Re: [HACKERS] lock related issues...
Chris Bowlby writes I'm looking for some details on how the locking system works in relation to transactions dealing with INSERTs and UPDATEs. The version of PostgreSQL is 7.3.2 p.152 of the 7.3.2 Users Guide, section 9.2.1 Read Committed Isolation Level applies to your situation as described A plpgsql function call is being made through the JDBC driver, auto-commits are off, and any commits or rollbacks are dependent on the results of the function. When more then one client evokes the function (again, through the JDBC driver), the first caller is able to gain a lock with out issue, via a SELECT ... FOR UPDATE.. clause. Any connections that are made during the select are obviously set in a wait queue. Once the first transaction has completed, then the next call in the wait queue is process, and so on. The issue that we are seeing is that if there is a update that takes place on a record, the results are available on any transactions that follow the initial update, regardless of whether they have been in a wait queue or not. However, if there are inserts that are mode during a transcation, those inserts are not becomming available if a transaction is already in motion (unlike the updates, which do show up). If the transaction is closed and a new one is reopened, after all of the inserts have been completed, then we can see them. Is this the standard behaviour associate to transactions? Does what it says on the tin. The manual doesn't explicitly draw attention to the situation you have recognized, but the described behaviour fits exactly what it says in the manual. The SELECT .. FOR UPDATE sees rows that were there when the transaction started, not when it eventually gets to read them, some time later. The lock prevents them from accessing those rows for some time, during which time other inserts are applied, which they cannot see. When they get the lock, they are able to access the rows they wanted to access, but because of this particular lock mode (read committed isolation level), you see the updated version of those rows (if they still match the WHERE clause). You can of course use the serializable isolation level, though this would cause your second and subsequent transactions to abort, allowing a retry. Use SET TRANSACTION ISOLATION LEVEL SERIALIZABLE as the first statement of the transaction, wherever that is. If you don't like these behaviours, you can make other design choices that prevent these situations from occurring. The locking mechanisms are designed to give various options of data protection/concurrency trade-offs. They aren't designed to provide general (or even that efficient) queuing mechanisms - it would be more appropriate to select a different form of queuing mechanism, probably within your Java - or just have a single connection do everybody's work for them. If you really must do this, lock the rows you wish to see earlier in the transaction using a stricter form of locking. An example of this might be to issue an explicit UPDATE using the same WHERE clause as you did for the SELECT..FOR UPDATE, though whether this was possible and desirable would require a wider view of the application before that advice is safe to take as-is. So, doesn't look like a bug to me, nor an awful hidden secret feature either. Best regards, Simon Riggs ---(end of broadcast)--- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faqs/FAQ.html
Re: [HACKERS] lock related issues...
Hi Simon, Thanks for the confirmation, I just wanted to make sure I was not going ape over it and getting confused. At 08:04 PM 1/28/04, Simon Riggs wrote: Chris Bowlby writes I'm looking for some details on how the locking system works in relation to transactions dealing with INSERTs and UPDATEs. The version of PostgreSQL is 7.3.2 p.152 of the 7.3.2 Users Guide, section 9.2.1 Read Committed Isolation Level applies to your situation as described A plpgsql function call is being made through the JDBC driver, auto-commits are off, and any commits or rollbacks are dependent on the results of the function. When more then one client evokes the function (again, through the JDBC driver), the first caller is able to gain a lock with out issue, via a SELECT ... FOR UPDATE.. clause. Any connections that are made during the select are obviously set in a wait queue. Once the first transaction has completed, then the next call in the wait queue is process, and so on. The issue that we are seeing is that if there is a update that takes place on a record, the results are available on any transactions that follow the initial update, regardless of whether they have been in a wait queue or not. However, if there are inserts that are mode during a transcation, those inserts are not becomming available if a transaction is already in motion (unlike the updates, which do show up). If the transaction is closed and a new one is reopened, after all of the inserts have been completed, then we can see them. Is this the standard behaviour associate to transactions? Does what it says on the tin. The manual doesn't explicitly draw attention to the situation you have recognized, but the described behaviour fits exactly what it says in the manual. The SELECT .. FOR UPDATE sees rows that were there when the transaction started, not when it eventually gets to read them, some time later. The lock prevents them from accessing those rows for some time, during which time other inserts are applied, which they cannot see. When they get the lock, they are able to access the rows they wanted to access, but because of this particular lock mode (read committed isolation level), you see the updated version of those rows (if they still match the WHERE clause). You can of course use the serializable isolation level, though this would cause your second and subsequent transactions to abort, allowing a retry. Use SET TRANSACTION ISOLATION LEVEL SERIALIZABLE as the first statement of the transaction, wherever that is. If you don't like these behaviours, you can make other design choices that prevent these situations from occurring. The locking mechanisms are designed to give various options of data protection/concurrency trade-offs. They aren't designed to provide general (or even that efficient) queuing mechanisms - it would be more appropriate to select a different form of queuing mechanism, probably within your Java - or just have a single connection do everybody's work for them. If you really must do this, lock the rows you wish to see earlier in the transaction using a stricter form of locking. An example of this might be to issue an explicit UPDATE using the same WHERE clause as you did for the SELECT..FOR UPDATE, though whether this was possible and desirable would require a wider view of the application before that advice is safe to take as-is. So, doesn't look like a bug to me, nor an awful hidden secret feature either. Best regards, Simon Riggs ---(end of broadcast)--- TIP 2: you can get off all lists at once with the unregister command (send unregister YourEmailAddressHere to [EMAIL PROTECTED])
Re: [HACKERS] lock related issues...
Chris Bowlby wrote: Hi Simon, Thanks for the confirmation, I just wanted to make sure I was not going ape over it and getting confused. At 08:04 PM 1/28/04, Simon Riggs wrote: Chris Bowlby writes I'm looking for some details on how the locking system works in relation to transactions dealing with INSERTs and UPDATEs. The version of PostgreSQL is 7.3.2 p.152 of the 7.3.2 Users Guide, section 9.2.1 Read Committed Isolation Level applies to your situation as described A great description of concurrency issues is Tom Lane's O'Reilly presentation. After installing PostgreSQL, a message should be output to read it: http://conferences.oreillynet.com/presentations/os2002/lane_tom.tar.gz Mike Mascari ---(end of broadcast)--- TIP 4: Don't 'kill -9' the postmaster
