[jira] [Created] (TAJO-1809) Change default value of several configurations

["Jihoon Son (JIRA)" <ji...@apache.org>]

Jihoon Son created TAJO-1809:
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             Summary: Change default value of several configurations
                 Key: TAJO-1809
                 URL: https://issues.apache.org/jira/browse/TAJO-1809
             Project: Tajo
          Issue Type: Task
          Components: conf and scripts
            Reporter: Jihoon Son
            Assignee: Jihoon Son
             Fix For: 0.11.0


Tajo has many configurations. For the sake of user convenience, they all have some default value. Most of default values make sense, but some are not. Here is my idea.

* Tajo can choose hash-based algorithm and disk-based algorithm according to the remaining resource and query workload. Here, users can give a hint using session variables whose name is HASH_*_SIZE_LIMIT. Their current default value is 256 MB, but it's too large thereby incurring GC so often, and even OOM sometimes.
* Query cache should be disabled by default. It will not be commonly used.
* Configurations for broadcast join should accept numbers in MB. 
* Some configurations allow different unit in tajo-site.xml and session variable. This must be consistent.



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