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The rule InlineSubplanInputForNestedTupleSourceRule is to remove SubplanOperators containing DataScan, InnerJoin, LeftOuterJoin, UnionAll or Distinct. Given a qualified Subplan operator called S1, Let's call its input operator O1.

General Cases

 We have the following rewritings for general cases:

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In the plan, v_lc_1, ..., v_lc_n are live "covering" variables at InputOp, while v_rc_1, ..., v_rc_n are their corresponding variables populated from the deepcopy of InputOp. "Covering" variables form a set of variables that can imply all live variables. v_l1, ....v_ln in the decoration part of the added group-by operator are all live variables at InputOp except the covering variables v_lc_1, ..., v_lc_n.  In the current implementation, we use "covering" variables as primary key variables. In the next version, we will use the real primary key variables, which will fix ASTERIXDB-1168.

Here is a concrete example of the general case rewriting (optimizerts/queries/nested_loj4.aql). 

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Here is a concrete example (optimizerts/queries/nested_loj2.aql). .

Before plan (nested_loj2):

distribute result [%0->$$17] -- |UNPARTITIONED|

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          left outer join (function-call: algebricks:eq, Args:[%0->$$22, %0->$$18]) -- |UNPARTITIONED|

            data-scan []<-[$$18, $$0] <- tpch:Customers -- |UNPARTITIONED|

              empty-tuple-source -- |UNPARTITIONED|

            assign [$$28] <- [TRUE] -- |UNPARTITIONED|

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