feat: add subquery representation

proto/substrait/algebra.proto

@@ -258,6 +258,7 @@ message Expression {
     MultiOrList multi_or_list = 9;
     Enum enum = 10;
     Cast cast = 11;
+    Subquery subquery = 12;
   }
 
   message Enum {
@@ -568,11 +569,96 @@ message Expression {
     oneof root_type {
       Expression expression = 3;
       RootReference root_reference = 4;
+      OuterReference outer_reference = 5;
     }
 
     // Singleton that expresses this FieldReference is rooted off the root
     // incoming record type
     message RootReference {}
+
+    // A root reference for the outer relation's subquery
+    message OuterReference {
+      // number of subquery boundaries to traverse up for this field's reference
+      //
+      // This value must be >= 1
+      uint32 steps_out = 1;
+    }
+  }
+
+  // Subquery relation expression
+  message Subquery {
+    oneof subquery_type {
+      // Scalar subquery
+      Scalar scalar = 1;
+      // x IN y predicate
+      InPredicate in_predicate = 2;
+      // EXISTS/UNIQUE predicate
+      SetPredicate set_predicate = 3;
+      // ANY/ALL predicate
+      SetComparison set_comparison = 4;
+    }
+
+    // A subquery with one row and one column. This is often an aggregate
+    // though not required to be.
+    message Scalar { Rel input = 1; }
+
+    // Predicate checking that the left expression is contained in the right
+    // subquery
+    //
+    // Examples:
+    //
+    // x IN (SELECT * FROM t)
+    // (x, y) IN (SELECT a, b FROM t)
+    message InPredicate {
+      repeated Expression needles = 1;
+      Rel haystack = 2;
+    }
+
+    // A predicate over a set of rows in the form of a subquery
+    // EXISTS and UNIQUE are common SQL forms of this operation.
+    message SetPredicate {
+      enum PredicateOp {
+        PREDICATE_OP_UNSPECIFIED = 0;
+        PREDICATE_OP_EXISTS = 1;
+        PREDICATE_OP_UNIQUE = 2;
+      }
+      // TODO: should allow expressions
+      PredicateOp predicate_op = 1;
+      Rel tuples = 2;
+    }
+
+    // A subquery comparison using ANY or ALL.
+    // Examples:
+    //
+    // SELECT *
+    // FROM t1
+    // WHERE x < ANY(SELECT y from t2)
+    message SetComparison {
+      enum ComparisonOp {
+        COMPARISON_OP_UNSPECIFIED = 0;
+        COMPARISON_OP_EQ = 1;
+        COMPARISON_OP_NE = 2;
+        COMPARISON_OP_LT = 3;
+        COMPARISON_OP_GT = 4;
+        COMPARISON_OP_LE = 5;
+        COMPARISON_OP_GE = 6;
+      }
+
+      enum ReductionOp {
+        REDUCTION_OP_UNSPECIFIED = 0;
+        REDUCTION_OP_ANY = 1;
+        REDUCTION_OP_ALL = 2;
+      }
+
+      // ANY or ALL
+      ReductionOp reduction_op = 1;
+      // A comparison operator
+      ComparisonOp comparison_op = 2;
+      // left side of the expression
+      Expression left = 3;
+      // right side of the expression
+      Rel right = 4;
+    }
   }
 }
 

site/docs/expressions/subqueries.md

@@ -0,0 +1,67 @@
+# Subqueries
+
+Subqueries are scalar expressions comprised of another query.
+
+## Forms
+
+### Scalar
+
+Scalar subqueries are subqueries that return one row and one column.
+
+| Property | Description    | Required |
+| -------- | -------------- | -------- |
+| Input    | Input relation | Yes      |
+
+### `IN` predicate
+
+An `IN` subquery predicate checks that the left expression is contained in the
+right subquery.
+
+#### Examples
+
+```sql
+SELECT *
+FROM t1
+WHERE x IN (SELECT * FROM t2)
+```
+
+```sql
+SELECT *
+FROM t1
+WHERE (x, y) IN (SELECT a, b FROM t2)
+```
+
+| Property | Description                               | Required |
+| -------- | ----------------------------------------- | -------- |
+| Needles  | Expressions who existence will be checked | Yes      |
+| Haystack | Subquery to check                         | Yes      |
+
+### Set predicates
+
+A set predicate is a predicate over a set of rows in the form of a subquery.
+
+`EXISTS` and `UNIQUE` are common SQL spellings of these kinds of predicates.
+
+| Property  | Description                                | Required |
+| --------- | ------------------------------------------ | -------- |
+| Operation | The operation to perform over the set      | Yes      |
+| Tuples    | Set of tuples to check using the operation | Yes      |
+
+### Set comparisons
+
+A set comparison subquery is a subquery comparison using `ANY` or `ALL` operations.
+
+#### Examples
+
+```sql
+SELECT *
+FROM t1
+WHERE x < ANY(SELECT y from t2)
+```
+
+| Property              | Description                                    | Required |
+| --------------------- | ---------------------------------------------- | -------- |
+| Reduction operation   | The kind of reduction to use over the subquery | Yes      |
+| Comparision operation | The kind of comparison operation to use        | Yes      |
+| Expression            | Left hand side expression to check             | Yes      |
+| Subquery              | Subquery to check                              | Yes      |