[ES|QL] Allow single fork branch (elastic#136805)

Author: pmpailis

docs/changelog/136805.yaml

@@ -0,0 +1,6 @@
+pr: 136805
+summary: Allow single fork branch
+area: ES|QL
+type: enhancement
+issues:
+ - 135825

x-pack/plugin/esql/src/internalClusterTest/java/org/elasticsearch/xpack/esql/action/ForkIT.java

@@ -1005,8 +1005,14 @@ public void testOneSubQuery() {
             | FORK
                ( WHERE content:"fox" )
             """;
-        var e = expectThrows(ParsingException.class, () -> run(query));
-        assertTrue(e.getMessage().contains("Fork requires at least 2 branches"));
+        try (var resp = run(query)) {
+            assertColumnTypes(resp.columns(), List.of("text", "integer", "keyword"));
+            assertColumnNames(resp.columns(), List.of("content", "id", "_fork"));
+            Iterable<Iterable<Object>> expectedValues = List.of(
+                Arrays.stream(new Object[] { "The quick brown fox jumps over the lazy dog", 6, "fork1" }).toList()
+            );
+            assertValues(resp.values(), expectedValues);
+        }
     }
 
     public void testForkWithinFork() {

x-pack/plugin/esql/src/internalClusterTest/java/org/elasticsearch/xpack/esql/action/FuseIT.java

@@ -11,6 +11,7 @@
 import org.elasticsearch.action.support.WriteRequest;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.plugins.Plugin;
+import org.elasticsearch.xpack.esql.plan.logical.fuse.Fuse;
 import org.junit.Before;
 
 import java.util.Collection;
@@ -163,6 +164,30 @@ public void testFuseLinearWithWeightsAndNormalizer() {
         }
     }
 
+    public void testFuseWithSingleFork() {
+        for (Fuse.FuseType type : Fuse.FuseType.values()) {
+            var query = """
+                FROM test METADATA _score, _id, _index
+                | WHERE id > 2
+                | FORK
+                   ( WHERE content:"fox" | SORT _score, _id DESC )
+                | FUSE
+                """ + type.name() + """
+                | SORT _score DESC, _id, _index
+                | EVAL _fork = mv_sort(_fork)
+                | EVAL _score = round(_score, 4)
+                | KEEP id, content, _fork
+                """;
+            try (var resp = run(query)) {
+                assertColumnNames(resp.columns(), List.of("id", "content", "_fork"));
+                assertColumnTypes(resp.columns(), List.of("integer", "keyword", "keyword"));
+                assertThat(getValuesList(resp.values()).size(), equalTo(1));
+                Iterable<Iterable<Object>> expectedValues = List.of(List.of(6, "The quick brown fox jumps over the lazy dog", "fork1"));
+                assertValues(resp.values(), expectedValues);
+            }
+        }
+    }
+
     private void createAndPopulateIndex() {
         var indexName = "test";
         var client = client().admin().indices();

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/parser/LogicalPlanBuilder.java

@@ -848,9 +848,6 @@ private void checkForRemoteClusters(LogicalPlan plan, Source source, String comm
     @SuppressWarnings("unchecked")
     public PlanFactory visitForkCommand(EsqlBaseParser.ForkCommandContext ctx) {
         List<PlanFactory> subQueries = visitForkSubQueries(ctx.forkSubQueries());
-        if (subQueries.size() < Fork.MIN_BRANCHES) {
-            throw new ParsingException(source(ctx), "Fork requires at least " + Fork.MIN_BRANCHES + " branches");
-        }
         if (subQueries.size() > Fork.MAX_BRANCHES) {
             throw new ParsingException(source(ctx), "Fork supports up to " + Fork.MAX_BRANCHES + " branches");
         }

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/plan/logical/Fork.java

@@ -39,14 +39,10 @@ public class Fork extends LogicalPlan implements PostAnalysisPlanVerificationAwa
 
     public static final String FORK_FIELD = "_fork";
     public static final int MAX_BRANCHES = 8;
-    public static final int MIN_BRANCHES = 2;
     private final List<Attribute> output;
 
     public Fork(Source source, List<LogicalPlan> children, List<Attribute> output) {
         super(source, children);
-        if (children.size() < MIN_BRANCHES) {
-            throw new IllegalArgumentException("FORK requires more than " + MIN_BRANCHES + " branches, got: " + children.size());
-        }
         if (children.size() > MAX_BRANCHES) {
             throw new IllegalArgumentException("FORK supports up to " + MAX_BRANCHES + " branches, got: " + children.size());
         }

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/parser/StatementParserTests.java

@@ -3861,11 +3861,12 @@ public void testForkAllCommands() {
     }
 
     public void testInvalidFork() {
-        expectError("FROM foo* | FORK (WHERE a:\"baz\")", "line 1:13: Fork requires at least 2 branches");
-        expectError("FROM foo* | FORK (LIMIT 10)", "line 1:13: Fork requires at least 2 branches");
-        expectError("FROM foo* | FORK (SORT a)", "line 1:13: Fork requires at least 2 branches");
-        expectError("FROM foo* | FORK (WHERE x>1 | LIMIT 5)", "line 1:13: Fork requires at least 2 branches");
-        expectError("FROM foo* | WHERE x>1 | FORK (WHERE a:\"baz\")", "Fork requires at least 2 branches");
+        expectError("""
+            FROM foo* | FORK
+            """, "line 2:1: mismatched input '<EOF>' expecting '('");
+        expectError("""
+            FROM foo* | FORK ()
+            """, "line 1:19: mismatched input ')'");
 
         expectError("""
             FROM foo*