Re: Reducing some DDL Locks to ShareLock

We can't do partial indexes on system tables. I forget exactly why nut
if you search for relevant comments it should pop up.

greg

On 7 Oct 2008, at 07:38 PM, Simon Riggs <simon(at)2ndQuadrant(dot)com> wrote:

>
> On Tue, 2008-10-07 at 11:46 -0400, Tom Lane wrote:
>> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
>>> On Tue, 2008-10-07 at 10:35 -0400, Tom Lane wrote:
>>>> I wonder whether this could be helped if we refactored
>>>> pg_constraint.
>>
>>> Sounds better. Doesn't make much sense as it is now.
>>
>> I looked at the code a bit, and it seems the only place where the
>> current design makes any sense is in ChooseConstraintName, which
>> explains itself thusly:
>>
>> * Select a nonconflicting name for a new constraint.
>> *
>> * The objective here is to choose a name that is unique within the
>> * specified namespace. Postgres does not require this, but the SQL
>> * spec does, and some apps depend on it. Therefore we avoid choosing
>> * default names that so conflict.
>> *
>> * Note: it is theoretically possible to get a collision anyway, if
>> someone
>> * else chooses the same name concurrently. This is fairly unlikely
>> to be
>> * a problem in practice, especially if one is holding an exclusive
>> lock on
>> * the relation identified by name1.
>>
>> (The last bit of the comment falls flat when you consider constraints
>> on domains...)
>>
>> Note that this policy is used for system-selected constraint names;
>> it's not enforced against user-selected names. We do attempt (in
>> ConstraintNameIsUsed) to reject duplicate user-selected constraint
>> names
>> *on the same object*, but that test is not bulletproof against
>> concurrent additions. The refactoring I suggested would make for
>> bulletproof enforcement via the unique indexes.
>>
>> To preserve the same behavior for system-selected constraint names
>> with
>> the new design, we'd still need to store namespace OIDs in the two
>> new
>> tables (I had been thinking those columns would go away), and still
>> have
>> nonunique indexes on (conname, connamespace), and probe both of the
>> new
>> catalogs via these indexes to look for a match to a proposed
>> constraint
>> name. So that's a bit of a PITA but certainly doable. Again, it's
>> not
>> bulletproof against concurrent insertions, but the existing code is
>> not
>> either.
>
> How about we put a partial unique index on instead?
>
> Dunno if its possible, but the above begins to sound too much froth
> for
> such a small error.
>
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> Simon Riggs www.2ndQuadrant.com
> PostgreSQL Training, Services and Support
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