dataflow-expression: Add JsonSubtract operator

This new operator represents the `-` operator as applied to JSONB. As
such, we now branch on the left operand's type during lowering time to
determine whether to convert the `Subtract` AST node to the existing
`Subtract` Dataflow operator or the new `JsonSubtract` Dataflow
operator.

For now, nothing is implemented to evaluate this operator; that will be
handled in a future commit.

Change-Id: Ib4327554af43f0edcff5560e0e066b3cac7f2f7d
Reviewed-on: https://gerrit.readyset.name/c/readyset/+/3692
Tested-by: Buildkite CI
Reviewed-by: Griffin Smith <griffin@readyset.io>

dataflow-expression/src/binary_operator.rs

@@ -104,14 +104,17 @@ pub enum BinaryOperator {
 
     /// `<@`
     JsonContainedIn,
+
+    /// `-` applied to the PostreSQL JSONB type
+    JsonSubtract,
 }
 
 impl BinaryOperator {
     /// Converts from a [`nom_sql::BinaryOperator`] within the context of a SQL [`Dialect`].
     pub fn from_sql_op(
         op: SqlBinaryOperator,
         dialect: Dialect,
-        _left_type: &DfType,
+        left_type: &DfType,
         _right_type: &DfType,
     ) -> ReadySetResult<Self> {
         use SqlBinaryOperator::*;
@@ -123,7 +126,13 @@ impl BinaryOperator {
             Less => Self::Less,
             LessOrEqual => Self::LessOrEqual,
             Add => Self::Add,
-            Subtract => Self::Subtract,
+            Subtract => {
+                if left_type.is_jsonb() {
+                    Self::JsonSubtract
+                } else {
+                    Self::Subtract
+                }
+            }
             Multiply => Self::Multiply,
             Divide => Self::Divide,
             Like => Self::Like,
@@ -289,7 +298,7 @@ impl fmt::Display for BinaryOperator {
             Self::Is => "IS",
             Self::IsNot => "IS NOT",
             Self::Add => "+",
-            Self::Subtract => "-",
+            Self::Subtract | Self::JsonSubtract => "-",
             Self::Multiply => "*",
             Self::Divide => "/",
             Self::JsonExists => "?",
@@ -309,6 +318,20 @@ impl fmt::Display for BinaryOperator {
 mod tests {
     use super::*;
 
+    #[test]
+    fn json_subtract_lowering() {
+        assert_eq!(
+            BinaryOperator::from_sql_op(
+                SqlBinaryOperator::Subtract,
+                Dialect::DEFAULT_POSTGRESQL,
+                &DfType::Jsonb,
+                &DfType::Int
+            )
+            .unwrap(),
+            BinaryOperator::JsonSubtract
+        );
+    }
+
     mod output_type {
         use super::*;
 

dataflow-expression/src/eval.rs

@@ -193,6 +193,7 @@ impl Expr {
                             Ok(serde_json::to_string(&res)?.into())
                         }
                     }
+                    JsonSubtract => unsupported!("'-' operator applied to JSONB not yet supported")
                 }
             }
             Expr::Cast { expr, ty, .. } => {