risingwave_frontend/optimizer/rule/
max_one_row_eliminate_rule.rs

1// Copyright 2025 RisingWave Labs
2//
3// Licensed under the Apache License, Version 2.0 (the "License");
4// you may not use this file except in compliance with the License.
5// You may obtain a copy of the License at
6//
7//     http://www.apache.org/licenses/LICENSE-2.0
8//
9// Unless required by applicable law or agreed to in writing, software
10// distributed under the License is distributed on an "AS IS" BASIS,
11// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12// See the License for the specific language governing permissions and
13// limitations under the License.
14
15use super::prelude::{PlanRef, *};
16use crate::optimizer::plan_node::{LogicalApply, LogicalLimit, LogicalMaxOneRow};
17use crate::optimizer::plan_visitor::{LogicalCardinalityExt, SoleSysTableVisitor};
18
19/// Eliminate max one row restriction from `LogicalApply`.
20///
21/// If we cannot guarantee that the right side of `Apply` will return at most one row
22/// in compile time, we will add a `MaxOneRow` that does runtime check to satisfy the
23/// max one row restriction.
24///
25/// As a result, the `max_one_row` flag of `LogicalApply` will always be `false`
26/// after applying this rule.
27pub struct MaxOneRowEliminateRule {}
28impl Rule<Logical> for MaxOneRowEliminateRule {
29    fn apply(&self, plan: PlanRef) -> Option<PlanRef> {
30        let apply: &LogicalApply = plan.as_logical_apply()?;
31        let (left, mut right, on, join_type, correlated_id, correlated_indices, max_one_row) =
32            apply.clone().decompose();
33
34        if !max_one_row {
35            return None;
36        }
37
38        if !right.max_one_row() {
39            right = if SoleSysTableVisitor::sys_table_only(right.clone()) {
40                // If the right side is just a `SysScan` (with `Values`), we add a `Limit 1` to enforce the max one row restriction.
41                // This is a workaround for the case where `SysScan` cannot be guaranteed to return at most one row in compile time,
42                // but to make the system queries work compatible with PostgreSQL, we need to enforce the max one row restriction at runtime.
43                LogicalLimit::create(right, 1, 0)
44            } else {
45                LogicalMaxOneRow::create(right)
46            };
47            debug_assert!(right.max_one_row());
48        }
49
50        Some(LogicalApply::create(
51            left,
52            right,
53            join_type,
54            on,
55            correlated_id,
56            correlated_indices,
57            false,
58        ))
59    }
60}
61
62impl MaxOneRowEliminateRule {
63    pub fn create() -> BoxedRule {
64        Box::new(MaxOneRowEliminateRule {})
65    }
66}
risingwave_frontend/optimizer/rule/max_one_row_eliminate_rule.rs

risingwave_frontend/optimizer/rule/
max_one_row_eliminate_rule.rs
