[jira] [Created] (HBASE-7342) Split operation without split key incorrectly finds the middle key in off-by-one error

["Aleksandr Shulman (JIRA)" <ji...@apache.org>]

Aleksandr Shulman created HBASE-7342:
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             Summary: Split operation without split key incorrectly finds the middle key in off-by-one error
                 Key: HBASE-7342
                 URL: https://issues.apache.org/jira/browse/HBASE-7342
             Project: HBase
          Issue Type: Bug
          Components: HFile, io
    Affects Versions: 0.94.3, 0.94.2, 0.94.1, 0.96.0, 0.94.4
            Reporter: Aleksandr Shulman
            Assignee: Aleksandr Shulman
            Priority: Minor
             Fix For: 0.96.0, 0.94.2


I took a deeper look into issues I was having using region splitting when specifying a region (but not a key for splitting).

The midkey calculation is off by one and when there are 2 rows, will pick the 0th one. This causes the firstkey to be the same as midkey and the split will fail. Removing the -1 causes it work correctly, as per the test I've added.

Looking into the code here is what goes on:

1. Split takes the largest storefile
2. It puts all the keys into a 2-dimensional array called blockKeys[][]. Key i resides as blockKeys[i]
3. Getting the middle root-level index should yield the key in the middle of the storefile
4. In step 3, we see that there is a possible erroneous (-1) to adjust for the 0-offset indexing.
5. In a result with where there are only 2 blockKeys, this yields the 0th block key. 
6. Unfortunately, this is the same block key that 'firstKey' will be.
7. This yields the result in HStore.java:1873 ("cannot split because midkey is the same as first or last row")
8. Removing the -1 solves the problem (in this case). 

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