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From "Fabian Hueske (JIRA)" <j...@apache.org>
Subject [jira] [Created] (FLINK-5226) Eagerly project unused attributes
Date Fri, 02 Dec 2016 09:10:58 GMT
Fabian Hueske created FLINK-5226:

             Summary: Eagerly project unused attributes
                 Key: FLINK-5226
                 URL: https://issues.apache.org/jira/browse/FLINK-5226
             Project: Flink
          Issue Type: Improvement
          Components: Table API & SQL
    Affects Versions: 1.2.0
            Reporter: Fabian Hueske

The optimizer does currently not eagerly remove unused attributes. 
For example given a table {{tab5}} with five attributes {{a, b, c, d, e}}, the following query

SELECT x.a, y.b FROM tab5 AS x, tab5 AS y WHERE x.a = y.a

would result in the non-optimized plan

LogicalProject(a=[$0], b=[$6])
  LogicalFilter(condition=[=($0, $5)])
    LogicalJoin(condition=[true], joinType=[inner])

and the optimized plan:

DataSetCalc(select=[a, b0 AS b])
  DataSetJoin(where=[=(a, a0)], join=[a, b, c, d, e, a0, b0, c0, d0, e0], joinType=[InnerJoin])

This plan is inefficient because it joins all ten attributes of both tables instead of eagerly
projecting out all unused fields ({{x.b, x.c, x.d, x.e, y.c, y.d, y.e}}).

Since this is one of the most common optimizations, I would assume that Calcite provides some
rules to extract eager projections. If this is the case, the issue can be solved by adding
such rules to {{FlinkRuleSets}}.

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