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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