[FEATURE] Enrichment Explain Extended Results

Is your feature request related to a problem?
First, read issue

There is currently no effective way to determine which field is actually being used when debugging this issue.

• The EXPLAIN results are not human-readable and require better formatting and refinement.
• The EXPLAIN output is missing key debugging information. For example, it may show a function like REX_EXTRACT($0, status=(?<statusCode>\\d+), 1), but it does not reveal how $0 is fetched from OpenSearch, which is essential for diagnosing the problem.

###
POST {{baseUrl}}/_plugins/_ppl
Content-Type: application/x-ndjson

{
  "query": "explain extended source=log* | rex field=msg 'status=(?<statusCode>\\d+)' | where statusCode=200"
}

{
  "calcite": {
    "logical": "LogicalSystemLimit(fetch=[10000], type=[QUERY_SIZE_LIMIT])\n  LogicalProject(msg=[$0], index=[$1], statusCode=[$8])\n    LogicalFilter(condition=[=(SAFE_CAST($8), 200.0E0)])\n      LogicalProject(msg=[$0], index=[$1], _id=[$2], _index=[$3], _score=[$4], _maxscore=[$5], _sort=[$6], _routing=[$7], statusCode=[REX_EXTRACT($0, 'status=(?<statusCode>\\d+)', 1)])\n        CalciteLogicalIndexScan(table=[[OpenSearch, log*]])\n",
    "physical": "EnumerableCalc(expr#0..1=[{inputs}], expr#2=['status=(?<statusCode>\\d+)'], expr#3=[1], expr#4=[REX_EXTRACT($t0, $t2, $t3)], proj#0..1=[{exprs}], $f2=[$t4])\n  CalciteEnumerableIndexScan(table=[[OpenSearch, log*]], PushDownContext=[[PROJECT->[msg, index], SCRIPT->=(SAFE_CAST(REX_EXTRACT($0, 'status=(?<statusCode>\\d+)', 1)), 200.0E0), LIMIT->10000], OpenSearchRequestBuilder(sourceBuilder={\"from\":0,\"size\":10000,\"timeout\":\"1m\",\"query\":{\"script\":{\"script\":{\"source\":\"{\\\"langType\\\":\\\"calcite\\\",\\\"script\\\":\\\"{\\\\n  \\\\\\\"op\\\\\\\": {\\\\n    \\\\\\\"name\\\\\\\": \\\\\\\"=\\\\\\\",\\\\n    \\\\\\\"kind\\\\\\\": \\\\\\\"EQUALS\\\\\\\",\\\\n    \\\\\\\"syntax\\\\\\\": \\\\\\\"BINARY\\\\\\\"\\\\n  },\\\\n  \\\\\\\"operands\\\\\\\": [\\\\n    {\\\\n      \\\\\\\"op\\\\\\\": {\\\\n        \\\\\\\"name\\\\\\\": \\\\\\\"SAFE_CAST\\\\\\\",\\\\n        \\\\\\\"kind\\\\\\\": \\\\\\\"SAFE_CAST\\\\\\\",\\\\n        \\\\\\\"syntax\\\\\\\": \\\\\\\"SPECIAL\\\\\\\"\\\\n      },\\\\n      \\\\\\\"operands\\\\\\\": [\\\\n        {\\\\n          \\\\\\\"op\\\\\\\": {\\\\n            \\\\\\\"name\\\\\\\": \\\\\\\"REX_EXTRACT\\\\\\\",\\\\n            \\\\\\\"kind\\\\\\\": \\\\\\\"OTHER_FUNCTION\\\\\\\",\\\\n            \\\\\\\"syntax\\\\\\\": \\\\\\\"FUNCTION\\\\\\\"\\\\n          },\\\\n          \\\\\\\"operands\\\\\\\": [\\\\n            {\\\\n              \\\\\\\"input\\\\\\\": 0,\\\\n              \\\\\\\"name\\\\\\\": \\\\\\\"$0\\\\\\\"\\\\n            },\\\\n            {\\\\n              \\\\\\\"literal\\\\\\\": \\\\\\\"status=(?<statusCode>\\\\\\\\\\\\\\\\d+)\\\\\\\",\\\\n              \\\\\\\"type\\\\\\\": {\\\\n                \\\\\\\"type\\\\\\\": \\\\\\\"CHAR\\\\\\\",\\\\n                \\\\\\\"nullable\\\\\\\": false,\\\\n                \\\\\\\"precision\\\\\\\": 25\\\\n              }\\\\n            },\\\\n            {\\\\n              \\\\\\\"literal\\\\\\\": 1,\\\\n              \\\\\\\"type\\\\\\\": {\\\\n                \\\\\\\"type\\\\\\\": \\\\\\\"INTEGER\\\\\\\",\\\\n                \\\\\\\"nullable\\\\\\\": false\\\\n              }\\\\n            }\\\\n          ],\\\\n          \\\\\\\"class\\\\\\\": \\\\\\\"org.opensearch.sql.expression.function.UserDefinedFunctionBuilder$1\\\\\\\",\\\\n          \\\\\\\"type\\\\\\\": {\\\\n            \\\\\\\"type\\\\\\\": \\\\\\\"VARCHAR\\\\\\\",\\\\n            \\\\\\\"nullable\\\\\\\": true,\\\\n            \\\\\\\"precision\\\\\\\": 2000\\\\n          },\\\\n          \\\\\\\"deterministic\\\\\\\": true,\\\\n          \\\\\\\"dynamic\\\\\\\": false\\\\n        }\\\\n      ],\\\\n      \\\\\\\"type\\\\\\\": {\\\\n        \\\\\\\"type\\\\\\\": \\\\\\\"DOUBLE\\\\\\\",\\\\n        \\\\\\\"nullable\\\\\\\": true\\\\n      }\\\\n    },\\\\n    {\\\\n      \\\\\\\"literal\\\\\\\": 200.0,\\\\n      \\\\\\\"type\\\\\\\": {\\\\n        \\\\\\\"type\\\\\\\": \\\\\\\"DOUBLE\\\\\\\",\\\\n        \\\\\\\"nullable\\\\\\\": false\\\\n      }\\\\n    }\\\\n  ]\\\\n}\\\"}\",\"lang\":\"opensearch_compounded_script\",\"params\":{\"utcTimestamp\":1761319006844113000}},\"boost\":1.0}},\"_source\":{\"includes\":[\"msg\",\"index\"],\"excludes\":[]}}, requestedTotalSize=10000, pageSize=null, startFrom=0)])\n",
    "extended": "public org.apache.calcite.linq4j.Enumerable bind(final org.apache.calcite.DataContext root) {\n  final org.opensearch.sql.opensearch.storage.scan.CalciteEnumerableIndexScan v1stashed = (org.opensearch.sql.opensearch.storage.scan.CalciteEnumerableIndexScan) root.get(\"v1stashed\");\n  final org.apache.calcite.linq4j.Enumerable _inputEnumerable = v1stashed.scan();\n  return new org.apache.calcite.linq4j.AbstractEnumerable(){\n      public org.apache.calcite.linq4j.Enumerator enumerator() {\n        return new org.apache.calcite.linq4j.Enumerator(){\n            public final org.apache.calcite.linq4j.Enumerator inputEnumerator = _inputEnumerable.enumerator();\n            public void reset() {\n              inputEnumerator.reset();\n            }\n\n            public boolean moveNext() {\n              return inputEnumerator.moveNext();\n            }\n\n            public void close() {\n              inputEnumerator.close();\n            }\n\n            public Object current() {\n              final Object[] current = (Object[]) inputEnumerator.current();\n              final String input_value1 = current[0] == null ? null : current[0].toString();\n              return new Object[] {\n                  current[0],\n                  current[1],\n                  input_value1 == null ? null : org.opensearch.sql.expression.function.udf.RexExtractFunction.extractGroup(input_value1, \"status=(?<statusCode>\\\\d+)\", 1)};\n            }\n\n          };\n      }\n\n    };\n}\n\n\npublic Class getElementType() {\n  return java.lang.Object[].class;\n}\n\n\n"
  }
}

What solution would you like?
opensearch_compounded_script results should

1. readable.
2. has type info.
   e.g. SAFE_CAST(REX_EXTRACT(docValue(msg), status=(?<statusCode>\\d+), 1), DOUBLE)

Start by reading issue 4659, then inspect the EXPLAIN EXTENDED output shown here, especially the opensearch_compounded_script result. Done means the result is readable, includes type information, and represents the source field as docValue(msg) in the requested form.