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https://issues.apache.org/jira/browse/HIVE-8038?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14132263#comment-14132263
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Pankit Thapar commented on HIVE-8038:
-------------------------------------
Hi Gopal,
Thanks for taking a look.
I have uploaded an updated diff on the cr with the changes recommended. And
also, commented on the feedback.
Please let me know your feedback on the same.
> Decouple ORC files split calculation logic from Filesystem's get file
> location implementation
> ---------------------------------------------------------------------------------------------
>
> Key: HIVE-8038
> URL: https://issues.apache.org/jira/browse/HIVE-8038
> Project: Hive
> Issue Type: Improvement
> Components: File Formats
> Affects Versions: 0.13.1
> Reporter: Pankit Thapar
> Fix For: 0.14.0
>
> Attachments: HIVE-8038.patch
>
>
> What is the Current Logic
> ======================
> 1.get the file blocks from FileSystem.getFileBlockLocations() which returns
> an array of BlockLocation
> 2.In SplitGenerator.createSplit(), check if split only spans one block or
> multiple blocks.
> 3.If split spans just one block, then using the array index (index =
> offset/blockSize), get the corresponding host having the blockLocation
> 4.If the split spans multiple blocks, then get all hosts that have at least
> 80% of the max of total data in split hosted by any host.
> 5.add the split to a list of splits
> Issue with Current Logic
> =====================
> Dependency on FileSystem API’s logic for block location calculations. It
> returns an array and we need to rely on FileSystem to
> make all blocks of same size if we want to directly access a block from the
> array.
>
> What is the Fix
> =============
> 1a.get the file blocks from FileSystem.getFileBlockLocations() which returns
> an array of BlockLocation
> 1b.convert the array into a tree map <offset, BlockLocation> and return it
> through getLocationsWithOffSet()
> 2.In SplitGenerator.createSplit(), check if split only spans one block or
> multiple blocks.
> 3.If split spans just one block, then using Tree.floorEntry(key), get the
> highest entry smaller than offset for the split and get the corresponding
> host.
> 4a.If the split spans multiple blocks, get a submap, which contains all
> entries containing blockLocations from the offset to offset + length
> 4b.get all hosts that have at least 80% of the max of total data in split
> hosted by any host.
> 5.add the split to a list of splits
> What are the major changes in logic
> ==============================
> 1. store BlockLocations in a Map instead of an array
> 2. Call SHIMS.getLocationsWithOffSet() instead of getLocations()
> 3. one block case is checked by "if(offset + length <= start.getOffset() +
> start.getLength())" instead of "if((offset % blockSize) + length <=
> blockSize)"
> What is the affect on Complexity (Big O)
> =================================
> 1. We add a O(n) loop to build a TreeMap from an array but its a one time
> cost and would not be called for each split
> 2. In case of one block case, we can get the block in O(logn) worst case
> which was O(1) before
> 3. Getting the submap is O(logn)
> 4. In case of multiple block case, building the list of hosts is O(m) which
> was O(n) & m < n as previously we were iterating
> over all the block locations but now we are only iterating only blocks
> that belong to that range go offsets that we need.
> What are the benefits of the change
> ==============================
> 1. With this fix, we do not depend on the blockLocations returned by
> FileSystem to figure out the block corresponding to the offset and blockSize
> 2. Also, it is not necessary that block lengths is same for all blocks for
> all FileSystems
> 3. Previously we were using blockSize for one block case and block.length for
> multiple block case, which is not the case now. We figure out the block
> depending upon the actual length and offset of the block
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