Details
Critical Description
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.