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From "Sergey Shelukhin (JIRA)" <>
Subject [jira] [Created] (HIVE-9805) LLAP: consider specialized "transient" metadata cache
Date Fri, 27 Feb 2015 01:35:05 GMT
Sergey Shelukhin created HIVE-9805:

             Summary: LLAP: consider specialized "transient" metadata cache
                 Key: HIVE-9805
             Project: Hive
          Issue Type: Sub-task
            Reporter: Sergey Shelukhin
             Fix For: llap

Due to the nature of cache now (metadata cache + disk cache), when data is read from ORC,
whole bunch of processing is still done with metadata, columns, streams, contexts, offsets,
etc. to get the data that is in cache. Essentially only the disk reads are eliminated, everything
else is as if we are reading an unknown file.
We could have a better metadata representation that is saved during first read - for example,
(file, stripe) -> DiskRange[] (incl. cache buffers that are not locked) + multi-dimensional
array per column per stream per RG pointing to offsets in DiskRange array. 
That way if such structure is found in cache, reader can avoid all the calculation and just
do dumb conversion into results to pass to decoder plus disk reading for missing parts. 

This java cache cannot figure in the main data eviction policy so it should be small. With
java objects no cache locking is needed, we can evict while someone is still using the structure,
and it will be GCed

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