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From arina-ielchiieva <...@git.apache.org>
Subject [GitHub] drill pull request #794: DRILL-5375: Nested loop join: return correct result...
Date Mon, 03 Apr 2017 12:15:13 GMT
Github user arina-ielchiieva commented on a diff in the pull request:

    --- Diff: exec/java-exec/src/main/java/org/apache/drill/exec/planner/physical/PlannerSettings.java
    @@ -105,6 +103,29 @@
       public static final PositiveLongValidator PARQUET_ROWGROUP_FILTER_PUSHDOWN_PLANNING_THRESHOLD
           Long.MAX_VALUE, 10000);
    +  /*
    +     Enables rules that re-write query joins in the most optimal way.
    +     Though its turned on be default and its value in query optimization is undeniable,
user may want turn off such
    +     optimization to leave join order indicated in sql query unchanged.
    +      For example:
    +      Currently only nested loop join allows non-equi join conditions usage.
    +      During planning stage nested loop join will be chosen when non-equi join is detected
    +      and {@link #NLJOIN_FOR_SCALAR} set to false. Though query performance may not be
the most optimal in such case,
    +      user may use such workaround to execute queries with non-equi joins.
    +      Nested loop join allows only INNER and LEFT join usage and implies that right input
is smaller that left input.
    +      During LEFT join when join optimization is enabled and detected that right input
is larger that left,
    +      join will be optimized: left and right inputs will be flipped and LEFT join type
will be changed to RIGHT one.
    +      If query contains non-equi joins, after such optimization it will fail, since nested
loop does not allow
    +      RIGHT join. In this case if user accepts probability of non optimal performance,
he may turn off join optimization.
    +      Turning off join optimization, makes sense only if user are not sure that right
output is less or equal to left,
    +      otherwise join optimization can be left turned on.
    +      Note: once hash and merge joins will allow non-equi join conditions,
    +      the need to turn off join optimization may go away.
    +   */
    +  public static final BooleanValidator JOIN_OPTIMIZATION = new BooleanValidator("planner.enable_join_optimization",
    --- End diff --
    Makes sense, I have opened the Jira https://issues.apache.org/jira/browse/DRILL-5408.

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