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From "Mike Matrigali (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-6300) row locks incorrectly taken for rows that do not match SELECT predicate
Date Thu, 25 Jul 2013 00:37:48 GMT

    [ https://issues.apache.org/jira/browse/DERBY-6300?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13719102#comment-13719102

Mike Matrigali commented on DERBY-6300:

After some more research trying to figure out why non-qualified rows were locked, determined
that for the
plan being debugged no qualifier was being pushed into STORE by the SQL engine.  Store is
designed to 
not hold locks in REPEATABLE READ for non-qualified rows, but ONLY in the case where STORE
is the
one that determines that they are not qualifed.  Once the rows leave store, and in this case
determined by
an upper level that the row is not qualified, they are not unlocked in repeatable read.  

I have filed DERBY-6301, an improvement request to push down the IN-LIST to store, in this
case.  I think
if that were done, then repeatable read would happen as expected.

Will file an improvement JIRA to create and implement use of a store interface to allow language
to indicate
a row did not qualify.  This is likely a medium hard project, especially to insure that that
the rest of the 
system does not slow down to support this.
> row locks incorrectly taken for rows that do not match SELECT predicate
> -----------------------------------------------------------------------
>                 Key: DERBY-6300
>                 URL: https://issues.apache.org/jira/browse/DERBY-6300
>             Project: Derby
>          Issue Type: Bug
>    Affects Versions:,
>         Environment: Windows, Linux
>            Reporter: H Zhang
>         Attachments: derby.log, RowLocksIssue.java
> Derby seems to be taking S-locks on all the rows in a table after a SELECT query, even
when none of the rows match the query predicate. For example, after running a query like
> 	SELECT col1, col2 FROM table1 WHERE col1 IN (?, ?, ?...) WITH RS
> and the query returns 0 rows, we still see S-locks being taken on all rows in the table.
> This issue seems to be dependent on which exact query plan gets chosen to be executed,
as changing some combination of the following factors seems to avoid the issue:
> 1) The number of total rows in the table is small. In the test case, we're using 10 rows.
> 2) There is an explicitly created composite index on the table that covers all the rows.
> 3) The number of values in the IN clause of the SELECT query is sufficiently large.
> What plan the optimizer chooses seems to be a factor. For example, in our actual database,
we've found we need about 5 or 6 parameters in the IN clause to reproduce the issue. In the
attached test case, it seems the issue can be seen with 3 or more parameters.
> The attached test results in a database deadlock if the row locking issue occurs. It
basically does the following:
> a) Have a table with 10 rows. The values are basically A0, A1, ...
> b) Have a transaction selecting for values C0, C1, ...
> c) Have a 2nd transaction selecting for values D0, D1, ...
> d) Execute SQL deletes from both transactions
> The test fails in (d) with a deadlock because after (b) and (c), both transactions have
S-locks on all the rows in the table.
> We've tested on 10.8.3 and, and both seem to exhibit the issue.

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