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https://issues.apache.org/jira/browse/TRAFODION-2408?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15795850#comment-15795850
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ASF GitHub Bot commented on TRAFODION-2408:
-------------------------------------------
Github user sandhyasun commented on a diff in the pull request:
https://github.com/apache/incubator-trafodion/pull/889#discussion_r94461094
--- Diff: core/sql/exp/ExpLOBaccess.cpp ---
@@ -1502,7 +1507,50 @@ Ex_Lob_Error ExLob::allocateDesc(ULng32 size, Int64
&descNum, Int64 &dataOffset,
char logBuf[4096];
lobDebugInfo("In ExLob::allocateDesc",0,__LINE__,lobTrace_);
Int32 openFlags = O_RDONLY ;
-
+ if (size == 0) //we are trying to empty this lob.
+ {
+ //rename lob datafile
+ char * saveLobDataFile = new(getLobGlobalHeap())
char[MAX_LOB_FILE_NAME_LEN+6];
--- End diff --
This is fine. We only need to allocate for "_save" suffix
str_sprintf(saveLobDataFile, "%s_save",lobDataFile_);
> 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
>
> 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.
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