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From "Kristian Waagan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-5367) Stale data retrieved when using new collation=TERRITORY_BASED:PRIMARY feature
Date Mon, 19 Sep 2011 17:53:08 GMT

    [ https://issues.apache.org/jira/browse/DERBY-5367?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13108026#comment-13108026
] 

Kristian Waagan commented on DERBY-5367:
----------------------------------------

Mike, what's your take on a fix along the lines of patch 4a?
I'd have to run the same sets of benchmarks on non-collated database, first but if those results
are looking okay I lean towards a commit.

I don't have a good sense of how often this code will be run, so it's hard for me to say if
we should just not optimize this at all. Judging by the performance numbers only, I would
definitely do the optimization.

I want to see a fix for this issue shortly, such that people affected can run off the head
of 10.8 with their own build if they want to do so (with the risks associated of course).

> Stale data retrieved when using new collation=TERRITORY_BASED:PRIMARY feature
> -----------------------------------------------------------------------------
>
>                 Key: DERBY-5367
>                 URL: https://issues.apache.org/jira/browse/DERBY-5367
>             Project: Derby
>          Issue Type: Bug
>          Components: Store
>    Affects Versions: 10.8.1.2, 10.9.0.0
>         Environment: Mac OS X, Windows
>            Reporter: Brett Wooldridge
>            Assignee: Kristian Waagan
>            Priority: Critical
>         Attachments: derby-5367-1a-update_row_fully.diff, derby-5367-1b-update_row_fully.diff,
derby-5367-1b-update_row_fully.stat, derby-5367-2a-minimal_fix.diff, derby-5367-3a-update_field_by_field_preview.diff,
derby-5367-4a-fix_with_optimization_improved.diff
>
>
> Our product recently upgraded to version 10.8.1.2 in order to take advantage of the new
'case-insensitive' mode offered by Derby in the form of the "collation=TERRITORY_BASED:PRIMARY"
connection parameter.
> Unfortunately, we have run into an issue whereby stale data appears to be retrieved from
an index, even though the data in the table itself has changed.
> You can see this issue in the IJ session below.  The database in question was created
using this Java parameter when invoking IJ:
> -Dij.database=jdbc:derby:test;create=true;collation=TERRITORY_BASED:PRIMARY
> Here is the IJ session:
> CONNECTION0* - 	jdbc:derby:test
> * = current connection
> ij> CREATE TABLE tag (
>     tag_id INTEGER GENERATED BY DEFAULT AS IDENTITY NOT NULL,
>     tag VARCHAR(255) NOT NULL,
>     CONSTRAINT tag_pk PRIMARY KEY (tag_id),
>     CONSTRAINT tag_tag_unique UNIQUE (tag)
> );
> 0 rows inserted/updated/deleted
> ij> -- first insert a value 'Test', note the upper-case 'T' in 'Test'
> ij> INSERT INTO tag (tag) VALUES ('Test');
> 1 row inserted/updated/deleted
> ij> SELECT * FROM tag;
> TAG_ID     |TAG                                                                     
                                                       
> --------------------------------------------------------------------------------------------------------------------------------------------
> 1          |Test                                                                    
                                                       
> 1 row selected
> ij> -- Now delete the row
> ij> DELETE FROM tag WHERE tag='Test';
> 1 row inserted/updated/deleted
> ij> -- You could run another SELECT here to verify it is gone, but it is.
> ij> -- Now insert a new value 'test', note the lower-case 't' in 'test'
> ij> INSERT INTO tag (tag) VALUES ('test');
> 1 row inserted/updated/deleted
> ij> -- Now verify that the table contains only the lower-case version: 'test'
> ij> SELECT * FROM tag;
> TAG_ID     |TAG                                                                     
                                                       
> --------------------------------------------------------------------------------------------------------------------------------------------
> 2          |test                                                                    
                                                       
> 1 row selected
> ij> -- Now, here is the bug.
> ij> SELECT tag FROM tag;
> TAG                                                                                 
                                           
> --------------------------------------------------------------------------------------------------------------------------------
> Test                                                                                
                                           
> 1 row selected
> ij> 
> Note in the last SELECT we specify the 'tag' column specifically.  When we 'SELECT *',
Derby performs a table-scan and the result is correct.  However, when we 'SELECT tag', Derby
appears to use the index created for the 'tag_tag_unique' unique constraint.  As an optimization
Derby, like many databases, will use values directly from the index in the case where the
index covers all requested columns.
> The bigger question is, why doesn't the DELETE action cause the entry in the tag_tag_unique
index to be deleted?  Is this a further optimization?  If so, it is imperative that the index
at least be updated when the new value is inserted.
> This is rather a severe bug for us that causes stale data to be returned.

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