refactor(ir): add JoinTable operation unique to JoinChain instead…

… of using the globally unique `SelfReference`

This enables us to maintain join expression equality:
`a.join(b).equals(a.join(b))`

So far we have been using SelfReference to make join tables unique, but
it was globally unique which broke the equality check above. Therefore
we need to restrict the uniqueness to the scope of the join chain. The
simplest solution for that is to simply enumerate the join tables in
the join chain, hence now all join participants must be
`ops.JoinTable(rel, index)` instances.

`ops.SelfReference` is still required to distinguish between two
identical tables at the API level, but it is now decoupled from the
join internal representation.

ibis/expr/decompile.py

@@ -184,6 +184,11 @@ def self_reference(op, parent, identifier):
     return parent
 
 
+@translate.register(ops.JoinTable)
+def join_table(op, parent, index):
+    return parent
+
+
 @translate.register(ops.JoinLink)
 def join_link(op, table, predicates, how):
     return f".{how}_join({table}, {_try_unwrap(predicates)})"
@@ -327,7 +332,7 @@ def isin(op, value, options):
 class CodeContext:
     always_assign = (ops.ScalarParameter, ops.UnboundTable, ops.Aggregate)
     always_ignore = (
-        ops.SelfReference,
+        ops.JoinTable,
         ops.Field,
         dt.Primitive,
         dt.Variadic,

ibis/expr/format.py

@@ -162,7 +162,7 @@ def pretty(node):
 
     def mapper(op, _, **kwargs):
         result = fmt(op, **kwargs)
-        if isinstance(op, ops.Relation):
+        if isinstance(op, ops.Relation) and not isinstance(op, ops.JoinTable):
             tables[op] = result
             result = f"r{next(refcnt)}"
         return Rendered(result)
@@ -337,6 +337,11 @@ def _self_reference(op, parent, **kwargs):
     return f"{op.__class__.__name__}[{parent}]"
 
 
+@fmt.register(ops.JoinTable)
+def _join_table(op, parent, index):
+    return parent
+
+
 @fmt.register(ops.Literal)
 def _literal(op, value, **kwargs):
     if op.dtype.is_interval():

ibis/expr/operations/relations.py

@@ -229,27 +229,38 @@ def name(self) -> str:
 ]
 
 
+@public
+class JoinTable(Simple):
+    index: int
+
+
 @public
 class JoinLink(Node):
     how: JoinKind
-    table: SelfReference
+    table: JoinTable
     predicates: VarTuple[Value[dt.Boolean]]
 
 
 @public
 class JoinChain(Relation):
-    first: SelfReference
+    first: JoinTable
     rest: VarTuple[JoinLink]
     values: FrozenDict[str, Unaliased[Value]]
 
     def __init__(self, first, rest, values):
         allowed_parents = {first}
+        assert first.index == 0
         for join in rest:
+            assert join.table.index == len(allowed_parents)
             allowed_parents.add(join.table)
             _check_integrity(join.predicates, allowed_parents)
         _check_integrity(values.values(), allowed_parents)
         super().__init__(first=first, rest=rest, values=values)
 
+    @property
+    def length(self):
+        return len(self.rest) + 1
+
     @attribute
     def schema(self):
         return Schema({k: v.dtype.copy(nullable=True) for k, v in self.values.items()})

ibis/expr/tests/snapshots/test_format/test_asof_join/repr.txt

@@ -6,21 +6,15 @@ r1 := UnboundTable: right
   time2  int32
   value2 float64
 
-r2 := SelfReference[r0]
-
-r3 := SelfReference[r1]
-
-r4 := SelfReference[r1]
-
-JoinChain[r2]
-  JoinLink[asof, r3]
-    r2.time1 == r3.time2
-  JoinLink[inner, r4]
-    r2.value == r4.value2
+JoinChain[r0]
+  JoinLink[asof, r1]
+    r0.time1 == r1.time2
+  JoinLink[inner, r1]
+    r0.value == r1.value2
   values:
-    time1:        r2.time1
-    value:        r2.value
-    time2:        r3.time2
-    value2:       r3.value2
-    time2_right:  r4.time2
-    value2_right: r4.value2
+    time1:        r0.time1
+    value:        r0.value
+    time2:        r1.time2
+    value2:       r1.value2
+    time2_right:  r1.time2
+    value2_right: r1.value2

ibis/expr/tests/snapshots/test_format/test_format_multiple_join_with_projection/repr.txt

@@ -12,24 +12,18 @@ r2 := UnboundTable: three
   bar_id string
   value2 float64
 
-r3 := SelfReference[r1]
-
-r4 := SelfReference[r2]
-
-r5 := Filter[r0]
+r3 := Filter[r0]
   r0.f > 0
 
-r6 := SelfReference[r5]
-
-JoinChain[r6]
-  JoinLink[left, r3]
-    r6.foo_id == r3.foo_id
-  JoinLink[inner, r4]
-    r6.bar_id == r4.bar_id
+JoinChain[r3]
+  JoinLink[left, r1]
+    r3.foo_id == r1.foo_id
+  JoinLink[inner, r2]
+    r3.bar_id == r2.bar_id
   values:
-    c:      r6.c
-    f:      r6.f
-    foo_id: r6.foo_id
-    bar_id: r6.bar_id
-    value1: r3.value1
-    value2: r4.value2
+    c:      r3.c
+    f:      r3.f
+    foo_id: r3.foo_id
+    bar_id: r3.bar_id
+    value1: r1.value1
+    value2: r2.value2

ibis/expr/tests/snapshots/test_format/test_memoize_filtered_tables_in_join/repr.txt

@@ -17,15 +17,11 @@ r2 := Filter[r1]
 r3 := Filter[r1]
   r1.kind == 'bar'
 
-r4 := SelfReference[r2]
-
-r5 := SelfReference[r3]
-
-JoinChain[r4]
-  JoinLink[inner, r5]
-    r4.region == r5.region
+JoinChain[r2]
+  JoinLink[inner, r3]
+    r2.region == r3.region
   values:
-    region:      r4.region
-    kind:        r4.kind
-    total:       r4.total
-    right_total: r5.total
+    region:      r2.region
+    kind:        r2.kind
+    total:       r2.total
+    right_total: r3.total

ibis/expr/tests/snapshots/test_format/test_table_count_expr/join_repr.txt

@@ -6,17 +6,13 @@ r1 := UnboundTable: t2
   a int64
   b float64
 
-r2 := SelfReference[r0]
-
-r3 := SelfReference[r1]
-
-r4 := JoinChain[r2]
-  JoinLink[inner, r3]
-    r2.a == r3.a
+r2 := JoinChain[r0]
+  JoinLink[inner, r1]
+    r0.a == r1.a
   values:
-    a:       r2.a
-    b:       r2.b
-    a_right: r3.a
-    b_right: r3.b
+    a:       r0.a
+    b:       r0.b
+    a_right: r1.a
+    b_right: r1.b
 
-CountStar(): CountStar(r4)
+CountStar(): CountStar(r2)

ibis/expr/tests/snapshots/test_format/test_two_inner_joins/repr.txt

@@ -8,24 +8,18 @@ r1 := UnboundTable: right
   value2 float64
   b      string
 
-r2 := SelfReference[r0]
-
-r3 := SelfReference[r1]
-
-r4 := SelfReference[r1]
-
-JoinChain[r2]
-  JoinLink[inner, r3]
-    r2.a == r3.b
-  JoinLink[inner, r4]
-    r2.value == r4.value2
+JoinChain[r0]
+  JoinLink[inner, r1]
+    r0.a == r1.b
+  JoinLink[inner, r1]
+    r0.value == r1.value2
   values:
-    time1:        r2.time1
-    value:        r2.value
-    a:            r2.a
-    time2:        r3.time2
-    value2:       r3.value2
-    b:            r3.b
-    time2_right:  r4.time2
-    value2_right: r4.value2
-    b_right:      r4.b
+    time1:        r0.time1
+    value:        r0.value
+    a:            r0.a
+    time2:        r1.time2
+    value2:       r1.value2
+    b:            r1.b
+    time2_right:  r1.time2
+    value2_right: r1.value2
+    b_right:      r1.b