How to wait during SELECT that pending INSERT commit?

Question

I'm using PostgreSQL 9.2 in a Windows environment. I'm in a 2PC (2 phase commit) environment using MSDTC.

I have a client application, that starts a transaction at the SERIALIZABLE isolation level, inserts a new row of data in a table for a specific foreign key value (there is an index on the column), and vote for completion of the transaction (The transaction is PREPARED). The transaction will be COMMITED by the Transaction Coordinator.

Immediatly after that, outside of a transaction, the same client requests all the rows for this same specific foreign key value.

Because there may be a delay before the previous transaction is really commited, the SELECT clause may return a previous snapshot of the data. In fact, it does happen sometimes, and this is problematic. Of course the application may be redesigned but until then, I'm looking for a lock solution. Advisory Lock ?

I already solved the problem while performing UPDATE on specific rows, then using SELECT...FOR SHARE, and it works well. The SELECT waits until the transaction commits and return old and new rows.

Now I'm trying to solve it for INSERT. SELECT...FOR SHARE does not block and return immediatley.

There is no concurrency issue here as only one client deals with a specific set of rows. I already know about MVCC.

Any help appreciated.

Answer 1

To wait for a not-yet-committed INSERT you'd need to take a predicate lock. There's limited predicate locking in PostgreSQL for the serializable support, but it's not exposed directly to the user.

Simple SERIALIZABLE isolation won't help you here, because SERIALIZABLE only requires that there be an order in which the transactions could've occurred to produce a consistent result. In your case this ordering is SELECT followed by INSERT.

The only option I can think of is to take an ACCESS EXCLUSIVE lock on the table before INSERTing. This will only get released at COMMIT PREPARED or ROLLBACK PREPARED time, and in the mean time any other queries will wait for the lock. You can enforce this via a BEFORE trigger to avoid the need to change the app. You'll probably get the odd deadlock and rollback if you do it that way, though, because INSERT will take a lower lock then you'll attempt lock promotion in the trigger. If possible it's better to run the LOCK TABLE ... IN ACCESS EXCLUSIVE MODE command before the INSERT.

As you've alluded to, this is mostly an application mis-design problem. Expecting to see not-yet-committed rows doesn't really make any sense.