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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