Hyunsik Choi created TAJO-1005:
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Summary: Sort on expression values including aggregation function
should be supported.
Key: TAJO-1005
URL: https://issues.apache.org/jira/browse/TAJO-1005
Project: Tajo
Issue Type: Improvement
Components: planner/optimizer
Reporter: Hyunsik Choi
Assignee: Hyunsik Choi
Fix For: 0.9.0
h3. Background
In Tajo, expression evaluation are performed at only three operators, Scan,
Join, and Projection. An expression is divided into multiple sub expressions,
and then they are placed into proper operators.
h3. Problem
See the following example:
{code}
SELECT sum(x) / 100 as b from lineitem order by b
{code}
Tajo evaluates expressions at three operators: scan, join, and projection.
Aggregation functions are evaluated at groupby operator, and expressions
including aggregation functions are evaluated at projection operator.
But, sort operator is placed between aggregation function and projection. So,
sort operator does not have any opportunity to take the result of some
expression including aggregation functions.
h3. Solution
So far, projection operator is only topmost operator of all operators. To solve
the above problem, we need to improve the order of operators. Especially,
projection operator can be interleaved between all operators.
More specially, we allow projection operator to be inserted between group-by
and sort operators. If so, the above problem will be solved.
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