ESQL: Remove redundant TopN

docs/changelog/131833.yaml

@@ -0,0 +1,6 @@
+pr: 131833
+summary: "ESQL: Remove redundant `TopN`"
+area: ES|QL
+type: enhancement
+issues:
+  - 131233

x-pack/plugin/esql/qa/testFixtures/src/main/resources/topN.csv-spec

@@ -168,3 +168,161 @@ FROM employees
 avg_worked_seconds:long | birth_date:date      | emp_no:i | gender:k | height:d | height.float:d     | height.half_float:d | height.scaled_float:d | hire_date:date       | is_rehired:bool      | job_positions:k     | languages:i | languages.byte:i | languages.long:l | languages.short:short | salary:i | salary_change:d | salary_change.int:i | salary_change.keyword:k | salary_change.long:l | still_hired:bool | name:k  | first_name:k | last_name:k
 349086555               | 1961-09-01T00:00:00Z | 10056    | F        | 1.57     | 1.5700000524520874 | 1.5703125           | 1.57                  | 1990-02-01T00:00:00Z | [false, false, true] | [Senior Team Lead]  | 2           | 2                | 2                | 2                     | 33370    | [-5.17, 10.99]  | [-5, 10]            | [-5.17, 10.99]          | [-5, 10]             | true             | Brendon | Bernini      | Brendon
 ;
+
+
+redundantTopNBasic
+from employees 
+| sort emp_no 
+| limit 100 
+| sort emp_no 
+| limit 10 
+| keep emp_no, first_name
+;
+
+emp_no:integer|first_name:keyword
+10001         |Georgi        
+10002         |Bezalel       
+10003         |Parto         
+10004         |Chirstian     
+10005         |Kyoichi       
+10006         |Anneke        
+10007         |Tzvetan       
+10008         |Saniya        
+10009         |Sumant        
+10010         |Duangkaew
+;
+
+
+redundantTopNWithFilter
+from employees 
+| sort emp_no desc 
+| limit 50 
+| where gender == "M" 
+| sort emp_no desc 
+| limit 5 
+| keep emp_no, first_name, gender
+;
+
+emp_no:integer | first_name:keyword | gender:keyword
+10097          | Remzi              | M
+10096          | Jayson             | M
+10095          | Hilari             | M
+10093          | Sailaja            | M
+10091          | Amabile            | M
+;
+
+
+redundantTopNWithProject
+from employees 
+| sort salary desc 
+| limit 20 
+| keep emp_no, first_name, salary 
+| sort salary desc 
+| limit 3
+;
+
+emp_no:integer | first_name:keyword | salary:integer
+10029          | Otmar              | 74999
+10045          | Moss               | 74970
+10007          | Tzvetan            | 74572
+;
+
+
+redundantTopNWithEval
+from employees 
+| sort emp_no 
+| limit 15 
+| eval double_emp_no = emp_no * 2 
+| sort emp_no 
+| limit 5 
+| keep emp_no, first_name, double_emp_no
+;
+
+emp_no:integer|first_name:keyword|double_emp_no:integer
+10001         |Georgi            |20002                
+10002         |Bezalel           |20004                
+10003         |Parto             |20006                
+10004         |Chirstian         |20008                
+10005         |Kyoichi           |20010
+;
+
+
+redundantTopNWithRename
+from employees 
+| sort first_name 
+| limit 20 
+| rename first_name as name 
+| sort name 
+| limit 8 
+| keep emp_no, name
+;
+
+emp_no:integer | name:keyword
+10059          | Alejandro
+10091          | Amabile
+10006          | Anneke
+10062          | Anoosh
+10094          | Arumugam
+10049          | Basil
+10058          | Berhard
+10014          | Berni
+;
+
+
+nonRedundantTopNDifferentOrder
+from employees 
+| sort emp_no 
+| limit 10 
+| sort salary desc 
+| limit 5 
+| keep emp_no, first_name, salary
+;
+
+emp_no:integer | first_name:keyword | salary:integer
+10007          | Tzvetan            | 74572
+10009          | Sumant             | 66174
+10005          | Kyoichi            | 63528
+10003          | Parto              | 61805
+10006          | Anneke             | 60335
+;
+
+
+redundantTopNMultipleFields
+from employees 
+| sort gender, emp_no 
+| limit 20 
+| where salary > 50000 
+| sort gender, emp_no 
+| limit 8 
+| keep emp_no, first_name, gender, salary
+;
+
+emp_no:integer | first_name:keyword | gender:keyword | salary:integer
+10002          | Bezalel            | F              | 56371
+10006          | Anneke             | F              | 60335
+10007          | Tzvetan            | F              | 74572
+10009          | Sumant             | F              | 66174
+10024          | Suzette            | F              | 64675
+10027          | Divier             | F              | 73851
+10032          | null               | F              | 62233
+10041          | Uri                | F              | 56415
+;
+
+
+redundantTopNWithStats
+from employees 
+| sort gender 
+| limit 10 
+| stats cnt = count(*) by emp_no, gender
+| sort gender
+| limit 5 
+| keep emp_no, gender, cnt
+;
+
+emp_no:integer | gender:keyword | cnt:long
+10023          | F              | 1
+10027          | F              | 1
+10006          | F              | 1
+10007          | F              | 1
+10040          | F              | 1
+;

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/optimizer/LogicalPlanOptimizer.java

@@ -37,6 +37,7 @@
 import org.elasticsearch.xpack.esql.optimizer.rules.logical.PruneLiteralsInOrderBy;
 import org.elasticsearch.xpack.esql.optimizer.rules.logical.PruneRedundantOrderBy;
 import org.elasticsearch.xpack.esql.optimizer.rules.logical.PruneRedundantSortClauses;
+import org.elasticsearch.xpack.esql.optimizer.rules.logical.PruneRedundantTopN;
 import org.elasticsearch.xpack.esql.optimizer.rules.logical.PruneUnusedIndexMode;
 import org.elasticsearch.xpack.esql.optimizer.rules.logical.PushDownAndCombineFilters;
 import org.elasticsearch.xpack.esql.optimizer.rules.logical.PushDownAndCombineLimits;
@@ -219,6 +220,7 @@ protected static Batch<LogicalPlan> cleanup() {
         return new Batch<>(
             "Clean Up",
             new ReplaceLimitAndSortAsTopN(),
+            new PruneRedundantTopN(),
             new HoistRemoteEnrichTopN(),
             new ReplaceRowAsLocalRelation(),
             new PropgateUnmappedFields(),

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/optimizer/rules/logical/PruneRedundantTopN.java

@@ -0,0 +1,102 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+package org.elasticsearch.xpack.esql.optimizer.rules.logical;
+
+import org.elasticsearch.xpack.esql.optimizer.LogicalOptimizerContext;
+import org.elasticsearch.xpack.esql.plan.logical.Drop;
+import org.elasticsearch.xpack.esql.plan.logical.Eval;
+import org.elasticsearch.xpack.esql.plan.logical.Filter;
+import org.elasticsearch.xpack.esql.plan.logical.Insist;
+import org.elasticsearch.xpack.esql.plan.logical.Limit;
+import org.elasticsearch.xpack.esql.plan.logical.LogicalPlan;
+import org.elasticsearch.xpack.esql.plan.logical.OrderBy;
+import org.elasticsearch.xpack.esql.plan.logical.Project;
+import org.elasticsearch.xpack.esql.plan.logical.Rename;
+import org.elasticsearch.xpack.esql.plan.logical.TopN;
+import org.elasticsearch.xpack.esql.plan.logical.inference.Completion;
+
+import java.util.ArrayDeque;
+import java.util.Collections;
+import java.util.Deque;
+import java.util.IdentityHashMap;
+import java.util.Set;
+
+/**
+ * Removes redundant TopN operations from the logical plan to improve execution efficiency.
+ * <p>
+ * Multiple TopN nodes may appear in a query plan—particularly after optimization passes—
+ * and some of them can be safely removed if they share the same sort order and are not separated
+ * by operations that disrupt sorting semantics.
+ * <p>
+ * For instance:
+ * <pre>
+ * from test | sort x | limit 100 | sort x | limit 10
+ * </pre>
+ * Both <code>sort x | limit 100</code> and <code>sort x | limit 10</code> will be transformed into TopN nodes.
+ * Since they sort by the same key and the latter applies a stricter (or equal) limit,
+ * the first TopN becomes redundant and can be pruned.
+ */
+public class PruneRedundantTopN extends OptimizerRules.ParameterizedOptimizerRule<TopN, LogicalOptimizerContext> {
+
+    public PruneRedundantTopN() {
+        super(OptimizerRules.TransformDirection.DOWN);
+    }
+
+    @Override
+    protected LogicalPlan rule(TopN plan, LogicalOptimizerContext ctx) {
+        Set<TopN> redundant = findRedundantTopN(plan, ctx);
+        if (redundant.isEmpty()) {
+            return plan;
+        }
+        return plan.transformDown(TopN.class, topN -> redundant.contains(topN) ? topN.child() : topN);
+    }
+
+    /**
+     * breadth-first recursion to find redundant TopNs in the children tree.
+     * Returns an identity set (we need to compare and prune the exact instances)
+     */
+    private Set<TopN> findRedundantTopN(TopN parentTopN, LogicalOptimizerContext ctx) {
+        Set<TopN> result = Collections.newSetFromMap(new IdentityHashMap<>());
+
+        Deque<LogicalPlan> toCheck = new ArrayDeque<>();
+        toCheck.push(parentTopN.child());
+
+        while (toCheck.isEmpty() == false) {
+            LogicalPlan p = toCheck.pop();
+            if (p instanceof TopN childTopN) {
+                // Check if a child TopN is redundant compared to a parent TopN.
+                // A child TopN is redundant if it matches the parent's sort order and has a greater or equal limit.
+                if (childTopN.order().equals(parentTopN.order())
+                    // Although `PushDownAndCombineLimits` is expected to have propagated the stricter (lower) limit,
+                    // we still compare limit values here to ensure correctness and avoid relying solely on prior optimizations.
+                    // This limit check should always pass, but we validate it explicitly for robustness.
+                    && (int) parentTopN.limit().fold(ctx.foldCtx()) <= (int) childTopN.limit().fold(ctx.foldCtx())) {
+                    result.add(childTopN);
+                    toCheck.push(childTopN.child());
+                }
+            } else if (canRemoveRedundantChildTopN(p)) {
+                for (LogicalPlan child : p.children()) {
+                    toCheck.push(child);
+                }
+            }
+        }
+        return result;
+    }
+
+    private boolean canRemoveRedundantChildTopN(LogicalPlan p) {
+        return p instanceof Completion
+            || p instanceof Drop
+            || p instanceof Eval
+            || p instanceof Rename
+            || p instanceof Filter
+            || p instanceof Insist
+            || p instanceof Limit
+            || p instanceof OrderBy
+            || p instanceof Project;
+    }
+}

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/optimizer/LogicalPlanOptimizerTests.java

@@ -8530,6 +8530,80 @@ public void testPruneRedundantOrderBy() {
         as(mvExpand2.child(), Row.class);
     }
 
+    /**
+     * <pre>{@code
+     * EsqlProject[[first_name{f}#6]]
+     * \_TopN[[Order[first_name{f}#6,ASC,LAST]],10[INTEGER]]
+     *   \_EsRelation[test][_meta_field{f}#11, emp_no{f}#5, first_name{f}#6, ge..]
+     * }</pre>
+     */
+    public void testPruneRedundantTopN() {
+        var plan = optimizedPlan("""
+            FROM test
+            | KEEP first_name
+            | SORT first_name
+            | LIMIT 100
+            | SORT first_name
+            | LIMIT 10
+            """);
+        var project = as(plan, Project.class);
+        var topN = as(project.child(), TopN.class);
+        as(topN.child(), EsRelation.class);
+    }
+
+    /**
+     * <pre>{@code
+     * EsqlProject[[first_name{f}#9, a{r}#6]]
+     * \_TopN[[Order[first_name{f}#9,ASC,LAST]],10[INTEGER]]
+     *   \_Eval[[12[INTEGER] AS a#6]]
+     *     \_EsRelation[test][_meta_field{f}#14, emp_no{f}#8, first_name{f}#9, ge..]
+     * }</pre>
+     */
+    public void testPruneRedundantTopNWithNodesInBetween() {
+        var plan = optimizedPlan("""
+            FROM test
+            | SORT first_name
+            | LIMIT 10
+            | DROP last_name
+            | KEEP first_name
+            | EVAL a = 12
+            | SORT first_name
+            | LIMIT 100
+            """);
+        var project = as(plan, Project.class);
+        var topN = as(project.child(), TopN.class);
+        var eval = as(topN.child(), Eval.class);
+        as(eval.child(), EsRelation.class);
+    }
+
+    /**
+     * <pre>{@code
+     * Project[[avg{r}#6, first_name{f}#9]]
+     * \_TopN[[Order[first_name{f}#9,ASC,LAST]],100[INTEGER]]
+     *   \_Eval[[$$SUM$avg$0{r$}#19 / $$COUNT$avg$1{r$}#20 AS avg#6]]
+     *     \_Aggregate[[first_name{f}#9],[SUM(salary{f}#13,true[BOOLEAN],compensated[KEYWORD]) AS $$SUM$avg$0#19, COUNT(salary{f}#13,t
+     * rue[BOOLEAN]) AS $$COUNT$avg$1#20, first_name{f}#9]]
+     *       \_TopN[[Order[first_name{f}#9,ASC,LAST]],10[INTEGER]]
+     *         \_EsRelation[test][_meta_field{f}#14, emp_no{f}#8, first_name{f}#9, ge..]
+     * }</pre>
+     */
+    public void testCanNotPruneRedundantTopNWithNodesInBetween() {
+        var plan = optimizedPlan("""
+            FROM test
+            | SORT first_name
+            | LIMIT 10
+            | STATS avg = AVG(salary) BY first_name
+            | SORT first_name
+            | LIMIT 100
+            """);
+        var project = as(plan, Project.class);
+        var topN = as(project.child(), TopN.class);
+        var eval = as(topN.child(), Eval.class);
+        var agg = as(eval.child(), Aggregate.class);
+        var topN2 = as(agg.child(), TopN.class);
+        as(topN2.child(), EsRelation.class);
+    }
+
     /**
      * <pre>{@code
      * Eval[[1[INTEGER] AS irrelevant1, 2[INTEGER] AS irrelevant2]]