[jira] [Commented] (KYLIN-4310) "select 1 from tb_name limit 1" seems to scan all segments and dicts

["Yaqian Zhang (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/KYLIN-4310?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17006642#comment-17006642 ] 

Yaqian Zhang commented on KYLIN-4310:
-------------------------------------

Hi Congling:

I execute this simple sql and found that Kylin does take lots of resources and time for it.

Maybe it's a meaningful job to find out the reason and optimize it.

 

> "select 1 from tb_name limit 1" seems to scan all segments and dicts
> --------------------------------------------------------------------
>
>                 Key: KYLIN-4310
>                 URL: https://issues.apache.org/jira/browse/KYLIN-4310
>             Project: Kylin
>          Issue Type: Improvement
>          Components: Query Engine
>            Reporter: Congling Xia
>            Priority: Major
>
> Hey! Some users of our kylin system tend to send  SQL like
>  
> {code:java}
> select 1 from <table_name> limit 1;
> {code}
> as a validate query. After checking the log, I found that Kylin seems to scans all segments of a cube and load all dictionaries of each columns. More HBase scans and HDFS I/O will take place in the process, and may lead to slow response.
> The dictionaries may cache in and out frequently in `DictionaryManager`. It is worse when dictionaries are big files and queries like that execute concurrently.
> It seems hard work for Kylin to execute that simple SQL...
> Any ideas for optimization?
>  
>  
>  



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