Refactor: create a isLogicalSortLimit method to enhance readability

Signed-off-by: Yuanchun Shen <yuanchu@amazon.com>

Author: yuancu

integ-test/src/test/resources/expectedOutput/calcite/explain_limit_push.json

@@ -1,6 +1,6 @@
 {
-  "calcite":{
-    "logical":"LogicalProject(ageMinus=[$17])\n  LogicalSort(fetch=[5])\n    LogicalProject(account_number=[$0], firstname=[$1], address=[$2], balance=[$3], gender=[$4], city=[$5], employer=[$6], state=[$7], age=[$8], email=[$9], lastname=[$10], _id=[$11], _index=[$12], _score=[$13], _maxscore=[$14], _sort=[$15], _routing=[$16], ageMinus=[-($8, 30)])\n      CalciteLogicalIndexScan(table=[[OpenSearch, opensearch-sql_test_index_account]])\n",
-    "physical":"EnumerableCalc(expr#0=[{inputs}], expr#1=[30], expr#2=[-($t0, $t1)], $f0=[$t2])\n  CalciteEnumerableIndexScan(table=[[OpenSearch, opensearch-sql_test_index_account]], PushDownContext=[[LIMIT->5, PROJECT->[age]], OpenSearchRequestBuilder(sourceBuilder={\"from\":0,\"size\":5,\"timeout\":\"1m\",\"_source\":{\"includes\":[\"age\"],\"excludes\":[]}}, requestedTotalSize=5, pageSize=null, startFrom=0)])\n"
+  "calcite": {
+    "logical": "LogicalProject(ageMinus=[$17])\n  LogicalSort(fetch=[5])\n    LogicalProject(account_number=[$0], firstname=[$1], address=[$2], balance=[$3], gender=[$4], city=[$5], employer=[$6], state=[$7], age=[$8], email=[$9], lastname=[$10], _id=[$11], _index=[$12], _score=[$13], _maxscore=[$14], _sort=[$15], _routing=[$16], ageMinus=[-($8, 30)])\n      CalciteLogicalIndexScan(table=[[OpenSearch, opensearch-sql_test_index_account]])\n",
+    "physical": "EnumerableCalc(expr#0=[{inputs}], expr#1=[30], expr#2=[-($t0, $t1)], $f0=[$t2])\n  CalciteEnumerableIndexScan(table=[[OpenSearch, opensearch-sql_test_index_account]], PushDownContext=[[LIMIT->5, PROJECT->[age]], OpenSearchRequestBuilder(sourceBuilder={\"from\":0,\"size\":5,\"timeout\":\"1m\",\"_source\":{\"includes\":[\"age\"],\"excludes\":[]}}, requestedTotalSize=5, pageSize=null, startFrom=0)])\n"
   }
 }

opensearch/src/main/java/org/opensearch/sql/opensearch/planner/physical/OpenSearchLimitIndexScanRule.java

@@ -30,10 +30,7 @@ public void onMatch(RelOptRuleCall call) {
     final LogicalSort sort = call.rel(0);
     final CalciteLogicalIndexScan scan = call.rel(1);
 
-    // The LogicalSort is a LIMIT that should be pushed down when its fetch field is not null and
-    // its collation is empty.
-    // For example: `sort name | head 5` should not be pushed down because it has a field collation.
-    if (sort.fetch != null && sort.getCollation().getFieldCollations().isEmpty()) {
+    if (isLogicalSortLimit(sort)) {
       Integer limitValue = extractLimitValue(sort.fetch);
       Integer offsetValue = extractOffsetValue(sort.offset);
       if (limitValue != null && offsetValue != null) {
@@ -45,6 +42,18 @@ public void onMatch(RelOptRuleCall call) {
     }
   }
 
+  /**
+   * The LogicalSort is a LIMIT that should be pushed down when its fetch field is not null and its
+   * collation is empty. For example: <code>sort name | head 5</code> should not be pushed down
+   * because it has a field collation.
+   *
+   * @param sort The LogicalSort to check.
+   * @return True if the LogicalSort is a LIMIT that can be pushed down, false otherwise.
+   */
+  private static boolean isLogicalSortLimit(LogicalSort sort) {
+    return sort.fetch != null && sort.getCollation().getFieldCollations().isEmpty();
+  }
+
   private static Integer extractLimitValue(RexNode fetch) {
     if (fetch instanceof RexLiteral) {
       return ((RexLiteral) fetch).getValueAs(Integer.class);
@@ -53,17 +62,17 @@ private static Integer extractLimitValue(RexNode fetch) {
   }
 
   /**
-   * Extracts the offset value from the given `RexNode`. If the offset is `null`, it defaults to 0.
-   * For example:
+   * Extracts the offset value from the given <code>RexNode</code>. If the offset is <code>null
+   * </code>, it defaults to 0. For example:
    *
    * <ul>
    *   <li><code>source=people | head 1</code> will have a <code>null</code> offset, which is
    *       converted to 0.
    *   <li><code>source=people | head 1 from 2</code> will have an offset of 2.
    * </ul>
    *
-   * @param offset The `RexNode` representing the offset.
-   * @return The extracted offset value, or `null` if it cannot be determined.
+   * @param offset The <code>RexNode</code> representing the offset.
+   * @return The extracted offset value, or <code>null</code> if it cannot be determined.
    */
   private static Integer extractOffsetValue(RexNode offset) {
     if (Objects.isNull(offset)) {