risingwave_frontend/optimizer/rule/
over_window_to_topn_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 fixedbitset::FixedBitSet;
16use risingwave_common::types::DataType;
17use risingwave_expr::window_function::WindowFuncKind;
18
19use super::prelude::{PlanRef, *};
20use crate::expr::{
21    Expr, ExprImpl, ExprRewriter, ExprType, FunctionCall, Literal, collect_input_refs,
22};
23use crate::optimizer::plan_node::generic::GenericPlanRef;
24use crate::optimizer::plan_node::{LogicalFilter, LogicalTopN, PlanTreeNodeUnary};
25use crate::optimizer::property::Order;
26use crate::planner::LIMIT_ALL_COUNT;
27use crate::utils::Condition;
28
29/// Transforms the following pattern to group `TopN` (No Ranking Output).
30///
31/// ```sql
32/// -- project - filter - over window
33/// SELECT .. FROM
34///   (SELECT .., ROW_NUMBER() OVER(PARTITION BY .. ORDER BY ..) rank FROM ..)
35/// WHERE rank [ < | <= | > | >= | = ] ..;
36/// ```
37///
38/// Transforms the following pattern to `OverWindow` + group `TopN` (Ranking Output).
39/// The `TopN` decreases the number of rows to be processed by the `OverWindow`.
40///
41/// ```sql
42/// -- filter - over window
43/// SELECT .., ROW_NUMBER() OVER(PARTITION BY .. ORDER BY ..) rank
44/// FROM ..
45/// WHERE rank [ < | <= | > | >= | = ] ..;
46/// ```
47///
48/// Also optimizes filter arithmetic expressions in the `Project <- Filter <- OverWindow` pattern,
49/// such as simplifying `(row_number - 1) = 0` to `row_number = 1`.
50pub struct OverWindowToTopNRule;
51
52impl OverWindowToTopNRule {
53    pub fn create() -> BoxedRule {
54        Box::new(OverWindowToTopNRule)
55    }
56}
57
58impl Rule<Logical> for OverWindowToTopNRule {
59    fn apply(&self, plan: PlanRef) -> Option<PlanRef> {
60        let ctx = plan.ctx();
61        let (project, plan) = {
62            if let Some(project) = plan.as_logical_project() {
63                (Some(project), project.input())
64            } else {
65                (None, plan)
66            }
67        };
68        let filter = plan.as_logical_filter()?;
69        let plan = filter.input();
70        // The filter is directly on top of the over window after predicate pushdown.
71        let over_window = plan.as_logical_over_window()?;
72
73        // First try to simplify filter arithmetic expressions
74        let filter = if let Some(simplified) = self.simplify_filter_arithmetic(filter) {
75            simplified
76        } else {
77            filter.clone()
78        };
79
80        if over_window.window_functions().len() != 1 {
81            // Queries with multiple window function calls are not supported yet.
82            return None;
83        }
84        let window_func = &over_window.window_functions()[0];
85        if !window_func.kind.is_numbering() {
86            // Only rank functions can be converted to TopN.
87            return None;
88        }
89
90        let output_len = over_window.schema().len();
91        let window_func_pos = output_len - 1;
92
93        let with_ties = match window_func.kind {
94            // Only `ROW_NUMBER` and `RANK` can be optimized to TopN now.
95            WindowFuncKind::RowNumber => false,
96            WindowFuncKind::Rank => true,
97            WindowFuncKind::DenseRank => {
98                ctx.warn_to_user("`dense_rank` is not supported in Top-N pattern, will fallback to inefficient implementation");
99                return None;
100            }
101            _ => unreachable!("window functions other than rank functions should not reach here"),
102        };
103
104        let (rank_pred, other_pred) = {
105            let predicate = filter.predicate();
106            let mut rank_col = FixedBitSet::with_capacity(output_len);
107            rank_col.set(window_func_pos, true);
108            predicate.clone().split_disjoint(&rank_col)
109        };
110
111        let (limit, offset) = handle_rank_preds(&rank_pred.conjunctions, window_func_pos)?;
112
113        if offset > 0 && with_ties {
114            tracing::warn!("Failed to optimize with ties and offset");
115            ctx.warn_to_user("group topN with ties and offset is not supported, see https://www.risingwave.dev/docs/current/sql-pattern-topn/ for more information");
116            return None;
117        }
118
119        let topn: PlanRef = LogicalTopN::new(
120            over_window.input(),
121            limit,
122            offset,
123            with_ties,
124            Order {
125                column_orders: window_func.order_by.to_vec(),
126            },
127            window_func.partition_by.iter().map(|i| i.index).collect(),
128        )
129        .into();
130        let filter = LogicalFilter::create(topn, other_pred);
131
132        let plan = if let Some(project) = project {
133            let referred_cols = collect_input_refs(output_len, project.exprs());
134            if !referred_cols.contains(window_func_pos) {
135                // No Ranking Output
136                project.clone_with_input(filter).into()
137            } else {
138                // Ranking Output, with project
139                project
140                    .clone_with_input(over_window.clone_with_input(filter).into())
141                    .into()
142            }
143        } else {
144            // Ranking Output, without project
145            ctx.warn_to_user("It can be inefficient to output ranking number in Top-N, see https://www.risingwave.dev/docs/current/sql-pattern-topn/ for more information");
146            over_window.clone_with_input(filter).into()
147        };
148        Some(plan)
149    }
150}
151
152impl OverWindowToTopNRule {
153    /// Simplify arithmetic expressions in filter conditions before TopN optimization
154    /// For example: `(row_number - 1) = 0` -> `row_number = 1`
155    fn simplify_filter_arithmetic(&self, filter: &LogicalFilter) -> Option<LogicalFilter> {
156        let new_predicate = self.simplify_filter_arithmetic_condition(filter.predicate())?;
157        Some(LogicalFilter::new(filter.input(), new_predicate))
158    }
159
160    /// Simplify arithmetic expressions in the filter condition
161    fn simplify_filter_arithmetic_condition(&self, predicate: &Condition) -> Option<Condition> {
162        let expr = predicate.as_expr_unless_true()?;
163        let mut rewriter = FilterArithmeticRewriter {};
164        let new_expr = rewriter.rewrite_expr(expr.clone());
165
166        if new_expr != expr {
167            Some(Condition::with_expr(new_expr))
168        } else {
169            None
170        }
171    }
172}
173
174/// Filter arithmetic simplification rewriter: simplifies `(col op const) = const2` to `col = (const2 reverse_op const)`
175struct FilterArithmeticRewriter {}
176
177impl ExprRewriter for FilterArithmeticRewriter {
178    fn rewrite_function_call(&mut self, func_call: FunctionCall) -> ExprImpl {
179        use ExprType::{
180            Equal, GreaterThan, GreaterThanOrEqual, LessThan, LessThanOrEqual, NotEqual,
181        };
182
183        // Check if this is a comparison operation
184        match func_call.func_type() {
185            Equal | NotEqual | LessThan | LessThanOrEqual | GreaterThan | GreaterThanOrEqual => {
186                let inputs = func_call.inputs();
187                if inputs.len() == 2 {
188                    // Check if left operand is an arithmetic expression and right operand is a constant
189                    if let ExprImpl::FunctionCall(left_func) = &inputs[0]
190                        && inputs[1].is_const()
191                        && let Some(simplified) = self.simplify_arithmetic_comparison(
192                            left_func,
193                            &inputs[1],
194                            func_call.func_type(),
195                        )
196                    {
197                        return simplified;
198                    }
199                }
200            }
201            _ => {}
202        }
203
204        // Recursively handle sub-expressions
205        let (func_type, inputs, ret_type) = func_call.decompose();
206        let new_inputs: Vec<_> = inputs
207            .into_iter()
208            .map(|input| self.rewrite_expr(input))
209            .collect();
210
211        FunctionCall::new_unchecked(func_type, new_inputs, ret_type).into()
212    }
213}
214
215impl FilterArithmeticRewriter {
216    /// Simplify arithmetic comparison: `(col op const1) comp const2` -> `col comp (const2 reverse_op const1)`
217    fn simplify_arithmetic_comparison(
218        &self,
219        arithmetic_func: &FunctionCall,
220        comparison_const: &ExprImpl,
221        comparison_op: ExprType,
222    ) -> Option<ExprImpl> {
223        use ExprType::{Add, Subtract};
224
225        // Check arithmetic operation
226        match arithmetic_func.func_type() {
227            Add | Subtract => {
228                let inputs = arithmetic_func.inputs();
229                if inputs.len() == 2 {
230                    // Find column reference and constant
231                    let (column_ref, arith_const, reverse_op) = if inputs[1].is_const() {
232                        // col op const
233                        let reverse_op = match arithmetic_func.func_type() {
234                            Add => Subtract,
235                            Subtract => Add,
236                            _ => unreachable!(),
237                        };
238                        (&inputs[0], &inputs[1], reverse_op)
239                    } else if inputs[0].is_const() && arithmetic_func.func_type() == Add {
240                        // const + col
241                        (&inputs[1], &inputs[0], Subtract)
242                    } else {
243                        return None;
244                    };
245
246                    // Calculate new constant value
247                    if let Ok(new_const_func) = FunctionCall::new(
248                        reverse_op,
249                        vec![comparison_const.clone(), arith_const.clone()],
250                    ) {
251                        let new_const_expr: ExprImpl = new_const_func.into();
252                        // Try constant folding
253                        if let Some(Ok(Some(folded_value))) = new_const_expr.try_fold_const() {
254                            let new_const =
255                                Literal::new(Some(folded_value), new_const_expr.return_type())
256                                    .into();
257
258                            // Construct new comparison expression
259                            if let Ok(new_comparison) = FunctionCall::new(
260                                comparison_op,
261                                vec![column_ref.clone(), new_const],
262                            ) {
263                                return Some(new_comparison.into());
264                            }
265                        }
266                    }
267                }
268            }
269            _ => {}
270        }
271
272        None
273    }
274}
275
276/// Returns `None` if the conditions are too complex or invalid. `Some((limit, offset))` otherwise.
277fn handle_rank_preds(rank_preds: &[ExprImpl], window_func_pos: usize) -> Option<(u64, u64)> {
278    if rank_preds.is_empty() {
279        return None;
280    }
281
282    // rank >= lb
283    let mut lb: Option<i64> = None;
284    // rank <= ub
285    let mut ub: Option<i64> = None;
286    // rank == eq
287    let mut eq: Option<i64> = None;
288
289    for cond in rank_preds {
290        if let Some((input_ref, cmp, v)) = cond.as_comparison_const() {
291            assert_eq!(input_ref.index, window_func_pos);
292            let v = v
293                .cast_implicit(&DataType::Int64)
294                .ok()?
295                .fold_const()
296                .ok()??;
297            let v = *v.as_int64();
298            match cmp {
299                ExprType::LessThanOrEqual => ub = ub.map_or(Some(v), |ub| Some(ub.min(v))),
300                ExprType::LessThan => ub = ub.map_or(Some(v - 1), |ub| Some(ub.min(v - 1))),
301                ExprType::GreaterThan => lb = lb.map_or(Some(v + 1), |lb| Some(lb.max(v + 1))),
302                ExprType::GreaterThanOrEqual => lb = lb.map_or(Some(v), |lb| Some(lb.max(v))),
303                _ => unreachable!(),
304            }
305        } else if let Some((input_ref, v)) = cond.as_eq_const() {
306            assert_eq!(input_ref.index, window_func_pos);
307            let v = v
308                .cast_implicit(&DataType::Int64)
309                .ok()?
310                .fold_const()
311                .ok()??;
312            let v = *v.as_int64();
313            if let Some(eq) = eq
314                && eq != v
315            {
316                tracing::warn!(
317                    "Failed to optimize rank predicate with conflicting equal conditions."
318                );
319                return None;
320            }
321            eq = Some(v)
322        } else {
323            // TODO: support between and in
324            tracing::warn!("Failed to optimize complex rank predicate {:?}", cond);
325            return None;
326        }
327    }
328
329    // Note: rank functions start from 1
330    if let Some(eq) = eq {
331        if eq < 1 {
332            tracing::warn!(
333                "Failed to optimize rank predicate with invalid predicate rank={}.",
334                eq
335            );
336            return None;
337        }
338        let lb = lb.unwrap_or(i64::MIN);
339        let ub = ub.unwrap_or(i64::MAX);
340        if !(lb <= eq && eq <= ub) {
341            tracing::warn!("Failed to optimize rank predicate with conflicting bounds.");
342            return None;
343        }
344        Some((1, (eq - 1) as u64))
345    } else {
346        match (lb, ub) {
347            (Some(lb), Some(ub)) => Some(((ub - lb + 1).max(0) as u64, (lb - 1).max(0) as u64)),
348            (Some(lb), None) => Some((LIMIT_ALL_COUNT, (lb - 1).max(0) as u64)),
349            (None, Some(ub)) => Some((ub.max(0) as u64, 0)),
350            (None, None) => unreachable!(),
351        }
352    }
353}
risingwave_frontend/optimizer/rule/over_window_to_topn_rule.rs

risingwave_frontend/optimizer/rule/
over_window_to_topn_rule.rs
