Modified ExplainAnalyzeOperator to support multiple substages in output

The EXPLAIN ANALYZE operator only supports one substage when returning the output.
When outputting in TEXT format, modify it to loop through the substages and return
the substage ID and its plan.
For JSON format, return a list of plans.

Resolves: #23798

presto-main/src/main/java/com/facebook/presto/operator/ExplainAnalyzeOperator.java

@@ -26,6 +26,7 @@
 import com.google.common.collect.ImmutableList;
 
 import java.util.List;
+import java.util.StringJoiner;
 import java.util.concurrent.TimeUnit;
 
 import static com.facebook.presto.common.type.VarcharType.VARCHAR;
@@ -149,18 +150,28 @@ public Page getOutput()
 
         QueryInfo queryInfo = queryPerformanceFetcher.getQueryInfo(operatorContext.getDriverContext().getTaskId().getQueryId());
         checkState(queryInfo.getOutputStage().isPresent(), "Output stage is missing");
-        checkState(queryInfo.getOutputStage().get().getSubStages().size() == 1, "Expected one sub stage of explain node");
 
         if (!hasFinalStageInfo(queryInfo.getOutputStage().get())) {
             return null;
         }
         String plan;
         switch (format) {
             case TEXT:
-                plan = textDistributedPlan(queryInfo.getOutputStage().get().getSubStages().get(0), functionAndTypeManager, operatorContext.getSession(), verbose);
+                StringBuilder planStringBuilder = new StringBuilder();
+                for (StageInfo stageInfo : queryInfo.getOutputStage().get().getSubStages()) {
+                    planStringBuilder.append("Stage ID: ").append(stageInfo.getStageId().toString()).append('\n')
+                            .append(textDistributedPlan(stageInfo, functionAndTypeManager, operatorContext.getSession(),
+                                    verbose));
+                }
+                plan = planStringBuilder.toString();
                 break;
             case JSON:
-                plan = jsonDistributedPlan(queryInfo.getOutputStage().get().getSubStages().get(0), functionAndTypeManager, operatorContext.getSession());
+                StringJoiner planStringJoiner = new StringJoiner(",", "[", "]");
+                for (StageInfo stageInfo : queryInfo.getOutputStage().get().getSubStages()) {
+                    planStringJoiner.add(jsonDistributedPlan(stageInfo, functionAndTypeManager,
+                            operatorContext.getSession()));
+                }
+                plan = planStringJoiner.toString();
                 break;
             default:
                 throw new PrestoException(GENERIC_INTERNAL_ERROR, "Explain format not supported: " + format);