Re: Problem JDBC, AutoCommit ON and SELECT FOR UPDATE

Philippe EMERIAUD wrote on 06.03.2015 17:27:
> Hi all,
>
> We have an application based on DB2 database, We are testing this
> same application on PostgreSQL database. By default we are in
> autocommit on mode.
>
> On DB2 (and Oracle), the query SELECT FOR UPDATE locks the row until
> the resultset is closed. On PostgreSQL database (all versions) this
> same query never locks (because we are in autocommit on mode). But
> this is a bad implementation of JDBC specification : "If a connection
> is in auto-commit mode, then all its SQL statements will be executed
> and committed as individual transactions.[...] The commit occurs when
> the statement completes or the next execute occurs, whichever comes
> first. In the case of statements returning a ResultSet object, the
> statement completes when the last row of the ResultSet object has
> been retrieved or the ResultSet object has been closed."

Maybe it's because the Postgres JDBC driver first reads the whole result into memory, which means
as soon as Statement.executeQuery() is finished, there is no cursor open any longer on the
server side.

Oracle (dont't know about DB2) will keep the cursor open until the last row has been processed.

Unfortunately, you can only switch to cursor-based retrieval when turning off autocommit:

https://jdbc.postgresql.org/documentation/94/query.html#query-with-cursor

> Is it possible to lock row in autocommit on mode on PostgreSQL
> database ?

In my eyes locking and autocommit don't really fit together.
What's the point in locking something if at the end of the statement (=control is returned to the caller)
the lock will be released anyway.

I'm surprised this works in Oracle actually.

Thomas