[jira] Commented: (DERBY-4477) Selecting / projecting a column whose value is represented by a stream more than once fails

["Dag H. Wanvik (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/DERBY-4477?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12803511#action_12803511 ] 

Dag H. Wanvik commented on DERBY-4477:
--------------------------------------

I notice that test repro for DERBY-3646 still fails with my version of the patch. The derby-4477-0a-prototype patch makes that repro pass, but only because the limit for materialization is set low: increase it from 32K to 64K , and that repro fails with that patch as well, so there seems to be more to be done here? Or maybe I misunderstood something of the prototype patch... In my version I marked for cloning all occurences which had duplicates in the RCL (not just occurence 2..n as the prototype patch does), but that does not seem to make a difference..

> Selecting / projecting a column whose value is represented by a stream more than once fails
> -------------------------------------------------------------------------------------------
>
>                 Key: DERBY-4477
>                 URL: https://issues.apache.org/jira/browse/DERBY-4477
>             Project: Derby
>          Issue Type: Bug
>          Components: Store
>    Affects Versions: 10.3.3.0, 10.4.2.0, 10.5.3.0
>            Reporter: Kristian Waagan
>            Assignee: Kristian Waagan
>         Attachments: derby-4477-0a-prototype.diff, derby-4477-partial.diff, derby-4477-partial.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 mytable");
> 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 Jiras).
> 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 (negligible?)
>  - 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 reasons:
>     * 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 / user
>        (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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