/*
 * Licensed to Elasticsearch under one or more contributor
 * license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright
 * ownership. Elasticsearch licenses this file to you under
 * the Apache License, Version 2.0 (the "License"); you may
 * not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.elasticsearch.search.fetch.subphase;

import org.apache.lucene.index.NumericDocValues;
import org.elasticsearch.ElasticsearchException;
import org.elasticsearch.common.lucene.uid.Versions;
import org.elasticsearch.index.mapper.VersionFieldMapper;
import org.elasticsearch.search.fetch.FetchSubPhase;
import org.elasticsearch.search.internal.SearchContext;

import java.io.IOException;

public final class VersionFetchSubPhase implements FetchSubPhase {

    @Override
    public void hitExecute(SearchContext context, HitContext hitContext) {
        if (context.version() == false ||
            (context.storedFieldsContext() != null && context.storedFieldsContext().fetchFields() == false)) {
            return;
        }
        long version = Versions.NOT_FOUND;
        try {
            NumericDocValues versions = hitContext.reader().getNumericDocValues(VersionFieldMapper.NAME);
            if (versions != null && versions.advanceExact(hitContext.docId())) {
                version = versions.longValue();
            }
        } catch (IOException e) {
            throw new ElasticsearchException("Could not retrieve version", e);
        }
        hitContext.hit().version(version < 0 ? -1 : version);
    }
}

/*
 * Licensed to Elasticsearch under one or more contributor
 * license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright
 * ownership. Elasticsearch licenses this file to you under
 * the Apache License, Version 2.0 (the "License"); you may
 * not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.elasticsearch.search.fetch.subphase;

import org.apache.lucene.index.LeafReaderContext;
import org.apache.lucene.index.NumericDocValues;
import org.apache.lucene.index.ReaderUtil;
import org.elasticsearch.common.lucene.uid.Versions;
import org.elasticsearch.index.mapper.VersionFieldMapper;
import org.elasticsearch.search.SearchHit;
import org.elasticsearch.search.fetch.FetchSubPhase;
import org.elasticsearch.search.internal.SearchContext;

import java.io.IOException;
import java.util.Arrays;
import java.util.Comparator;

public final class VersionFetchSubPhase implements FetchSubPhase {
    @Override
    public void hitsExecute(SearchContext context, SearchHit[] hits) throws IOException {
        if (context.version() == false ||
            (context.storedFieldsContext() != null && context.storedFieldsContext().fetchFields() == false)) {
            return;
        }

        hits = hits.clone(); // don't modify the incoming hits
        Arrays.sort(hits, Comparator.comparingInt(SearchHit::docId));

        int lastReaderId = -1;
        NumericDocValues versions = null;
        for (SearchHit hit : hits) {
            int readerId = ReaderUtil.subIndex(hit.docId(), context.searcher().getIndexReader().leaves());
            LeafReaderContext subReaderContext = context.searcher().getIndexReader().leaves().get(readerId);
            if (lastReaderId != readerId) {
                versions = subReaderContext.reader().getNumericDocValues(VersionFieldMapper.NAME);
                lastReaderId = readerId;
            }
            int docId = hit.docId() - subReaderContext.docBase;
            long version = Versions.NOT_FOUND;
            if (versions != null && versions.advanceExact(docId)) {
                version = versions.longValue();
            }
            hit.version(version < 0 ? -1 : version);
        }
    }
}

Change VersionFetchSubPhase to create doc values iterator once per segment (#26809)