risingwave_frontend/expr/
expr_visitor.rs

// Copyright 2024 RisingWave Labs
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use risingwave_common::util::recursive::{tracker, Recurse};

use super::{
    AggCall, CorrelatedInputRef, ExprImpl, FunctionCall, FunctionCallWithLambda, InputRef, Literal,
    Now, Parameter, Subquery, TableFunction, UserDefinedFunction, WindowFunction,
    EXPR_DEPTH_THRESHOLD, EXPR_TOO_DEEP_NOTICE,
};
use crate::session::current::notice_to_user;

/// The default implementation of [`ExprVisitor::visit_expr`] that simply dispatches to other
/// methods based on the type of the expression.
///
/// You can use this function as a helper to reduce boilerplate code when implementing the trait.
// TODO: This is essentially a mimic of `super` pattern from OO languages. Ideally, we should
// adopt the style proposed in https://github.com/risingwavelabs/risingwave/issues/13477.
pub fn default_visit_expr<V: ExprVisitor + ?Sized>(visitor: &mut V, expr: &ExprImpl) {
    // TODO: Implementors may choose to not use this function at all, in which case we will fail
    // to track the recursion and grow the stack as necessary. The current approach is only a
    // best-effort attempt to prevent stack overflow.
    tracker!().recurse(|t| {
        if t.depth_reaches(EXPR_DEPTH_THRESHOLD) {
            notice_to_user(EXPR_TOO_DEEP_NOTICE);
        }

        match expr {
            ExprImpl::InputRef(inner) => visitor.visit_input_ref(inner),
            ExprImpl::Literal(inner) => visitor.visit_literal(inner),
            ExprImpl::FunctionCall(inner) => visitor.visit_function_call(inner),
            ExprImpl::FunctionCallWithLambda(inner) => {
                visitor.visit_function_call_with_lambda(inner)
            }
            ExprImpl::AggCall(inner) => visitor.visit_agg_call(inner),
            ExprImpl::Subquery(inner) => visitor.visit_subquery(inner),
            ExprImpl::CorrelatedInputRef(inner) => visitor.visit_correlated_input_ref(inner),
            ExprImpl::TableFunction(inner) => visitor.visit_table_function(inner),
            ExprImpl::WindowFunction(inner) => visitor.visit_window_function(inner),
            ExprImpl::UserDefinedFunction(inner) => visitor.visit_user_defined_function(inner),
            ExprImpl::Parameter(inner) => visitor.visit_parameter(inner),
            ExprImpl::Now(inner) => visitor.visit_now(inner),
        }
    })
}

/// Traverse an expression tree.
///
/// Each method of the trait is a hook that can be overridden to customize the behavior when
/// traversing the corresponding type of node. By default, every method recursively visits the
/// subtree.
///
/// Note: The default implementation for `visit_subquery` is a no-op, i.e., expressions inside
/// subqueries are not traversed.
pub trait ExprVisitor {
    fn visit_expr(&mut self, expr: &ExprImpl) {
        default_visit_expr(self, expr)
    }
    fn visit_function_call(&mut self, func_call: &FunctionCall) {
        func_call
            .inputs()
            .iter()
            .for_each(|expr| self.visit_expr(expr));
    }
    fn visit_function_call_with_lambda(&mut self, func_call: &FunctionCallWithLambda) {
        self.visit_function_call(func_call.base())
    }
    fn visit_agg_call(&mut self, agg_call: &AggCall) {
        agg_call
            .args()
            .iter()
            .for_each(|expr| self.visit_expr(expr));
        agg_call.order_by().visit_expr(self);
        agg_call.filter().visit_expr(self);
    }
    fn visit_parameter(&mut self, _: &Parameter) {}
    fn visit_literal(&mut self, _: &Literal) {}
    fn visit_input_ref(&mut self, _: &InputRef) {}
    fn visit_subquery(&mut self, _: &Subquery) {}
    fn visit_correlated_input_ref(&mut self, _: &CorrelatedInputRef) {}

    fn visit_table_function(&mut self, func_call: &TableFunction) {
        func_call.args.iter().for_each(|expr| self.visit_expr(expr));
    }
    fn visit_window_function(&mut self, func_call: &WindowFunction) {
        func_call.args.iter().for_each(|expr| self.visit_expr(expr));
    }
    fn visit_user_defined_function(&mut self, func_call: &UserDefinedFunction) {
        func_call.args.iter().for_each(|expr| self.visit_expr(expr));
    }
    fn visit_now(&mut self, _: &Now) {}
}
risingwave_frontend/expr/expr_visitor.rs

risingwave_frontend/expr/
expr_visitor.rs
