ESQL: Make field fusion generic

docs/changelog/137382.yaml

@@ -0,0 +1,5 @@
+pr: 137382
+summary: Make field fusion generic
+area: ES|QL
+type: enhancement
+issues: []

server/src/main/java/module-info.java

@@ -501,4 +501,5 @@
     exports org.elasticsearch.index.codec.vectors.es93 to org.elasticsearch.test.knn;
     exports org.elasticsearch.search.crossproject;
     exports org.elasticsearch.index.mapper.blockloader.docvalues;
+    exports org.elasticsearch.index.mapper.blockloader;
 }

server/src/main/java/org/elasticsearch/index/mapper/KeywordFieldMapper.java

@@ -55,7 +55,9 @@
 import org.elasticsearch.index.fielddata.SourceValueFetcherSortedBinaryIndexFieldData;
 import org.elasticsearch.index.fielddata.StoredFieldSortedBinaryIndexFieldData;
 import org.elasticsearch.index.fielddata.plain.SortedSetOrdinalsIndexFieldData;
+import org.elasticsearch.index.mapper.blockloader.BlockLoaderFunctionConfig;
 import org.elasticsearch.index.mapper.blockloader.docvalues.BytesRefsFromOrdsBlockLoader;
+import org.elasticsearch.index.mapper.blockloader.docvalues.Utf8CodePointsFromOrdsBlockLoader;
 import org.elasticsearch.index.query.AutomatonQueryWithDescription;
 import org.elasticsearch.index.query.SearchExecutionContext;
 import org.elasticsearch.index.similarity.SimilarityProvider;
@@ -813,10 +815,19 @@ NamedAnalyzer normalizer() {
         @Override
         public BlockLoader blockLoader(BlockLoaderContext blContext) {
             if (hasDocValues() && (blContext.fieldExtractPreference() != FieldExtractPreference.STORED || isSyntheticSourceEnabled())) {
-                return new BytesRefsFromOrdsBlockLoader(name());
+                return switch (blContext.blockLoaderFunctionConfig()) {
+                    case null -> new BytesRefsFromOrdsBlockLoader(name());
+                    case BlockLoaderFunctionConfig.Named named -> switch (named.name()) {
+                        case "LENGTH" -> new Utf8CodePointsFromOrdsBlockLoader(named.warnings(), name());
+                        default -> throw new UnsupportedOperationException("unknown fusion config [" + named.name() + "]");
+                    };
+                    default -> throw new UnsupportedOperationException(
+                        "unknown fusion config [" + blContext.blockLoaderFunctionConfig() + "]"
+                    );
+                };
             }
-            if (isStored()) {
-                return new BlockStoredFieldsReader.BytesFromBytesRefsBlockLoader(name());
+            if (blContext.blockLoaderFunctionConfig() != null) {
+                throw new UnsupportedOperationException("function fusing only supported for doc values");
             }
 
             // Multi fields don't have fallback synthetic source.

server/src/main/java/org/elasticsearch/index/mapper/MappedFieldType.java

@@ -35,6 +35,7 @@
 import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.index.fielddata.FieldDataContext;
 import org.elasticsearch.index.fielddata.IndexFieldData;
+import org.elasticsearch.index.mapper.blockloader.BlockLoaderFunctionConfig;
 import org.elasticsearch.index.query.DistanceFeatureQueryBuilder;
 import org.elasticsearch.index.query.QueryRewriteContext;
 import org.elasticsearch.index.query.QueryShardException;
@@ -710,11 +711,4 @@ default BlockLoaderFunctionConfig blockLoaderFunctionConfig() {
         }
     }
 
-    /**
-     * Marker interface that contains the configuration needed to transform loaded values into blocks.
-     * Is retrievable from the {@link BlockLoaderContext}. The {@link MappedFieldType} can use this configuration to choose the appropriate
-     * implementation for transforming loaded values into blocks.
-     */
-    public interface BlockLoaderFunctionConfig {}
-
 }

server/src/main/java/org/elasticsearch/index/mapper/blockloader/BlockLoaderFunctionConfig.java

@@ -0,0 +1,23 @@
+/*
+ * 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", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+package org.elasticsearch.index.mapper.blockloader;
+
+import org.elasticsearch.index.mapper.MappedFieldType;
+
+/**
+ * Configuration needed to transform loaded values into blocks.
+ * {@link MappedFieldType}s will find me in
+ * {@link MappedFieldType.BlockLoaderContext#blockLoaderFunctionConfig()} and
+ * use this configuration to choose the appropriate implementation for
+ * transforming loaded values into blocks.
+ */
+public interface BlockLoaderFunctionConfig {
+    record Named(String name, Warnings warnings) implements BlockLoaderFunctionConfig {}
+}

server/src/main/java/org/elasticsearch/index/mapper/blockloader/docvalues/Warnings.java → server/src/main/java/org/elasticsearch/index/mapper/blockloader/Warnings.java

@@ -7,7 +7,7 @@
  * License v3.0 only", or the "Server Side Public License, v 1".
  */
 
-package org.elasticsearch.index.mapper.blockloader.docvalues;
+package org.elasticsearch.index.mapper.blockloader;
 
 /**
  * Warnings returned when loading values for ESQL. These are returned as HTTP 299 headers like so:
@@ -17,7 +17,7 @@
  *  < Warning: 299 Elasticsearch-${ver} "Line 1:27: java.lang.IllegalArgumentException: single-value function encountered multi-value"
  * }</pre>
  */
-interface Warnings {
+public interface Warnings {
     /**
      * Register a warning. ESQL deduplicates and limits the number of warnings returned so it should
      * be fine to blast as many warnings into this as you encounter.

server/src/main/java/org/elasticsearch/index/mapper/blockloader/docvalues/Utf8CodePointsFromOrdsBlockLoader.java

@@ -16,11 +16,12 @@
 import org.apache.lucene.util.BytesRef;
 import org.apache.lucene.util.RamUsageEstimator;
 import org.apache.lucene.util.UnicodeUtil;
+import org.elasticsearch.index.mapper.blockloader.Warnings;
 
 import java.io.IOException;
 import java.util.Arrays;
 
-import static org.elasticsearch.index.mapper.blockloader.docvalues.Warnings.registerSingleValueWarning;
+import static org.elasticsearch.index.mapper.blockloader.Warnings.registerSingleValueWarning;
 
 /**
  * A count of utf-8 code points for {@code keyword} style fields that are stored as a lookup table.

server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java

@@ -73,6 +73,7 @@
 import org.elasticsearch.index.mapper.SourceLoader;
 import org.elasticsearch.index.mapper.SourceValueFetcher;
 import org.elasticsearch.index.mapper.ValueFetcher;
+import org.elasticsearch.index.mapper.blockloader.BlockLoaderFunctionConfig;
 import org.elasticsearch.index.mapper.blockloader.docvalues.DenseVectorBlockLoader;
 import org.elasticsearch.index.mapper.blockloader.docvalues.DenseVectorBlockLoaderProcessor;
 import org.elasticsearch.index.mapper.blockloader.docvalues.DenseVectorFromBinaryBlockLoader;
@@ -3173,10 +3174,10 @@ public interface SimilarityFunction {
     }
 
     /**
-     * Configuration for a {@link MappedFieldType.BlockLoaderFunctionConfig} that calculates vector similarity.
+     * Configuration for a {@link BlockLoaderFunctionConfig} that calculates vector similarity.
      * Functions that use this config should use SIMILARITY_FUNCTION_NAME as their name.
      */
-    public static class VectorSimilarityFunctionConfig implements MappedFieldType.BlockLoaderFunctionConfig {
+    public static class VectorSimilarityFunctionConfig implements BlockLoaderFunctionConfig {
 
         private final SimilarityFunction similarityFunction;
         private final float[] vector;
@@ -3185,7 +3186,6 @@ public static class VectorSimilarityFunctionConfig implements MappedFieldType.Bl
         public VectorSimilarityFunctionConfig(SimilarityFunction similarityFunction, float[] vector) {
             this.similarityFunction = similarityFunction;
             this.vector = vector;
-
         }
 
         /**

server/src/test/java/org/elasticsearch/index/mapper/blockloader/docvalues/MockWarnings.java

@@ -9,6 +9,8 @@
 
 package org.elasticsearch.index.mapper.blockloader.docvalues;
 
+import org.elasticsearch.index.mapper.blockloader.Warnings;
+
 import java.util.ArrayList;
 import java.util.List;
 

x-pack/plugin/esql-core/src/main/java/org/elasticsearch/xpack/esql/core/type/FunctionEsField.java

@@ -8,23 +8,23 @@
 package org.elasticsearch.xpack.esql.core.type;
 
 import org.elasticsearch.common.io.stream.StreamOutput;
-import org.elasticsearch.index.mapper.MappedFieldType;
+import org.elasticsearch.index.mapper.blockloader.BlockLoaderFunctionConfig;
 
 import java.io.IOException;
 import java.util.Map;
 import java.util.Objects;
 
 /**
  * EsField that represents a function being applied to a field on extraction. It receives a
- * {@link org.elasticsearch.index.mapper.MappedFieldType.BlockLoaderFunctionConfig} that will be passed down to the block loading process
+ * {@link BlockLoaderFunctionConfig} that will be passed down to the block loading process
  * to apply the function at data load time.
  */
 public class FunctionEsField extends EsField {
 
     // Not serialized as it will be created on the data node
-    private final transient MappedFieldType.BlockLoaderFunctionConfig functionConfig;
+    private final transient BlockLoaderFunctionConfig functionConfig;
 
-    public FunctionEsField(EsField esField, DataType dataType, MappedFieldType.BlockLoaderFunctionConfig functionConfig) {
+    public FunctionEsField(EsField esField, DataType dataType, BlockLoaderFunctionConfig functionConfig) {
         this(
             esField.getName(),
             dataType,
@@ -43,7 +43,7 @@ private FunctionEsField(
         boolean aggregatable,
         boolean isAlias,
         TimeSeriesFieldType timeSeriesFieldType,
-        MappedFieldType.BlockLoaderFunctionConfig functionConfig
+        BlockLoaderFunctionConfig functionConfig
     ) {
         super(name, esDataType, properties, aggregatable, isAlias, timeSeriesFieldType);
         this.functionConfig = functionConfig;
@@ -54,7 +54,7 @@ public void writeTo(StreamOutput out) throws IOException {
         throw new UnsupportedOperationException("FunctionEsField is not serializable, should be created on data nodes");
     }
 
-    public MappedFieldType.BlockLoaderFunctionConfig functionConfig() {
+    public BlockLoaderFunctionConfig functionConfig() {
         return functionConfig;
     }
 

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/lucene/ShardContext.java

@@ -13,6 +13,7 @@
 import org.elasticsearch.index.mapper.BlockLoader;
 import org.elasticsearch.index.mapper.MappedFieldType;
 import org.elasticsearch.index.mapper.SourceLoader;
+import org.elasticsearch.index.mapper.blockloader.BlockLoaderFunctionConfig;
 import org.elasticsearch.search.sort.SortAndFormats;
 import org.elasticsearch.search.sort.SortBuilder;
 
@@ -58,7 +59,7 @@ BlockLoader blockLoader(
         String name,
         boolean asUnsupportedSource,
         MappedFieldType.FieldExtractPreference fieldExtractPreference,
-        MappedFieldType.BlockLoaderFunctionConfig blockLoaderFunctionConfig
+        BlockLoaderFunctionConfig blockLoaderFunctionConfig
     );
 
     /**

x-pack/plugin/esql/compute/src/test/java/org/elasticsearch/compute/lucene/LuceneSourceOperatorTests.java

@@ -43,6 +43,7 @@
 import org.elasticsearch.index.mapper.MappedFieldType;
 import org.elasticsearch.index.mapper.NumberFieldMapper;
 import org.elasticsearch.index.mapper.SourceLoader;
+import org.elasticsearch.index.mapper.blockloader.BlockLoaderFunctionConfig;
 import org.elasticsearch.indices.CrankyCircuitBreakerService;
 import org.elasticsearch.search.internal.ContextIndexSearcher;
 import org.elasticsearch.search.sort.SortAndFormats;
@@ -467,7 +468,7 @@ public BlockLoader blockLoader(
             String name,
             boolean asUnsupportedSource,
             MappedFieldType.FieldExtractPreference fieldExtractPreference,
-            MappedFieldType.BlockLoaderFunctionConfig blockLoaderFunctionConfig
+            BlockLoaderFunctionConfig blockLoaderFunctionConfig
         ) {
             throw new UnsupportedOperationException();
         }

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/blockloader/BlockLoaderExpression.java

@@ -7,17 +7,50 @@
 
 package org.elasticsearch.xpack.esql.expression.function.blockloader;
 
-import org.elasticsearch.index.mapper.MappedFieldType;
+import org.elasticsearch.compute.data.Block;
+import org.elasticsearch.core.Nullable;
+import org.elasticsearch.index.mapper.blockloader.BlockLoaderFunctionConfig;
+import org.elasticsearch.xpack.esql.core.expression.Expression;
+import org.elasticsearch.xpack.esql.core.expression.FieldAttribute;
+import org.elasticsearch.xpack.esql.stats.SearchStats;
 
 /**
- * {@link org.elasticsearch.xpack.esql.core.expression.Expression}s that can be implemented as part of value loading implement this
- * interface to provide the {@link MappedFieldType.BlockLoaderFunctionConfig} that will be used to load and
- * transform the value of the field.
+ * {@link Expression} that can be "fused" into value loading. Most of the time
+ * we load values into {@link Block}s and then run the expressions on them, but
+ * sometimes it's worth short-circuiting this process and running the expression
+ * in the tight loop we use for loading:
+ * <ul>
+ *     <li>
+ *         {@code V_COSINE(vector, [constant_vector])} - vector is ~512 floats
+ *         and V_COSINE is one double.
+ *     </li>
+ *     <li>
+ *         {@code ST_CENTROID(shape)} - shapes can be quite large. Centroids
+ *         are just one point.
+ *     </li>
+ *     <li>
+ *         {@code LENGTH(string)} - strings can be quite long, but string length
+ *         is always an int. For more fun, {@code keyword}s are usually stored
+ *         using a dictionary, and it's <strong>fairly</strong> easy to optimize
+ *         running {@code LENGTH} once per dictionary entry.
+ *     </li>
+ *     <li>
+ *         {@code MV_COUNT(anything)} - counts are always integers.
+ *     </li>
+ * </ul>
  */
 public interface BlockLoaderExpression {
+    /**
+     * The field and loading configuration that replaces this expression, effectively
+     * "fusing" the expression into the load. Or null if the fusion isn't possible.
+     */
+    @Nullable
+    Fuse tryFuse(SearchStats stats);
 
     /**
-     * Returns the configuration that will be used to load the value of the field and transform it
+     * Fused load configuration.
+     * @param field the field whose load we're fusing into
+     * @param config the fusion configuration
      */
-    MappedFieldType.BlockLoaderFunctionConfig getBlockLoaderFunctionConfig();
+    record Fuse(FieldAttribute field, BlockLoaderFunctionConfig config) {}
 }

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

@@ -13,22 +13,26 @@
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.compute.ann.Evaluator;
 import org.elasticsearch.compute.operator.EvalOperator.ExpressionEvaluator;
+import org.elasticsearch.index.mapper.blockloader.BlockLoaderFunctionConfig;
 import org.elasticsearch.xpack.esql.core.expression.Expression;
+import org.elasticsearch.xpack.esql.core.expression.FieldAttribute;
 import org.elasticsearch.xpack.esql.core.tree.NodeInfo;
 import org.elasticsearch.xpack.esql.core.tree.Source;
 import org.elasticsearch.xpack.esql.core.type.DataType;
 import org.elasticsearch.xpack.esql.expression.function.Example;
 import org.elasticsearch.xpack.esql.expression.function.FunctionInfo;
 import org.elasticsearch.xpack.esql.expression.function.Param;
+import org.elasticsearch.xpack.esql.expression.function.blockloader.BlockLoaderExpression;
 import org.elasticsearch.xpack.esql.expression.function.scalar.UnaryScalarFunction;
+import org.elasticsearch.xpack.esql.stats.SearchStats;
 
 import java.io.IOException;
 import java.util.List;
 
 import static org.elasticsearch.xpack.esql.core.expression.TypeResolutions.ParamOrdinal.DEFAULT;
 import static org.elasticsearch.xpack.esql.core.expression.TypeResolutions.isString;
 
-public class Length extends UnaryScalarFunction {
+public class Length extends UnaryScalarFunction implements BlockLoaderExpression {
     public static final NamedWriteableRegistry.Entry ENTRY = new NamedWriteableRegistry.Entry(Expression.class, "Length", Length::new);
 
     @FunctionInfo(
@@ -90,4 +94,17 @@ protected NodeInfo<? extends Expression> info() {
     public ExpressionEvaluator.Factory toEvaluator(ToEvaluator toEvaluator) {
         return new LengthEvaluator.Factory(source(), toEvaluator.apply(field()));
     }
+
+    @Override
+    public Fuse tryFuse(SearchStats stats) {
+        if (field instanceof FieldAttribute f) {
+            if (stats.hasDocValues(f.fieldName()) == false) {
+                return null;
+            }
+            return new Fuse(f, new BlockLoaderFunctionConfig.Named("LENGTH", (exceptionClass, message) -> {
+                throw new IllegalStateException("NOCOMMIT");
+            }));
+        }
+        return null;
+    }
 }