ES|QL: Validate TOP_SNIPPETS query argument is foldable

docs/changelog/142462.yaml

@@ -0,0 +1,6 @@
+pr: 142462
+summary: "ES|QL: Validate TOP_SNIPPETS query argument is foldable at verification time"
+area: ES|QL
+type: bug
+issues:
+ - 142462

docs/changelog/142763.yaml

@@ -0,0 +1,6 @@
+area: ES|QL
+issues:
+ - 142462
+pr: 142763
+summary: Validate TOP_SNIPPETS query argument is foldable at verification
+type: enhancement

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

@@ -208,3 +208,39 @@ ROW content = "Sauron, the Dark Lord, has gathered to him all the Rings of Power
 snippets:keyword
 [All he lacks in his plans for dominion is the One Ring - the ring that rules them all - which has fallen into the hands of, ring that rules them all - which has fallen into the hands of the hobbit\, Bilbo Baggins.]
 ;
+
+topSnippetsWithConstantQuery
+required_capability: top_snippets_function
+
+FROM employees
+| EVAL snippets = TOP_SNIPPETS(first_name, "John")
+| KEEP emp_no, first_name, snippets
+| SORT emp_no
+| LIMIT 5
+;
+
+emp_no:integer | first_name:keyword | snippets:keyword
+10001          | Georgi             | null
+10002          | Bezalel            | null
+10003          | Parto              | null
+10004          | Chirstian          | null
+10005          | Kyoichi            | null
+;
+
+topSnippetsWithFoldableConcatQuery
+required_capability: top_snippets_function
+
+FROM employees
+| EVAL snippets = TOP_SNIPPETS(first_name, CONCAT("Jo", "hn"))
+| KEEP emp_no, first_name, snippets
+| SORT emp_no
+| LIMIT 5
+;
+
+emp_no:integer | first_name:keyword | snippets:keyword
+10001          | Georgi             | null
+10002          | Bezalel            | null
+10003          | Parto              | null
+10004          | Chirstian          | null
+10005          | Kyoichi            | null
+;

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/scalar/string/TopSnippets.java

@@ -20,6 +20,8 @@
 import org.elasticsearch.xpack.core.common.chunks.MemoryIndexChunkScorer;
 import org.elasticsearch.xpack.core.common.chunks.ScoredChunk;
 import org.elasticsearch.xpack.core.inference.chunking.SentenceBoundaryChunkingSettings;
+import org.elasticsearch.xpack.esql.capabilities.PostOptimizationVerificationAware;
+import org.elasticsearch.xpack.esql.common.Failures;
 import org.elasticsearch.xpack.esql.core.InvalidArgumentException;
 import org.elasticsearch.xpack.esql.core.expression.Expression;
 import org.elasticsearch.xpack.esql.core.expression.MapExpression;
@@ -49,11 +51,13 @@
 import static org.elasticsearch.xpack.esql.core.expression.TypeResolutions.ParamOrdinal.SECOND;
 import static org.elasticsearch.xpack.esql.core.expression.TypeResolutions.ParamOrdinal.THIRD;
 import static org.elasticsearch.xpack.esql.core.expression.TypeResolutions.isString;
+import static org.elasticsearch.xpack.esql.expression.Foldables.TypeResolutionValidator.forPostOptimizationValidation;
+import static org.elasticsearch.xpack.esql.expression.Foldables.resolveTypeQuery;
 import static org.elasticsearch.xpack.esql.expression.function.Options.resolve;
 import static org.elasticsearch.xpack.esql.expression.function.scalar.util.ChunkUtils.chunkText;
 import static org.elasticsearch.xpack.esql.expression.function.scalar.util.ChunkUtils.emitChunks;
 
-public class TopSnippets extends EsqlScalarFunction implements OptionalArgument {
+public class TopSnippets extends EsqlScalarFunction implements OptionalArgument, PostOptimizationVerificationAware {
 
     public static final NamedWriteableRegistry.Entry ENTRY = new NamedWriteableRegistry.Entry(
         Expression.class,
@@ -277,6 +281,11 @@ static void process(
         emitChunks(builder, snippets);
     }
 
+    @Override
+    public void postOptimizationVerification(Failures failures) {
+        resolveTypeQuery(query(), sourceText(), forPostOptimizationValidation(query(), failures));
+    }
+
     @Override
     public EvalOperator.ExpressionEvaluator.Factory toEvaluator(ToEvaluator toEvaluator) {
         int numSnippets;

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/expression/function/scalar/string/TopSnippetsTests.java

@@ -27,7 +27,6 @@
 
 import java.util.ArrayList;
 import java.util.List;
-import java.util.Locale;
 import java.util.Objects;
 import java.util.function.Supplier;
 import java.util.stream.Collectors;
@@ -38,7 +37,9 @@
 import static org.elasticsearch.xpack.esql.expression.function.scalar.string.TopSnippets.DEFAULT_NUM_SNIPPETS;
 import static org.elasticsearch.xpack.esql.expression.function.scalar.string.TopSnippets.DEFAULT_WORD_SIZE;
 import static org.elasticsearch.xpack.esql.expression.function.scalar.util.ChunkUtils.chunkText;
+import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.hasSize;
 
 public class TopSnippetsTests extends AbstractScalarFunctionTestCase {
 
@@ -245,14 +246,9 @@ public void testSnippetsReturnedInScoringOrder() {
 
         List<String> result = process(combinedText, query, 3, 50);
 
-        assertNotNull("Should return results for matching query", result);
-        assertFalse("Should have at least one result", result.isEmpty());
-
-        assertTrue(
-            "First snippet should be from the most relevant chunk (contains 'Elasticsearch' multiple times)",
-            result.get(0).toLowerCase(Locale.ROOT).contains("elasticsearch")
-                && (result.get(0).contains("powerful") || result.get(0).contains("supports") || result.get(0).contains("companies"))
-        );
+        assertThat(result, hasSize(2));
+        assertThat(result.get(0), containsString("Elasticsearch is a powerful search engine"));
+        assertThat(result.get(1), containsString("Elasticsearch is one option among several alternatives"));
     }
 
     private void verifySnippets(String query, Integer numSnippets, Integer numWords, int expectedNumChunksReturned) {
@@ -295,5 +291,4 @@ private List<String> process(String str, String query, int numSnippets, int numW
             }
         }
     }
-
 }

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/expression/function/scalar/string/TopSnippetsValidationTests.java

@@ -0,0 +1,55 @@
+/*
+ * 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.expression.function.scalar.string;
+
+import org.apache.lucene.util.BytesRef;
+import org.elasticsearch.test.ESTestCase;
+import org.elasticsearch.xpack.esql.common.Failures;
+import org.elasticsearch.xpack.esql.core.expression.Expression;
+import org.elasticsearch.xpack.esql.core.expression.FieldAttribute;
+import org.elasticsearch.xpack.esql.core.expression.Literal;
+import org.elasticsearch.xpack.esql.core.tree.Source;
+import org.elasticsearch.xpack.esql.core.type.DataType;
+import org.elasticsearch.xpack.esql.core.type.EsField;
+
+import java.util.Map;
+
+import static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.empty;
+import static org.hamcrest.Matchers.hasSize;
+import static org.hamcrest.Matchers.is;
+
+public class TopSnippetsValidationTests extends ESTestCase {
+
+    public void testValidateWithLiteralQuery() {
+        Expression field = fieldAttribute("body", DataType.TEXT);
+        Expression query = new Literal(Source.EMPTY, new BytesRef("search terms"), DataType.KEYWORD);
+        TopSnippets topSnippets = new TopSnippets(Source.synthetic("TOP_SNIPPETS(body, \"search terms\")"), field, query, null);
+
+        Failures failures = new Failures();
+        topSnippets.postOptimizationVerification(failures);
+
+        assertThat(failures.failures(), is(empty()));
+    }
+
+    public void testValidateWithFieldQuery() {
+        Expression field = fieldAttribute("body", DataType.TEXT);
+        Expression query = fieldAttribute("title", DataType.KEYWORD);
+        TopSnippets topSnippets = new TopSnippets(Source.synthetic("TOP_SNIPPETS(body, title)"), field, query, null);
+
+        Failures failures = new Failures();
+        topSnippets.postOptimizationVerification(failures);
+
+        assertThat(failures.failures(), hasSize(1));
+        assertThat(failures.failures().iterator().next().message(), containsString("Query must be a valid string"));
+    }
+
+    private static FieldAttribute fieldAttribute(String name, DataType type) {
+        return new FieldAttribute(Source.EMPTY, name, new EsField(name, type, Map.of(), true, EsField.TimeSeriesFieldType.NONE));
+    }
+}

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

@@ -9967,6 +9967,23 @@ STATS max(max_over_time(network.bytes_in)) by p = pod, bucket(@timestamp, 1 minu
         as(eval3.child(), EsRelation.class);
     }
 
+    public void testTopSnippetsQueryMustBeFoldable() {
+        var e = expectThrows(VerificationException.class, () -> optimizedPlan("FROM test | EVAL x = TOP_SNIPPETS(first_name, last_name)"));
+        assertThat(e.getMessage(), containsString("Query must be a valid string"));
+    }
+
+    public void testTopSnippetsQueryFoldableAfterOptimization() {
+        var plan = optimizedPlan("FROM test | EVAL x = TOP_SNIPPETS(first_name, \"search terms\")");
+        var failures = LogicalVerifier.INSTANCE.verify(plan, plan.output());
+        assertThat(failures.failures(), is(empty()));
+    }
+
+    public void testTopSnippetsQueryFoldableConcatConstants() {
+        var plan = optimizedPlan("FROM test | EVAL x = TOP_SNIPPETS(first_name, CONCAT(\"search\", \" terms\"))");
+        var failures = LogicalVerifier.INSTANCE.verify(plan, plan.output());
+        assertThat(failures.failures(), is(empty()));
+    }
+
     public void testPushDownSampleAndLimitThroughUriParts() {
         assumeTrue("requires compound output capability", EsqlCapabilities.Cap.URI_PARTS_COMMAND.isEnabled());
         var query = "FROM test | URI_PARTS parts = \"http://example.com/foo/bar?baz=qux\" | SAMPLE .5";