[ES|QL] Non-Correlated Subquery in FROM command

docs/changelog/135744.yaml

@@ -0,0 +1,5 @@
+pr: 135744
+summary: Non-Correlated Subquery in FROM command
+area: ES|QL
+type: enhancement
+issues: []

x-pack/plugin/esql-core/src/main/java/org/elasticsearch/xpack/esql/core/expression/Attribute.java

@@ -35,7 +35,9 @@ public abstract class Attribute extends NamedExpression {
     /**
      * Changing this will break bwc with 8.15, see {@link FieldAttribute#fieldName()}.
      */
-    protected static final String SYNTHETIC_ATTRIBUTE_NAME_PREFIX = "$$";
+    public static final String SYNTHETIC_ATTRIBUTE_NAME_PREFIX = "$$";
+
+    public static final String SYNTHETIC_ATTRIBUTE_NAME_SEPARATOR = "$";
 
     private static final TransportVersion ESQL_QUALIFIERS_IN_ATTRIBUTES = TransportVersion.fromName("esql_qualifiers_in_attributes");
 
@@ -77,7 +79,7 @@ public Attribute(
     }
 
     public static String rawTemporaryName(String... parts) {
-        var name = String.join("$", parts);
+        var name = String.join(SYNTHETIC_ATTRIBUTE_NAME_SEPARATOR, parts);
         return name.isEmpty() || name.startsWith(SYNTHETIC_ATTRIBUTE_NAME_PREFIX) ? name : SYNTHETIC_ATTRIBUTE_NAME_PREFIX + name;
     }
 

x-pack/plugin/esql/qa/server/multi-clusters/src/javaRestTest/java/org/elasticsearch/xpack/esql/ccq/MultiClusterSpecIT.java

@@ -56,6 +56,7 @@
 import static org.elasticsearch.xpack.esql.action.EsqlCapabilities.Cap.JOIN_LOOKUP_V12;
 import static org.elasticsearch.xpack.esql.action.EsqlCapabilities.Cap.JOIN_PLANNING_V1;
 import static org.elasticsearch.xpack.esql.action.EsqlCapabilities.Cap.METADATA_FIELDS_REMOTE_TEST;
+import static org.elasticsearch.xpack.esql.action.EsqlCapabilities.Cap.SUBQUERY_IN_FROM_COMMAND;
 import static org.elasticsearch.xpack.esql.action.EsqlCapabilities.Cap.UNMAPPED_FIELDS;
 import static org.elasticsearch.xpack.esql.qa.rest.RestEsqlTestCase.hasCapabilities;
 import static org.mockito.ArgumentMatchers.any;
@@ -172,6 +173,12 @@ protected void shouldSkipTest(String testName) throws IOException {
                 hasCapabilities(adminClient(), List.of(ENABLE_FORK_FOR_REMOTE_INDICES.capabilityName()))
             );
         }
+        // Subqueries in FROM are not fully tested in CCS yet
+        // And convertToRemoteIndices does not generate correct queries with subqueries in the FROM command yet
+        assumeFalse(
+            "Subqueries in FROM command not yet supported in CCS",
+            testCase.requiredCapabilities.contains(SUBQUERY_IN_FROM_COMMAND.capabilityName())
+        );
     }
 
     private TestFeatureService remoteFeaturesService() throws IOException {

x-pack/plugin/esql/qa/server/src/main/java/org/elasticsearch/xpack/esql/qa/rest/generative/GenerativeForkRestTest.java

@@ -55,6 +55,11 @@ protected void shouldSkipTest(String testName) throws IOException {
             testCase.requiredCapabilities.contains(UNMAPPED_FIELDS.capabilityName())
         );
 
+        assumeFalse(
+            "Tests using subqueries are skipped since we don't support nested subqueries",
+            testCase.requiredCapabilities.contains(SUBQUERY_IN_FROM_COMMAND.capabilityName())
+        );
+
         assumeTrue("Cluster needs to support FORK", hasCapabilities(adminClient(), List.of(FORK_V9.capabilityName())));
     }
 }