Tom Lane wrote:
This isn't going to be changed, because the likely direction of future
development is that the planner will start making use of constraints
even for SELECT queries. This means that a DROP CONSTRAINT operation
could invalidate the plan of a SELECT query, so the locking will be
essential.
Hi!
I also think the constraints can increase performance of queries, if the
planner can use them. It will be a great feature in the future! But I
have more questions about the coherency between a constraint and a
transaction. Can a constraint live in differenet isolation levels? If I
drop a constraint in a transaction (T1), it doesn't seem after the drop
operation in T1. But it should seem in another transaction (T2) in line
with T1 (if T2 is started between T1's begin and commit!). If T1 start
after T1's commit, our constraint doesn't have to seem in T2, so the
planner cannot use it. If I think well, these predicates means the
constraint follows its isolation level of the transaction.
How does it works in the current release?
If the constraints adapt its transaction why could it invalidate the
plan of a SELECT query? A SELECT could use a given constraint, if it's
dropped without comitting or exists when the SELECT or the tansaction of
the SELECT starts. I know we have to examine which rows can affect the
result of the SELECT. The main question in this case is that: A wrong
row (which break the dropped constraint) can affect the result of the
SELECT? In my opininon there isn't wrong rows. Do you know such special
case when it can happen? So some wrong rows can seem in the SELECT?
I know my original problem is not too common, but the parallel
performance of the PostgreSQL is very important in multiprocessor
environment. I see, you follow this direction! So you make better
locking conditions in 8.2 in more cases. Generally the drop constraints
are running in itself or in short transactions.
We have an optimalization trick when we have to insert more million rows
into a table in same transaction. Before inserting them we drop the
foreign key constraints after the begin of the transaction, and remake
tem after insertations. This method is faster then the conventional
solution. These trasactions are longer (5-40 minutes on a SunFireV40z).
I read the TODO list and I found more features about deferrability.
Would you like to implement the deferrable foreign key constraints? If
you want, in my opinion my posings will thouch it.
Thank you in anticipation!
Regards,
Antal Attila
