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

["Congling Xia (Jira)" <ji...@apache.org>]

Congling Xia created KYLIN-4310:
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             Summary: "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


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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