[Discussion] Support the LIMIT operator for show segments command

[haomarch]

We support show segment as query in the CarbonData 2.0.1, in the type of
 'SHOW SEGMENT on table {tablename} AS 'select * from tablename_segments'.

But in order to build the TEMP VIEW {tablename_segments}, it will list all
the segments and fetch the partitions info of every segment. When the number
of segments is large, the time cost is very high.
*
Solution 1:*
Parse LIMIT operations when query contains LIMIT operations. Then Only Load
the specified number of segments.
Disadvantage:
In order to support both
"show segments for table limit 10" and
"show segments on table as select * from table_segment limit 10".

Two different meaning of LIMIT operator in the 'SHOW SEGMENT'. need two
parsing flow to handle that.


Solution 2:
Add an LIMIT operater in the SHOW SEGMENT command.

SHOW [HISTORY] SEGMENTS
[FOR TABLE | ON] [db_name.]table_name* [LIMIT number_of_segments]*
[AS (select query from table_name_segments)]




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[David CaiQiang]

+1 for solution 1

but the limit statment will get the head or tail of segment list? or need
order by some columns? Please describe the details.



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Best Regards
David Cai
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