[jira] [Created] (IMPALA-11679) Inconsistent push down of limit with unpartitioned row_number()

["Csaba Ringhofer (Jira)" <ji...@apache.org>]

Csaba Ringhofer created IMPALA-11679:
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             Summary: Inconsistent push down of limit with unpartitioned row_number()
                 Key: IMPALA-11679
                 URL: https://issues.apache.org/jira/browse/IMPALA-11679
             Project: IMPALA
          Issue Type: Improvement
          Components: Frontend
            Reporter: Csaba Ringhofer


In case of row_number() having a <= predicate on row number and limit means the same, so these two queries should lead to an equivalent plan:

a:
select * from (select l_orderkey, row_number() OVER (ORDER by l_orderkey) as rnum from tpch_parquet.lineitem) s 
where rnum <= 10000;

b:
select * from (select l_orderkey, row_number() OVER (ORDER by l_orderkey) as rnum from
tpch_parquet.lineitem) s
limit 10000;

Currently a. will use to a top-n node while b. will use a sort node.
For rnum <= 1000 a. will also use a top-n node

Meanwhile if there is also a rnum > X clause (essentially an OFFSET), then limit has lower bounds for using top-n:

c:
select * from (select l_orderkey, row_number() OVER (ORDER by l_orderkey) as rnum fromtpch_parquet.lineitem) s
where rnum > 900 and rnum <= 1000

d:
select * from (select l_orderkey, row_number() OVER (ORDER by l_orderkey) as rnum from tpch_parquet.lineitem) s
where rnum > 900 limit 1000

c. will use a top-n node while d. will use a sort node

Besides not using the more optimal top-n (for low limits) another problem is that the analyitic-eval-node will process all rows, even when all further rows will be dropped by the predicate on row_number(). This is problematic as it runs on a single node/thread.

A solution could be to recognize < and > predicates on unpartitioned row_number() as limit and  offset.




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