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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-3769) Make LOBStoredProcedure on the server side smarter about the read buffer size
Date Thu, 10 Jul 2008 12:20:32 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3769?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12612455#action_12612455

Knut Anders Hatlen commented on DERBY-3769:

I think it is correct to fix this on the server side only. If we want the fix on the client
side, we need to give the client knowledge about maximum sizes for different server versions
(not a big issue right now, since the max varchar/varbinary size hasn't changed) which sounds
like unnecessary complexity if it only fixes performance issues with the combination of old
server and new client.

I think the fix looks good. I checked the code in CallableLocatorProcedures, and it seems
to correctly request more data if the string is truncated, so the fix should be fine. You
may consider the following cosmetic changes:

1) Instead of using Limits.DB2_VARCHAR_MAXWIDTH, we could have a dedicated constant (CLOBGETSUBSTRING_RETURN_LENGTH,
or something) that we used both when we declared the stored procedures (in DataDictionaryImpl)
and to truncate the return value in the procedures. This way it is easier to keep them consistent
if we at some point change the definition of the procedures.

2) The javadoc comments have paragraphs marked as implementation notes which say that the
length may be shorter than specified. I don't think these aren't actually implementation notes,
but crucial points of the API that anyone programming against these procedures should be aware

3) The comments say that the length may be shorter because of client/server communication.
I believe it is more correct to say that the length may be shorter than specified because
the stored procedure returns a VARCHAR (or VARBINARY) and can therefore not return strings
whose length exceed the max length for the data type.

4) Typo in javadoc for BLOBGETBYTES: smallar -> smaller

> Make LOBStoredProcedure on the server side smarter about the read buffer size
> -----------------------------------------------------------------------------
>                 Key: DERBY-3769
>                 URL: https://issues.apache.org/jira/browse/DERBY-3769
>             Project: Derby
>          Issue Type: Improvement
>          Components: Network Server
>    Affects Versions:,,
>            Reporter: Kristian Waagan
>            Assignee: Kristian Waagan
>         Attachments: derby-3769-1a-buffer_size_adjustment.diff
> Derby has a max length for VARBINARY and VARCHAR, which is 32'672 bytes or characters
> When working with LOBs represented by locators, using a read buffer larger than the max
value causes the server to process far more data than necessary.
> Say the read buffer is 33'000 bytes, and these bytes are requested by the client. This
requests ends up in LOBStoredProcedure.BLOBGETBYTES.
> Assume the stream position is 64'000, and this is where we want to read from. The following
>  a) BLOBGETBYTES instructs EmbedBlob to read 33'000 bytes, advancing the stream position
to 97'000.
>  b) Derby fetches/receives the 33'000 bytes, but can only send 32'672. The rest of the
data (328 bytes) is discarded.
>  c) The client receives the 32'672 bytes, recalculates the position and length arguments
and sends another request.
>  d) BLOBGETBYTES(locator, 96672, 328) is executed. EmbedBlob detects that the stream
position has advanced too far, so it resets the stream to position zero and skips/reads until
position 96'672 has been reached.
>  e) The remaining 328 bytes are sent to the client.
> This issue deals with points b) and d), by avoiding the need to reset the stream.
> Points a) and e) are also problematic if a large number of bytes are going to be read,
say hundreds of megabytes, but that's another issue.
> It is unfortunate that using 32 K (32 * 1024) as the buffer size is almost the worst
case; 32'768 - 32'672 = 96 bytes.

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