Block loaders for MV_MIN and MV_MAX for keywords

Adds special purpose `BlockLoader` implementations for the `MV_MIN` and
`MV_MAX` functions for `keyword` fields with doc values. These are a
noop for single valued keywords but should be *much* faster for
multivalued keywords.

These aren't plugged in yet. We can plug them in and performance test
them in elastic#137382. And they give us two more functions we can use to
demonstrate elastic#137382.

Author: nik9000

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

@@ -0,0 +1,206 @@
+/*
+ * 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.docvalues;
+
+import org.apache.lucene.index.DocValues;
+import org.apache.lucene.index.LeafReaderContext;
+import org.apache.lucene.index.SortedDocValues;
+import org.apache.lucene.index.SortedSetDocValues;
+import org.apache.lucene.util.BytesRef;
+
+import java.io.IOException;
+
+/**
+ * Loads {@code keyword} style fields that are stored as a lookup table.
+ */
+public abstract class AbstractBytesRefsFromOrdsBlockLoader extends BlockDocValuesReader.DocValuesBlockLoader {
+    protected final String fieldName;
+
+    public AbstractBytesRefsFromOrdsBlockLoader(String fieldName) {
+        this.fieldName = fieldName;
+    }
+
+    @Override
+    public BytesRefBuilder builder(BlockFactory factory, int expectedCount) {
+        return factory.bytesRefs(expectedCount);
+    }
+
+    @Override
+    public AllReader reader(LeafReaderContext context) throws IOException {
+        SortedSetDocValues docValues = context.reader().getSortedSetDocValues(fieldName);
+        if (docValues != null) {
+            SortedDocValues singleton = DocValues.unwrapSingleton(docValues);
+            if (singleton != null) {
+                return singletonReader(singleton);
+            }
+            return sortedSetReader(docValues);
+        }
+        SortedDocValues singleton = context.reader().getSortedDocValues(fieldName);
+        if (singleton != null) {
+            return singletonReader(singleton);
+        }
+        return new ConstantNullsReader();
+    }
+
+    protected abstract AllReader singletonReader(SortedDocValues docValues);
+
+    protected abstract AllReader sortedSetReader(SortedSetDocValues docValues);
+
+    protected static class Singleton extends BlockDocValuesReader {
+        private final SortedDocValues ordinals;
+
+        Singleton(SortedDocValues ordinals) {
+            this.ordinals = ordinals;
+        }
+
+        private Block readSingleDoc(BlockFactory factory, int docId) throws IOException {
+            if (ordinals.advanceExact(docId)) {
+                BytesRef v = ordinals.lookupOrd(ordinals.ordValue());
+                // the returned BytesRef can be reused
+                return factory.constantBytes(BytesRef.deepCopyOf(v), 1);
+            } else {
+                return factory.constantNulls(1);
+            }
+        }
+
+        @Override
+        public Block read(BlockFactory factory, Docs docs, int offset, boolean nullsFiltered) throws IOException {
+            if (docs.count() - offset == 1) {
+                return readSingleDoc(factory, docs.get(offset));
+            }
+            if (ordinals instanceof OptionalColumnAtATimeReader direct) {
+                Block block = direct.tryRead(factory, docs, offset, nullsFiltered, null, false);
+                if (block != null) {
+                    return block;
+                }
+            }
+            try (var builder = factory.singletonOrdinalsBuilder(ordinals, docs.count() - offset, false)) {
+                for (int i = offset; i < docs.count(); i++) {
+                    int doc = docs.get(i);
+                    if (ordinals.advanceExact(doc)) {
+                        builder.appendOrd(ordinals.ordValue());
+                    } else {
+                        builder.appendNull();
+                    }
+                }
+                return builder.build();
+            }
+        }
+
+        @Override
+        public void read(int docId, StoredFields storedFields, Builder builder) throws IOException {
+            if (ordinals.advanceExact(docId)) {
+                ((BytesRefBuilder) builder).appendBytesRef(ordinals.lookupOrd(ordinals.ordValue()));
+            } else {
+                builder.appendNull();
+            }
+        }
+
+        @Override
+        public int docId() {
+            return ordinals.docID();
+        }
+
+        @Override
+        public String toString() {
+            return "BytesRefsFromOrds.Singleton";
+        }
+    }
+
+    protected static class SortedSet extends BlockDocValuesReader {
+        private final SortedSetDocValues ordinals;
+
+        SortedSet(SortedSetDocValues ordinals) {
+            this.ordinals = ordinals;
+        }
+
+        @Override
+        public Block read(BlockFactory factory, Docs docs, int offset, boolean nullsFiltered) throws IOException {
+            if (docs.count() - offset == 1) {
+                return readSingleDoc(factory, docs.get(offset));
+            }
+            try (var builder = factory.sortedSetOrdinalsBuilder(ordinals, docs.count() - offset)) {
+                for (int i = offset; i < docs.count(); i++) {
+                    int doc = docs.get(i);
+                    if (doc < ordinals.docID()) {
+                        throw new IllegalStateException("docs within same block must be in order");
+                    }
+                    if (ordinals.advanceExact(doc) == false) {
+                        builder.appendNull();
+                        continue;
+                    }
+                    int count = ordinals.docValueCount();
+                    if (count == 1) {
+                        builder.appendOrd(Math.toIntExact(ordinals.nextOrd()));
+                    } else {
+                        builder.beginPositionEntry();
+                        for (int c = 0; c < count; c++) {
+                            builder.appendOrd(Math.toIntExact(ordinals.nextOrd()));
+                        }
+                        builder.endPositionEntry();
+                    }
+                }
+                return builder.build();
+            }
+        }
+
+        @Override
+        public void read(int docId, StoredFields storedFields, Builder builder) throws IOException {
+            read(docId, (BytesRefBuilder) builder);
+        }
+
+        private Block readSingleDoc(BlockFactory factory, int docId) throws IOException {
+            if (ordinals.advanceExact(docId) == false) {
+                return factory.constantNulls(1);
+            }
+            int count = ordinals.docValueCount();
+            if (count == 1) {
+                BytesRef v = ordinals.lookupOrd(ordinals.nextOrd());
+                return factory.constantBytes(BytesRef.deepCopyOf(v), 1);
+            }
+            try (var builder = factory.bytesRefsFromDocValues(count)) {
+                builder.beginPositionEntry();
+                for (int c = 0; c < count; c++) {
+                    BytesRef v = ordinals.lookupOrd(ordinals.nextOrd());
+                    builder.appendBytesRef(v);
+                }
+                builder.endPositionEntry();
+                return builder.build();
+            }
+        }
+
+        private void read(int docId, BytesRefBuilder builder) throws IOException {
+            if (false == ordinals.advanceExact(docId)) {
+                builder.appendNull();
+                return;
+            }
+            int count = ordinals.docValueCount();
+            if (count == 1) {
+                builder.appendBytesRef(ordinals.lookupOrd(ordinals.nextOrd()));
+                return;
+            }
+            builder.beginPositionEntry();
+            for (int v = 0; v < count; v++) {
+                builder.appendBytesRef(ordinals.lookupOrd(ordinals.nextOrd()));
+            }
+            builder.endPositionEntry();
+        }
+
+        @Override
+        public int docId() {
+            return ordinals.docID();
+        }
+
+        @Override
+        public String toString() {
+            return "BytesRefsFromOrds.SortedSet";
+        }
+    }
+}