Adjust random_score default field to _seq_no field (#118671)

In an effort to improve performance and continue to provide unique
seeded scores for documents in the same index, we are switching from _id
to _seq_no.

Requiring a field that is "unique" for a field and to help with random
scores is burdensome for the user. So, we should default to a unique
field (per index) when the user provides a seed. 

Using `_seq_no` should be better as:

 - We don't have to grab stored fields values
 - Bytes used are generally smaller

Additionally this removes the deprecation warning.

Marking as "breaking" as it does change the scores & behavior, but the
API provide is the same.

Author: benwtrent

build-tools-internal/src/main/resources/changelog-schema.json

@@ -295,6 +295,7 @@
         "Painless",
         "REST API",
         "Rollup",
+        "Search",
         "System requirement",
         "Transform"
       ]

docs/changelog/118671.yaml

@@ -0,0 +1,11 @@
+pr: 118671
+summary: Adjust `random_score` default field to `_seq_no` field
+area: Search
+type: breaking
+issues: []
+breaking:
+  title: Adjust `random_score` default field to `_seq_no` field
+  area: Search
+  details: When providing a 'seed' parameter to a 'random_score' function in the 'function_score' query but NOT providing a 'field', the default 'field' is switched from '_id' to '_seq_no'.
+  impact: The random scoring and ordering may change when providing a 'seed' and not providing a 'field' to a 'random_score' function. 
+  notable: false

server/src/main/java/org/elasticsearch/index/query/functionscore/RandomScoreFunctionBuilder.java

@@ -13,12 +13,10 @@
 import org.elasticsearch.common.ParsingException;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
-import org.elasticsearch.common.logging.DeprecationCategory;
-import org.elasticsearch.common.logging.DeprecationLogger;
 import org.elasticsearch.common.lucene.search.function.RandomScoreFunction;
 import org.elasticsearch.common.lucene.search.function.ScoreFunction;
-import org.elasticsearch.index.mapper.IdFieldMapper;
 import org.elasticsearch.index.mapper.MappedFieldType;
+import org.elasticsearch.index.mapper.SeqNoFieldMapper;
 import org.elasticsearch.index.query.SearchExecutionContext;
 import org.elasticsearch.xcontent.XContentBuilder;
 import org.elasticsearch.xcontent.XContentParser;
@@ -30,7 +28,6 @@
  * A function that computes a random score for the matched documents
  */
 public class RandomScoreFunctionBuilder extends ScoreFunctionBuilder<RandomScoreFunctionBuilder> {
-    private static final DeprecationLogger deprecationLogger = DeprecationLogger.getLogger(RandomScoreFunctionBuilder.class);
 
     public static final String NAME = "random_score";
     private String field;
@@ -140,17 +137,7 @@ protected ScoreFunction doToFunction(SearchExecutionContext context) {
             // DocID-based random score generation
             return new RandomScoreFunction(hash(context.nowInMillis()), salt, null);
         } else {
-            String fieldName;
-            if (field == null) {
-                deprecationLogger.warn(
-                    DeprecationCategory.QUERIES,
-                    "seed_requires_field",
-                    "As of version 7.0 Elasticsearch will require that a [field] parameter is provided when a [seed] is set"
-                );
-                fieldName = IdFieldMapper.NAME;
-            } else {
-                fieldName = field;
-            }
+            final String fieldName = Objects.requireNonNullElse(field, SeqNoFieldMapper.NAME);
             if (context.isFieldMapped(fieldName) == false) {
                 if (context.hasMappings() == false) {
                     // no mappings: the index is empty anyway

server/src/test/java/org/elasticsearch/index/query/functionscore/ScoreFunctionBuilderTests.java

@@ -66,7 +66,6 @@ public void testRandomScoreFunctionWithSeedNoField() throws Exception {
         Mockito.when(context.getFieldType(IdFieldMapper.NAME)).thenReturn(new KeywordFieldMapper.KeywordFieldType(IdFieldMapper.NAME));
         Mockito.when(context.isFieldMapped(IdFieldMapper.NAME)).thenReturn(true);
         builder.toFunction(context);
-        assertWarnings("As of version 7.0 Elasticsearch will require that a [field] parameter is provided when a [seed] is set");
     }
 
     public void testRandomScoreFunctionWithSeed() throws Exception {