[
https://issues.apache.org/jira/browse/TRAFODION-2408?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16705130#comment-16705130
]
ASF GitHub Bot commented on TRAFODION-2408:
-------------------------------------------
GitHub user sandhyasun opened a pull request:
https://github.com/apache/trafodion/pull/1754
[TRAFODION-2408] Remove code that empties the LOB data file when creating
an empty_blob.
Remove code that renames the lob data file when an empty_blob is created.
When allocating the descriptor for a new empty_blob entry, the lob data
file was being renamed. That is incorrect since the lob data file can contain
data from other lob entries in the column. (There is one single lob data file
for each LOB column and it can hold several blob entries in it). So remove
the code that does this and simply update the descriptor files to have an
entry which keeps the LOB offset and leave the LOB length as 0. This way we
know the LOB is empty . If data is appended to it, the data will go into a new
offset within that data file.
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/sandhyasun/trafodion traf_call_lib
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/trafodion/pull/1754.patch
To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:
This closes #1754
----
commit fb0ab72b30124418eba9473df5ac24743ffbc204
Author: Sandhya Sundaresan <sandhya@...>
Date: 2018-11-30T18:26:31Z
Remove code that empties the LOB data file when creating an empty_blob.
----
> Support for empty_blob() and update syntax using lobhandle
> ----------------------------------------------------------
>
> Key: TRAFODION-2408
> URL: https://issues.apache.org/jira/browse/TRAFODION-2408
> Project: Apache Trafodion
> Issue Type: Sub-task
> Components: sql-exe
> Reporter: Sandhya Sundaresan
> Assignee: Sandhya Sundaresan
> Priority: Major
> Fix For: 2.1-incubating
>
>
> This subtask in SQL is to help JDBC implementation for LOBs. They require the
> eimplementation of 2 interfaces
> .EMPTY_BLOB() , EMPTY_CLOB()
> update done directly to a lobhandle . This update syntax is different form
> the regular updates to lob columns using position.
> Assume a lob table of type :
> Create table tlob(c1 int , c2 blob, primary key (c1));
> Insert :
> Insert into tlob values (1, empty_blob());
> Insert into tlob values (1,empty_clob());
>
>
> This will create a lob handle . It will also create an empty descriptor
> handle entry and 1 chunk in the descriptor chunks table that is of size 0.
>
> The “type” of the lob handle will be the same as the underlying lob type of
> the column.
>
> Eg if we had created the table to hold external lobs
> Create table tlob(c1 int , c2 blob storage ‘external’ , primary key
> (c1));
> Insert into tlob values (1, empty_blob());
>
> The lob handle for this tlobext will contain the type ‘8’ instead of ‘2’ (as
> is the case for tlob.)
>
> But looking at the lob handle you will not be able to say if it’s empty or
> not. But if lob data is selected out it will be empty.
>
> Update:
> Assume tlob has some lob data in it.
>
> Update tlob set c2=empty_blob();
> This will replace all lob data in the table with the empty lob.
> The lob handle remains the same as before.
>
> Update tlob set c2 = stringtolob(‘xxxx’);
> This will replace the empty lob entry inth
> descriptor handle and descriptor chunks table with a new entry.
> OR
> Update tlob set c2 – stringtolob(‘xxxx’, append);
> This will append the new data to the empty lob. So
> it will leave 2 entries in the descriptor handle and descriptor chunks table.
>
> But the result when you select the data will be the same in both cases
> above.
>
>
> Update directly using a lobhandle using a buffer programmatically :
> New syntax has been introduced to allow updating a lob entry using lob
> handle directory. This is not positional syntax.
>
> The syntax is as follows :
>
> update lob (LOB ‘<lob handle>’, LOCATION < int64 address>, SIZE <
> int64 size>);
> This will take a lob handle as input (it could be an empty lob or have
> existing data). It will replace the contents with the data specified in
> LOCATION.
>
> update lob (LOB ‘<lob handle>’, LOCATION < int64 address>, SIZE <
> int64 size>, APPEND);
> This will take a lob handle as input (it could be an empty lob or have
> existing data). It will append the contents with the data specified in
> LOCATION.
>
> update lob (LOB ‘<lob handle>’, TRUNCATE);
> This will truncate the existing data and initialize the lob to empty_lob
> for the provided lob handle.
--
This message was sent by Atlassian JIRA
(v7.6.3#76005)
