[jira] [Commented] (DERBY-6300) row locks incorrectly taken for rows that do not match SELECT predicate

["Mike Matrigali (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/DERBY-6300?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13716619#comment-13716619 ] 

Mike Matrigali commented on DERBY-6300:
---------------------------------------

have not looked at test case yet, but likely this issue is more about query plans than locking.  So posting
the query plan of your test case will help the community comment on your issue.  This will then include
transaction isolation levels being used and indexes vs. table scans.  

>From your description it seems like you are pushing the edge where derby decides to use IN-list optimization
or not.  
                
> 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: 10.8.3.0, 10.10.1.1
>         Environment: Windows, Linux
>            Reporter: H Zhang
>         Attachments: 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 10.10.1.1, and both seem to exhibit the issue.

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