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From "Kathey Marsden (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-4477) Selecting / projecting a column whose value is represented by a stream more than once fails
Date Tue, 13 Jul 2010 22:41:51 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4477?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12888064#action_12888064

Kathey Marsden commented on DERBY-4477:

Myrna pointed out that back porting the change introduced the following javadoc warnings.

	  [javadoc] D:\svnnightlies\v10_5\src\opensource\java\testing\org\apache\derbyTesting\functionTests\tests\jdbcapi\BLOBTest.java:398:
warning - Tag @see: can't find testDerby4477_3645_3646_Repro_lowmem in org.apache.derbyTesting.functionTests.tests.memory.BlobMemTest
		  [javadoc] D:\svnnightlies\v10_5\src\opensource\java\testing\org\apache\derbyTesting\functionTests\tests\jdbcapi\BLOBTest.java:398:
warning - Tag @see: can't find testDerby4477_3645_3646_Repro_lowmem_clob in org.apache.derbyTesting.functionTests.tests.memory.ClobMemTest

Based on Kristian's earlier comment
I am unclear on whether this means I should also backport revison 908563 along with the lowmem
tests and hopefully in doing so resolve the difference in trunk and 10.5 or just fix up the
javadoc not to refer to these methods which are not currently in 10.5.

> Selecting / projecting a column whose value is represented by a stream more than once
> -------------------------------------------------------------------------------------------
>                 Key: DERBY-4477
>                 URL: https://issues.apache.org/jira/browse/DERBY-4477
>             Project: Derby
>          Issue Type: Bug
>          Components: SQL, Store
>    Affects Versions:,,
>            Reporter: Kristian Waagan
>            Assignee: Dag H. Wanvik
>             Fix For:,
>         Attachments: derby-4477-0a-prototype.diff, derby-4477-lowmem-2.diff, derby-4477-lowmem-2.stat,
derby-4477-lowmem-followup.diff, derby-4477-lowmem-followup.stat, derby-4477-lowmem.diff,
derby-4477-lowmem.stat, derby-4477-partial-2.diff, derby-4477-partial-2.stat, derby-4477-partial.diff,
derby-4477-partial.stat, derby-4477-useCloning.diff, derby-4477-useCloning.stat
> Selecting / projecting a column whose value is represented as a stream more than once
crashes Derby, i.e.:
> ResultSet rs = stmt.executeQuery("SELECT clobValue AS clobOne, clobValue AS clobTwo FROM
> rs.getString(1);
> rs.getString(2);
> After having looked at the class of bugs having to do with reuse of stream data types,
I now have a possible fix. It fixes DERBY-3645, DERBY-3646 and DERBY-2349 (there may be more
> The core of the fix is cloning certain DVDs being selected/projected in multiple columns.
There are two types of cloning:
>  A) materializing clone
>  B) stream clone
> (A) can be implemented already, (B) requires code to clone a stream without materializing
it. Note that the streams I'm talking about are streams originating from the store.
> Testing revealed the following:
>  - the cost of the checks performed to figure out if cloning is required seems acceptable
>  - in some cases (A) has better performance than (B) because the raw data only has to
be decoded once
>  - stream clones are preferred when the data value is above a certain size for several
>     * avoids potential out-of-memory errors (and in case of a server environment, it
lowers the memory pressure)
>     * avoids decoding the whole value if the JDBC streaming APIs are used to access only
parts of the value
>     * avoids decoding overall in cases where the value isn't accessed by the client /
>        (this statement conflicts with the performance observation above)
> We don't always know the size of a value, and since the fix code deals with all kinds
of data types, it is slightly more costly to try to obtain the size.
> What do people think about the following goal statement?
> Goals:
> ----- Phase 1
>  1) No crashes or wrong results due to stream reuse when executing duplicate column selections
(minus goal 4)
>  2) Minimal performance degradation for non-duplicate column selections
>  3) Only a minor performance degradation for duplicate [[LONG] VAR]CHAR [FOR BIT DATA]
column selections
> ----- Phase 2
>  4) No out-of-memory exceptions during execution of duplicate column selections of BLOB/CLOB
>  5) Optimize BLOB/CLOB cloning
> I think phase 1 can proceed by reviewing and discussing the prototype patch. Phase 2
requires more discussion and work (see DERBY-3650).
> A note about the bug behavior facts:
> Since this issue is the underlying cause for several other reported issues, I have decided
to be liberal when setting the bug behavior facts. Depending on where the duplicate column
selection is used, it can cause both crashes, wrong results and data corruption.

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