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From "Lars Hofhansl (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (PHOENIX-153) Implement TABLESAMPLE clause
Date Sat, 01 Jul 2017 06:35:00 GMT

    [ https://issues.apache.org/jira/browse/PHOENIX-153?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16071041#comment-16071041
] 

Lars Hofhansl commented on PHOENIX-153:
---------------------------------------

The default guidepost width is 300MB. Maybe we could go down to 10MB, once we have guidepost
combining.
Less than that will be a huge management burden to the system.

Still a good thing to do! On small tables you do not need to sample in the first place, and
for large tables - where it matters - we'll have sufficiently many guide posts. (A 1TB table
has over 3000 300MB guideposts, i.e. you'll have a resolution of 0.03%, which is plenty good!)


> Implement TABLESAMPLE clause
> ----------------------------
>
>                 Key: PHOENIX-153
>                 URL: https://issues.apache.org/jira/browse/PHOENIX-153
>             Project: Phoenix
>          Issue Type: Task
>            Reporter: James Taylor
>            Assignee: Ethan Wang
>              Labels: enhancement
>         Attachments: Sampling_Accuracy_Performance.jpg
>
>
> Support the standard SQL TABLESAMPLE clause by implementing a filter that uses a skip
next hint based on the region boundaries of the table to only return n rows per region.



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