Follow the outer side of two-input nodes through an abstract type

get_source and _collect_nodes! needed a method per node type that combines two
sources, because those carry `outer`/`inner` rather than `source`. With outer
joins, seven set operators and zip arriving, that is eleven more pairs of
near-identical methods.

Introduce QueryableBinary instead, and subtype QueryableJoin and
QueryableGroupJoin under it. The tree walk now has one method covering every
two-input node, and a new one only has to pick the right supertype.

QueryableBinary <: Queryable and the concrete types are unchanged, so
`node isa QueryableJoin` and anything dispatching on Queryable still behave
exactly as before.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: davidanthoff

src/query_tree.jl

@@ -1,17 +1,14 @@
 """
     get_source(q::Queryable)
 
-Follow the `.source` (or `.outer`) chain to the root `QueryableSource`.
+Follow the `.source` (or, for two-input nodes, `.outer`) chain to the root
+`QueryableSource`.
 """
 function get_source(q::QueryableSource)
     return q
 end
 
-function get_source(q::QueryableJoin)
-    return get_source(q.outer)
-end
-
-function get_source(q::QueryableGroupJoin)
+function get_source(q::QueryableBinary)
     return get_source(q.outer)
 end
 
@@ -34,12 +31,7 @@ function _collect_nodes!(nodes, q::QueryableSource)
     push!(nodes, q)
 end
 
-function _collect_nodes!(nodes, q::QueryableJoin)
-    push!(nodes, q)
-    _collect_nodes!(nodes, q.outer)
-end
-
-function _collect_nodes!(nodes, q::QueryableGroupJoin)
+function _collect_nodes!(nodes, q::QueryableBinary)
     push!(nodes, q)
     _collect_nodes!(nodes, q.outer)
 end

src/queryable/queryable.jl

@@ -1,5 +1,15 @@
 abstract type Queryable end
 
+"""
+    QueryableBinary <: Queryable
+
+Nodes that combine two sources — joins, set operations, `zip`. They carry
+`outer` and `inner` fields, and the tree walk follows `outer`, so the query
+plan reads as a chain hanging off the primary input with the secondary input
+attached at the node.
+"""
+abstract type QueryableBinary <: Queryable end
+
 QueryOperators.query(x::Queryable) = x
 
 IteratorInterfaceExtensions.isiterable(x::Queryable) = true

src/queryable/queryable_groupjoin.jl

@@ -1,4 +1,4 @@
-struct QueryableGroupJoin <: Queryable
+struct QueryableGroupJoin <: QueryableBinary
     outer
     inner
     outerKeySelector_func

src/queryable/queryable_join.jl

@@ -1,4 +1,4 @@
-struct QueryableJoin <: Queryable
+struct QueryableJoin <: QueryableBinary
     outer
     inner
     outerKeySelector_func