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
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