db-derby-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Mike Matrigali <mikem_...@sbcglobal.net>
Subject Re: SELECT ... FOR UPDATE not working?
Date Thu, 18 Aug 2005 21:49:23 GMT
In order to understand and compare across multiple db's there are 2
important things we need to know:  transaction isolation and autocommit
behavior.

The default in derby is read committed isolation level, and
autocommit=true.  I don't know what the defaults are for the other
databases.  To be safe when comparing I suggest any test explicitly
set both as part of the test.  From your description it sounds like
you have autocommit=false, but it is good to be explicit.

Does your test do any next() calls on the result set?  Derby almost
always streams rows to the application, so locking for all rows
affected by a query is only guaranteed if the application actually
does a next() through all rows.  Because your table does not have
any indexes as described, derby will use row locking for repeatable
read, and read committed, but will use table locking for serializable.

Derby in read committed will release locks at end of statement for
any row which is actually not updated as part of select for update,
in read committed isolation level.
During processing of the query it actually gets update locks which
are either upgraded to exclusive and held to end of transaction or
released if the row is not updated.   If your application needs
to hold locks until end transaction on selects it must use either
repeatable read or serializable isolation.

Did you see the described behavior using repeatable read and doing
next() calls in both the first and second select?

Kristian Waagan wrote:

> Hello,
> 
> I stumbled across a possible bug with the SELECT ... FOR UPDATE clause.
> I have found several sources of information regarding this statement,
> with partly conflicting contents.
> 
> First of all, I wrote a simple JDBC test application making use of the
> statement. I ran this on Derby and two other database systems. Derby did
> not behave as the two others (more on this later).
> 
> Second, the reference manual states that the statement is supported, and
> that it must be used to obtain updateable resultsets.
> 
> Third, the JIRA issue 231
> (http://issues.apache.org/jira/browse/DERBY-231) is regarding support
> for SELECT ... FOR UPDATE. It is unresolved and unassigned. Is this a
> stale issue?
> 
> The JDBC application I wrote consisted of two threads accessing a single
> table: locktesttbl (ID INT, VALUE INT). I inserted 5 rows
> (1,1),(2,2),...,(5,5). The first thread executes 'SELECT * FROM
> locktesttbl WHERE ID=1 FOR UPDATE', waits 10 seconds, closes the
> resultset, executes 'SELECT * FROM locktesttbl WHERE ID=1', closes
> resultset and commits. The second thread, which is started 2 seconds
> after the first one, executes 'UPDATE locktesttbl SET VALUE=100 WHERE
> ID=1' then commits.
> 
> The only time the selected VALUE field in the first thread was equal at
> the beginning and the end of the transcation, was when the transcation
> isolation level was set to SERIALIZABLE. At all other levels, VALUE was
> 100 at the end of the transaction (before commit). When I did this with
> the two other systems (MySQL and PostgreSQL), VALUE was always 1 within
> the transaction. This suggests SELECT .. FOR UPDATE is broken in Derby,
> and that the single instance of correct behavior seen is due to the
> transaction isolation level alone. I have not looked into the source
> code on this.
> 
> Does anyone have any comments on this?
> 
> I will add a JIRA bug issue under category SQL for this one in a few
> days (awaiting comments).
> 
> 
> -- 
> Kristian
> 
> 

Mime
View raw message