). Any sort of ALTER seems quite > problematical as well; how will it know to update both sets of catalog > entries? > > A view seems like a better idea, especially since you can do it without > any backend changes. > > regards, tom lane >
Marc Lavergne writes: > The question is, since CREATE SYNONYM appears to be a SQL extension, is > this something the group would want to incorporate? Well, you could start by explaining what exactly this concept is and what it would be useful for. I imagine that it is mostly equivalent to symlinks. If so, I can see a use for it, but it would probably have to be a separate type of object altogether, so you could also create synonyms/symlinks to functions, types, etc. -- Peter Eisentraut peter_e@gmx.net
Like you said, it's really just a symlink for db objects. If memory serves me right, synonyms can only refer to tables and views in Oracle. The most common use is for simplifying access to objects outside your schema (eg. create synonym TABLEX for JOHN.TABLEX) or for simplifying access to objects across database links (eg. create synonym TABLEX for TABLEX@DBY). Quick note: I know that PostgreSQL doesn't have the same concept of schemas, this is just an example. There are two types of synonyms, private and public. Public synonyms only exist within the current user's schema while public synonyms apply system wide to all users. From a PostgreSQL perspective, there is little *current* value that synonyms bring to the table, save for reducing an object name like THIS_IS_WAY_TOO_LONG to TOO_LONG. That said, it's still a very commonly used construct in other commercial DBMSs. I guess the place to implement this would be somewhere shortly after the parser has done it's thing, probably as part of the OID resolution. I should make this my standard disclaimer ;-) but again ... my goal with all this Oracle compatibility stuff is the SQL*Net compatibility listed in the TODO. Peter Eisentraut wrote: > Marc Lavergne writes: > > >>The question is, since CREATE SYNONYM appears to be a SQL extension, is >>this something the group would want to incorporate? > > > Well, you could start by explaining what exactly this concept is and what > it would be useful for. I imagine that it is mostly equivalent to > symlinks. If so, I can see a use for it, but it would probably have to be > a separate type of object altogether, so you could also create > synonyms/symlinks to functions, types, etc. >
On Fri, 2002-07-26 at 02:54, Marc Lavergne wrote: > Like you said, it's really just a symlink for db objects. If memory > serves me right, synonyms can only refer to tables and views in Oracle. > The most common use is for simplifying access to objects outside your > schema (eg. create synonym TABLEX for JOHN.TABLEX) or for simplifying > access to objects across database links (eg. create synonym TABLEX for > TABLEX@DBY). For quick answers on SYNONYM use in DB2 see http://members.aol.com/cronid/db2.htm (search for synonym) I found no SYNONYM in SQL99 but for a similar construct ALIAS, there is a reserved word in SQL99, but I could find no definition using it. ---------------- Hannu
On Fri, 26 Jul 2002 07:55:47 +1000, Marc Lavergne wrote: > Like you said, it's really just a symlink for db objects. If memory serves me > right, synonyms can only refer to tables and views in Oracle. In oracle (8i) you can create synonyms for tables, views, sequences, procedures, functions, packages, materialised views, java class objects and other synonyms.