On Tue, Jul 10, 2012 at 2:40 AM, Amit Kapila amit.kap...@huawei.com wrote:
Today I learnt [1,2,3] that the feature ALTER TABLE .. ADD CONSTRAINT
... USING INDEX we added back in the day is not so useful in the field. **
**
Having to drop foreign key constraints before this command, and
recreate them afterwards makes this command useless to most database
setups. I feel sorry
that no one brought this up when we were implementing the feature;
maybe we could've done something about it right then.
** **
Will it impact user such that it will block its operation or something
similar or it is a usability issue?
Yes, it will have to take an exclusive lock on the index, and possibly the
table too, but the operation should be quick to be even noticeable in low
load conditions.
However, if the x-lock is waiting for some other long running query to
finish, then lock queuing logic in Postgres will make new queries to wait
for this x-lock to be taken and released before any new query can begin
processing. This is my recollection of the logic from an old conversation,
others can weigh in to confirm.
All we need to do is allow swapping of pg_class.relfilenode of two
indexes. This will let the dependency entries stand as they are and allow
us to drop the
bloated primary key index structure without having to rebuild the
foreign key constraints.
** **
I have noticed is that currently Oid and pg_class.relfilenode are same for
user created tables and indexes. But after your implementation that will
not remain same, I am not sure whether it can impact any other path of
code.
They start off as same, but some operations, like REINDEX, changes the
relfilenode; that's the purpose of relfilenode: to map the oid to a
filename on disk.
As for the syntactical sugar, this can be added to either ALTER TABLE or
to ALTER INDEX. Although under no normal circumstances one would need to
use ALTER INDEX to swap two indexes' relfilenode (because one can easily
create a duplicate index and drop/rename-in-place the old one), I think it
would make more sense here since it is just an operation on two indexes
and has nothing to do with the constraints, apart from the fact that we
want to use this feature to
meddle with the constraints.
Syntax options:
ALTER TABLE tbl REPLACE [CONSTRAINT constr] {PRIMARY KEY | UNIQUE}
USING INDEX new_index;
ALTER INDEX ind REPLACE WITH new_index;
After this new syntax there will be 2 ways for users to do the replacement
of index, won’t it confuse users for which syntax to use?
Yes, I forgot to mention this in the original post. This feature will be a
superset of the feature we introduced in ALTER TABLE. I don't see a way
around that, except for slowly deprecating the older feature.
--
Gurjeet Singh
EnterpriseDB Corporation
The Enterprise PostgreSQL Company
