Error on data reading from the connection

[FabioG]

Hi all, I'm working with UniDac 5.0.1 and PostgreSQL version 9.1 server.
I just tried to lock with a select for update query some records (QUERY1).
Then I tried to do the same with a second connection (QUERY2) and instead of receiving a lock error I receive this exception:

Error on data reading from the connection

After the exception the UniConnection object is disconnected.
Is it normal?

Even if I close the second application with QUERY2, the postgres connection and the pending query stay resident on the postgres server until the first (QUERY1) app unlocks the records.

Is it possible to receive a different error on QUERY2 and possibly cancel this query if QUERY1 is locking same records?

Thank you.

[DemetrionQ]

Hello.

The "Error on data reading from the connection" error means that an error occurred on reading data from the socket, e.g., server connection break. In this case, connection closing will be a kind of standard behaviour.

If a record is already locked, when executing a query like

Code: Select all

SELECT * FROM table1 WHERE id = 1 FOR UPDATE;

client will be waiting for record unlock. If the connection breaks for some reason while waiting, you will get the "Error on data reading from the connection: ..." error.
In order for client not to wait for record unlock, but to get the "could not obtain lock on row ..." error immediately, a NOWAIT sql instruction should be added to the SELECT FOR UPDATE query, e.g.:

Code: Select all

SELECT * FROM table1 WHERE id = 1 FOR UPDATE NOWAIT;

If you work with an open dataset, it will be more convenient for you to lock a record using the TUniQuery.Lock method, which executes a similar SQL query (SELECT ... FOR UPDATE NOWAIT) for a current record. The detailed information about the TUniQuery.Lock method can be found at http://www.devart.com/unidac/docs/devar ... lock().htm