Thread: ACCESS EXCLUSIVE lock

Hi!

I have a problem with ACCESS EXCLUSIVE lock when I drop a reference in
transaction. I have 2 tables:
create table  a(id SERIAL primary key);
create table  b(id SERIAL primary key references a(id));

After that I have 2 processes: P1, P2
In P1:
begin;
ALTER TABLE b DROP CONSTRAINT  b_id_fkey;

In P2:
SELECT * FROM a;

And I'm waiting for the result, but I don't get until P1 finishes.
I know the DROP CONSTRAINT put an ACCESS EXCLUSIVE table LOCK into the
TABLE a, and the SELECT is stopped by this LOCK in P2.
Note: I cannot commit the P1 earlier, because it's a very long
transaction (more hours, data conversion transaction)
My question:  Why need this strict locking?

In my opinion there isn't exclusion between the DROP CONSTRAINT and the
SELECT.

Thanks for your suggestions!
Regards,
Antal Attila

On Wed, 2006-10-18 at 09:24, Atesz wrote:
> Hi!
>
> I have a problem with ACCESS EXCLUSIVE lock when I drop a reference in
> transaction. I have 2 tables:
> create table  a(id SERIAL primary key);
> create table  b(id SERIAL primary key references a(id));
>
> After that I have 2 processes: P1, P2
> In P1:
> begin;
> ALTER TABLE b DROP CONSTRAINT  b_id_fkey;
>
> In P2:
> SELECT * FROM a;
>
> And I'm waiting for the result, but I don't get until P1 finishes.
> I know the DROP CONSTRAINT put an ACCESS EXCLUSIVE table LOCK into the
> TABLE a, and the SELECT is stopped by this LOCK in P2.
> Note: I cannot commit the P1 earlier, because it's a very long
> transaction (more hours, data conversion transaction)
> My question:  Why need this strict locking?
>
> In my opinion there isn't exclusion between the DROP CONSTRAINT and the
> SELECT.

What if, a minute or two after the drop contraint, you issue a rollback?

Atesz <atesz@ritek.hu> writes:
> My question:  Why need this strict locking?
> In my opinion there isn't exclusion between the DROP CONSTRAINT and the
> SELECT.

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.

            regards, tom lane

Scott Marlowe wrote:
> What if, a minute or two after the drop contraint, you issue a rollback?
>
After the DROP CONSTRAINT I insert 4 million rekords into the TABLE b.
After the inserts I remake the dropped constraints, and commit the
transaction (P1). This solution is faster then the conventional method
without the constraint's trick.
In my work the table A is a dictionary table (key-value pairs) with
100-200 records, and the TABLE b has 20 columns with 10 references to
TABLE a. So my experience is that I have to drop constraints before the
4 million inserts and remake those after it.

If there is an error in my transaction (P1) and I have to rollback,
there isn't problem, because my inserts lost from TABLE b and the
dropped constraints may be rolled back. In my opinion there isn't
exclusion between  a dropped constraint (reference from b to a) and a
select on TABLE a. If I think well the dropped constraint have to seem
in other transation (for example: P2). And it doesn't have to seem in my
transaction, because it has already dropped.


Thanks your suggestions!
Regards,
Antal Attila

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