Table.AddIndexColumn Folded for Non-Left Nested Join - Why? :-)

[bgribaudo]

Hello,

If I do a Table.NestedJoin with a kind other than LeftOuter and I don't expand the nested join column, PQ folds an Table.AddIndexColumn onto the left table in the join.

What is this index column used for?

Example Code:

let
    BaseTable = GetTable("BaseTable),
    OtherTable = GetTable("OtherTable"),
    Joined = Table.NestedJoin(BaseTable, {"A"}, OtherTable, {"B"}, "Nested", JoinKind.LeftAnti)
in
    Joined

Folded Operations:

[view base]: GetType (args: [  ] )
[view other]: GetType (args: [  ] )
[view base]: GetType (args: [  ] )
[view base]: OnRenameColumns (args: [ renames = {[ OldName = "A", NewName = "A2" ] , [ OldName = "B", NewName = "B2" ] }  ] )
[view base]: GetType (args: [  ] )
[view base]: GetRows (args: [  ] )
[view base]: OnInvoke (args: [ function = Table.AddIndexColumn, arguments = {[view base] , "row_index"} , index = 0 ] )
[view other]: GetType (args: [  ] )
[view other]: GetRows (args: [  ] )
[view other]: OnJoin (args: [ joinSide = 1, leftTable = [view base] , rightTable = [view other]  , joinKeys = #table( type table [Left = any, Right = any, EqualityComparer = any] , {{"A2", "B", Value.Equals} } ) , joinKind = 4 ] )
[view base]: GetType (args: [  ] )
[view base]: GetRows (args: [  ] )
[view base]: OnJoin (args: [ joinSide = 0, leftTable = [view base] , rightTable = [view other]  , joinKeys = #table( type table [Left = any, Right = any, EqualityComparer = any] , {{"A2", "B", Value.Equals} } ) , joinKind = 4 ] )
[view other]: GetType (args: [  ] )
[view other]: OnJoin (args: [ joinSide = 1, leftTable = [view base] , rightTable = [view other] , joinKeys = #table( type table [Left = any, Right = any, EqualityComparer = any] , {{"A2", "B", Value.Equals} } ) , joinKind = 4 ] )
[view base]: GetRows (args: [  ] )
[view other]: GetRows (args: [  ] )

Thanks,
Ben