Use index-prefix fields for terms of length min_chars - 1 (#36703)

The default index_prefix settings will index prefixes of between 2 and 5 characters in length. 
Currently, if a prefix search falls outside of this range at either end we fall back to a standard prefix 
expansion, which is still very expensive for single character prefixes. However, we have an option 
here to use a wildcard expansion rather than a prefix expansion, so that a query of a* gets remapped 
to a? against the _index_prefix field - likely to be a very small set of terms, and certain to be much
smaller than a* against the whole index.

This commit adds this extra level of mapping for any prefix term whose length is one less than
the min_chars parameter of the index_prefixes field.

rest-api-spec/src/main/resources/rest-api-spec/test/search/190_index_prefix_search.yml

@@ -58,6 +58,21 @@ setup:
   - match: {hits.max_score: 2}
   - match: {hits.hits.0._score: 2}
 
+  - do:
+      search:
+        rest_total_hits_as_int: true
+        index: test
+        body:
+          query:
+            query_string:
+              default_field: text
+              query: s*
+              boost: 2
+
+  - match: {hits.total: 1}
+  - match: {hits.max_score: 2}
+  - match: {hits.hits.0._score: 2}
+
   - do:
       search:
         rest_total_hits_as_int: true

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

@@ -32,6 +32,7 @@ import org.apache.lucene.document.Field;
 import org.apache.lucene.index.IndexOptions;
 import org.apache.lucene.index.IndexableField;
 import org.apache.lucene.index.Term;
+import org.apache.lucene.search.AutomatonQuery;
 import org.apache.lucene.search.ConstantScoreQuery;
 import org.apache.lucene.search.MultiPhraseQuery;
 import org.apache.lucene.search.MultiTermQuery;
@@ -40,6 +41,9 @@ import org.apache.lucene.search.PhraseQuery;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.search.TermQuery;
 import org.apache.lucene.search.intervals.IntervalsSource;
+import org.apache.lucene.util.automaton.Automata;
+import org.apache.lucene.util.automaton.Automaton;
+import org.apache.lucene.util.automaton.Operations;
 import org.elasticsearch.Version;
 import org.elasticsearch.common.collect.Iterators;
 import org.elasticsearch.common.settings.Settings;
@@ -360,7 +364,7 @@ public class TextFieldMapper extends FieldMapper {
         }
 
         boolean accept(int length) {
-            return length >= minChars && length <= maxChars;
+            return length >= minChars - 1 && length <= maxChars;
         }
 
         void doXContent(XContentBuilder builder) throws IOException {
@@ -370,6 +374,22 @@ public class TextFieldMapper extends FieldMapper {
             builder.endObject();
         }
 
+        @Override
+        public Query prefixQuery(String value, MultiTermQuery.RewriteMethod method, QueryShardContext context) {
+            if (value.length() >= minChars) {
+                return super.termQuery(value, context);
+            }
+            List<Automaton> automata = new ArrayList<>();
+            automata.add(Automata.makeString(value));
+            for (int i = value.length(); i < minChars; i++) {
+                automata.add(Automata.makeAnyChar());
+            }
+            Automaton automaton = Operations.concatenate(automata);
+            AutomatonQuery query = new AutomatonQuery(new Term(name(), value + "*"), automaton);
+            query.setRewriteMethod(method);
+            return query;
+        }
+
         @Override
         public PrefixFieldType clone() {
             return new PrefixFieldType(name(), minChars, maxChars);
@@ -402,7 +422,6 @@ public class TextFieldMapper extends FieldMapper {
 
         @Override
         public int hashCode() {
-
             return Objects.hash(super.hashCode(), minChars, maxChars);
         }
     }
@@ -564,7 +583,7 @@ public class TextFieldMapper extends FieldMapper {
             if (prefixFieldType == null || prefixFieldType.accept(value.length()) == false) {
                 return super.prefixQuery(value, method, context);
             }
-            Query tq = prefixFieldType.termQuery(value, context);
+            Query tq = prefixFieldType.prefixQuery(value, method, context);
             if (method == null || method == MultiTermQuery.CONSTANT_SCORE_REWRITE
                 || method == MultiTermQuery.CONSTANT_SCORE_BOOLEAN_REWRITE) {
                 return new ConstantScoreQuery(tq);

server/src/test/java/org/elasticsearch/index/mapper/TextFieldMapperTests.java

@@ -31,10 +31,8 @@ import org.apache.lucene.index.LeafReader;
 import org.apache.lucene.index.PostingsEnum;
 import org.apache.lucene.index.Term;
 import org.apache.lucene.index.TermsEnum;
-import org.apache.lucene.search.ConstantScoreQuery;
 import org.apache.lucene.search.MultiPhraseQuery;
 import org.apache.lucene.search.PhraseQuery;
-import org.apache.lucene.search.PrefixQuery;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.search.TermQuery;
 import org.apache.lucene.util.BytesRef;
@@ -71,10 +69,8 @@ import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 
-import static org.apache.lucene.search.MultiTermQuery.CONSTANT_SCORE_REWRITE;
 import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
-import static org.hamcrest.Matchers.instanceOf;
 import static org.hamcrest.core.Is.is;
 
 public class TextFieldMapperTests extends ESSingleNodeTestCase {
@@ -817,18 +813,13 @@ public class TextFieldMapperTests extends ESSingleNodeTestCase {
 
     public void testIndexPrefixMapping() throws IOException {
 
-        QueryShardContext queryShardContext = indexService.newQueryShardContext(
-            randomInt(20), null, () -> {
-                throw new UnsupportedOperationException();
-            }, null);
-
         {
             String mapping = Strings.toString(XContentFactory.jsonBuilder().startObject().startObject("type")
                 .startObject("properties").startObject("field")
                 .field("type", "text")
                 .field("analyzer", "standard")
                 .startObject("index_prefixes")
-                .field("min_chars", 1)
+                .field("min_chars", 2)
                 .field("max_chars", 10)
                 .endObject()
                 .endObject().endObject()
@@ -837,16 +828,7 @@ public class TextFieldMapperTests extends ESSingleNodeTestCase {
             DocumentMapper mapper = parser.parse("type", new CompressedXContent(mapping));
             assertEquals(mapping, mapper.mappingSource().toString());
 
-            assertThat(mapper.mappers().getMapper("field._index_prefix").toString(), containsString("prefixChars=1:10"));
-
-            FieldMapper fieldMapper = (FieldMapper) mapper.mappers().getMapper("field");
-            MappedFieldType fieldType = fieldMapper.fieldType;
-
-            Query q = fieldType.prefixQuery("goin", CONSTANT_SCORE_REWRITE, queryShardContext);
-
-            assertEquals(new ConstantScoreQuery(new TermQuery(new Term("field._index_prefix", "goin"))), q);
-            q = fieldType.prefixQuery("internationalisatio", CONSTANT_SCORE_REWRITE, queryShardContext);
-            assertEquals(new PrefixQuery(new Term("field", "internationalisatio")), q);
+            assertThat(mapper.mappers().getMapper("field._index_prefix").toString(), containsString("prefixChars=2:10"));
 
             ParsedDocument doc = mapper.parse(SourceToParse.source("test", "type", "1", BytesReference
                     .bytes(XContentFactory.jsonBuilder()
@@ -870,17 +852,8 @@ public class TextFieldMapperTests extends ESSingleNodeTestCase {
             CompressedXContent json = new CompressedXContent(mapping);
             DocumentMapper mapper = parser.parse("type", json);
 
-            FieldMapper fieldMapper = (FieldMapper) mapper.mappers().getMapper("field");
-            MappedFieldType fieldType = fieldMapper.fieldType;
-
-            Query q1 = fieldType.prefixQuery("g", CONSTANT_SCORE_REWRITE, queryShardContext);
-            assertThat(q1, instanceOf(PrefixQuery.class));
-            Query q2 = fieldType.prefixQuery("go", CONSTANT_SCORE_REWRITE, queryShardContext);
-            assertThat(q2, instanceOf(ConstantScoreQuery.class));
-            Query q5 = fieldType.prefixQuery("going", CONSTANT_SCORE_REWRITE, queryShardContext);
-            assertThat(q5, instanceOf(ConstantScoreQuery.class));
-            Query q6 = fieldType.prefixQuery("goings", CONSTANT_SCORE_REWRITE, queryShardContext);
-            assertThat(q6, instanceOf(PrefixQuery.class));
+            assertThat(mapper.mappers().getMapper("field._index_prefix").toString(), containsString("prefixChars=2:5"));
+
         }
 
         {
@@ -898,10 +871,8 @@ public class TextFieldMapperTests extends ESSingleNodeTestCase {
                 .endObject().endObject()
                 .endObject().endObject());
 
-            IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> {
-                indexService.mapperService()
-                    .merge("type", new CompressedXContent(illegalMapping), MergeReason.MAPPING_UPDATE);
-            });
+            IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () ->
+                indexService.mapperService().merge("type", new CompressedXContent(illegalMapping), MergeReason.MAPPING_UPDATE));
             assertThat(e.getMessage(), containsString("Field [field._index_prefix] is defined twice in [type]"));
 
         }

server/src/test/java/org/elasticsearch/index/mapper/TextFieldTypeTests.java

@@ -20,11 +20,18 @@ package org.elasticsearch.index.mapper;
 
 import org.apache.lucene.index.IndexOptions;
 import org.apache.lucene.index.Term;
+import org.apache.lucene.search.AutomatonQuery;
+import org.apache.lucene.search.ConstantScoreQuery;
 import org.apache.lucene.search.FuzzyQuery;
+import org.apache.lucene.search.PrefixQuery;
+import org.apache.lucene.search.Query;
 import org.apache.lucene.search.RegexpQuery;
 import org.apache.lucene.search.TermInSetQuery;
 import org.apache.lucene.search.TermQuery;
 import org.apache.lucene.util.BytesRef;
+import org.apache.lucene.util.automaton.Automata;
+import org.apache.lucene.util.automaton.Automaton;
+import org.apache.lucene.util.automaton.Operations;
 import org.elasticsearch.common.unit.Fuzziness;
 import org.junit.Before;
 
@@ -32,6 +39,8 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
 
+import static org.apache.lucene.search.MultiTermQuery.CONSTANT_SCORE_REWRITE;
+
 public class TextFieldTypeTests extends FieldTypeTestCase {
     @Override
     protected MappedFieldType createDefaultFieldType() {
@@ -143,4 +152,21 @@ public class TextFieldTypeTests extends FieldTypeTestCase {
                 () -> ft.fuzzyQuery("foo", Fuzziness.fromEdits(2), 1, 50, true));
         assertEquals("Cannot search on field [field] since it is not indexed.", e.getMessage());
     }
+
+    public void testIndexPrefixes() {
+        TextFieldMapper.TextFieldType ft = new TextFieldMapper.TextFieldType();
+        ft.setName("field");
+        ft.setPrefixFieldType(new TextFieldMapper.PrefixFieldType("field._index_prefix", 2, 10));
+
+        Query q = ft.prefixQuery("goin", CONSTANT_SCORE_REWRITE, null);
+        assertEquals(new ConstantScoreQuery(new TermQuery(new Term("field._index_prefix", "goin"))), q);
+
+        q = ft.prefixQuery("internationalisatio", CONSTANT_SCORE_REWRITE, null);
+        assertEquals(new PrefixQuery(new Term("field", "internationalisatio")), q);
+
+        q = ft.prefixQuery("g", CONSTANT_SCORE_REWRITE, null);
+        Automaton automaton
+            = Operations.concatenate(Arrays.asList(Automata.makeChar('g'), Automata.makeAnyChar()));
+        assertEquals(new ConstantScoreQuery(new AutomatonQuery(new Term("field._index_prefix", "g*"), automaton)), q);
+    }
 }