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https://issues.apache.org/jira/browse/TRAFODION-2408?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16707639#comment-16707639
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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/trafodion/pull/1754#discussion_r238387612
--- Diff: core/sql/exp/ExpLOBaccess.cpp ---
@@ -1779,77 +1779,23 @@ Ex_Lob_Error ExLob::allocateDesc(ULng32 size, Int64
&descNum, Int64 &dataOffset,
lobDebugInfo("In ExLob::allocateDesc",0,__LINE__,lobTrace_);
Int32 openFlags = O_RDONLY ;
HDFS_Client_RetCode hdfsClientRetcode;
+ hdfsFileInfo *fInfo = NULL;
if (! useLibHdfs_) {
- if (size == 0) {
- // Delete and Create the Hdfs file by passing overwrite to TRUE
- hdfsClientRetcode = hdfsClient_->hdfsCreate(lobDataFile_.data(),
TRUE, FALSE, FALSE);
- if (hdfsClientRetcode != HDFS_CLIENT_OK)
- return LOB_DATA_FILE_WRITE_ERROR;
- else {
- dataOffset = 0;
- return LOB_OPER_OK;
- }
- }
- else {
+
dataOffset = hdfsClient_->hdfsSize(hdfsClientRetcode);
--- End diff --
The dataOffset still needs to be set to the EOD of the datafile for any
empty lob. For eg if we already have one lob entry int he data file and we are
inserting an empty lob in the next row, then we need to retrieve dataoffset and
in the descriptor we will set the size to 0 for that offset. It makes
bookkeeping of the lob offsets and lengths consistent.
> 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.
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