Add AnalyticsExplainIT with expected output files

Add dedicated explain integration test class following the CalciteExplainIT
pattern: each test compares actual explain JSON against expected output
files in resources/expectedOutput/analytics/. Tests cover simple scan,
project, filter+project, aggregation, sort (with collation propagation
to LogicalSystemLimit), and eval.

Signed-off-by: Kai Huang <kaihuang@amazon.com>
Signed-off-by: Kai Huang <ahkcs@amazon.com>

Author: ahkcs

integ-test/src/test/java/org/opensearch/sql/ppl/AnalyticsExplainIT.java

@@ -0,0 +1,86 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.sql.ppl;
+
+import static org.opensearch.sql.util.MatcherUtils.assertJsonEqualsIgnoreId;
+
+import java.io.IOException;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+import org.junit.Test;
+
+/**
+ * Explain integration tests for queries routed through the analytics engine path (Project Mustang).
+ * Validates that PPL queries targeting "parquet_*" indices produce correct logical plans via the
+ * _plugins/_ppl/_explain endpoint.
+ *
+ * <p>Expected output files are in resources/expectedOutput/analytics/. Each test compares the
+ * explain JSON output against its expected file, following the same pattern as CalciteExplainIT.
+ *
+ * <p>Since the analytics engine is not yet available, physical and extended plans are null. Only
+ * the logical plan (Calcite RelNode tree) is verified.
+ */
+public class AnalyticsExplainIT extends PPLIntegTestCase {
+
+  private static final Logger LOG = LogManager.getLogger(AnalyticsExplainIT.class);
+
+  @Override
+  protected void init() throws Exception {
+    // No index loading needed -- stub schema and data are hardcoded
+  }
+
+  private String loadAnalyticsExpectedPlan(String fileName) {
+    return loadFromFile("expectedOutput/analytics/" + fileName);
+  }
+
+  @Test
+  public void testExplainSimpleScan() throws IOException {
+    String query = "source = opensearch.parquet_logs";
+    String result = explainQueryToString(query);
+    LOG.info("[testExplainSimpleScan] query: {}\nresult:\n{}", query, result);
+    assertJsonEqualsIgnoreId(loadAnalyticsExpectedPlan("explain_simple_scan.json"), result);
+  }
+
+  @Test
+  public void testExplainProject() throws IOException {
+    String query = "source = opensearch.parquet_logs | fields ts, message";
+    String result = explainQueryToString(query);
+    LOG.info("[testExplainProject] query: {}\nresult:\n{}", query, result);
+    assertJsonEqualsIgnoreId(loadAnalyticsExpectedPlan("explain_project.json"), result);
+  }
+
+  @Test
+  public void testExplainFilterAndProject() throws IOException {
+    String query = "source = opensearch.parquet_logs | where status = 200 | fields ts, message";
+    String result = explainQueryToString(query);
+    LOG.info("[testExplainFilterAndProject] query: {}\nresult:\n{}", query, result);
+    assertJsonEqualsIgnoreId(loadAnalyticsExpectedPlan("explain_filter_project.json"), result);
+  }
+
+  @Test
+  public void testExplainAggregation() throws IOException {
+    String query = "source = opensearch.parquet_logs | stats count() by status";
+    String result = explainQueryToString(query);
+    LOG.info("[testExplainAggregation] query: {}\nresult:\n{}", query, result);
+    assertJsonEqualsIgnoreId(loadAnalyticsExpectedPlan("explain_aggregation.json"), result);
+  }
+
+  @Test
+  public void testExplainSort() throws IOException {
+    String query = "source = opensearch.parquet_logs | sort ts";
+    String result = explainQueryToString(query);
+    LOG.info("[testExplainSort] query: {}\nresult:\n{}", query, result);
+    assertJsonEqualsIgnoreId(loadAnalyticsExpectedPlan("explain_sort.json"), result);
+  }
+
+  @Test
+  public void testExplainEval() throws IOException {
+    String query = "source = opensearch.parquet_logs | eval error = status = 500";
+    String result = explainQueryToString(query);
+    LOG.info("[testExplainEval] query: {}\nresult:\n{}", query, result);
+    assertJsonEqualsIgnoreId(loadAnalyticsExpectedPlan("explain_eval.json"), result);
+  }
+}

integ-test/src/test/resources/expectedOutput/analytics/explain_aggregation.json

@@ -0,0 +1,5 @@
+{
+  "calcite": {
+    "logical": "LogicalSystemLimit(fetch=[10000], type=[QUERY_SIZE_LIMIT])\n  LogicalProject(count()=[$1], status=[$0])\n    LogicalAggregate(group=[{0}], count()=[COUNT()])\n      LogicalProject(status=[$1])\n        LogicalTableScan(table=[[opensearch, parquet_logs]])\n"
+  }
+}

integ-test/src/test/resources/expectedOutput/analytics/explain_eval.json

@@ -0,0 +1,5 @@
+{
+  "calcite": {
+    "logical": "LogicalSystemLimit(fetch=[10000], type=[QUERY_SIZE_LIMIT])\n  LogicalProject(ts=[$0], status=[$1], message=[$2], ip_addr=[$3], error=[=($1, 500)])\n    LogicalTableScan(table=[[opensearch, parquet_logs]])\n"
+  }
+}

integ-test/src/test/resources/expectedOutput/analytics/explain_filter_project.json

@@ -0,0 +1,5 @@
+{
+  "calcite": {
+    "logical": "LogicalSystemLimit(fetch=[10000], type=[QUERY_SIZE_LIMIT])\n  LogicalProject(ts=[$0], message=[$2])\n    LogicalFilter(condition=[=($1, 200)])\n      LogicalTableScan(table=[[opensearch, parquet_logs]])\n"
+  }
+}

integ-test/src/test/resources/expectedOutput/analytics/explain_project.json

@@ -0,0 +1,5 @@
+{
+  "calcite": {
+    "logical": "LogicalSystemLimit(fetch=[10000], type=[QUERY_SIZE_LIMIT])\n  LogicalProject(ts=[$0], message=[$2])\n    LogicalTableScan(table=[[opensearch, parquet_logs]])\n"
+  }
+}

integ-test/src/test/resources/expectedOutput/analytics/explain_simple_scan.json

@@ -0,0 +1,5 @@
+{
+  "calcite": {
+    "logical": "LogicalSystemLimit(fetch=[10000], type=[QUERY_SIZE_LIMIT])\n  LogicalTableScan(table=[[opensearch, parquet_logs]])\n"
+  }
+}

integ-test/src/test/resources/expectedOutput/analytics/explain_sort.json

@@ -0,0 +1,5 @@
+{
+  "calcite": {
+    "logical": "LogicalSystemLimit(sort0=[$0], dir0=[ASC-nulls-first], fetch=[10000], type=[QUERY_SIZE_LIMIT])\n  LogicalSort(sort0=[$0], dir0=[ASC-nulls-first])\n    LogicalTableScan(table=[[opensearch, parquet_logs]])\n"
+  }
+}