update doc

docs/user/optimization/optimization.rst

@@ -85,6 +85,31 @@ Elasticsearch Specific Optimization
 
 The Elasticsearch `Query DSL <https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl.html>`_ and `Aggregation <https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations.html>`_ also enabling the storage engine specific optimization.
 
+Push Project Into Query DSL
+---------------------------
+The Project list will push down to Query DSL to `filter the source <https://www.elastic.co/guide/en/elasticsearch/reference/7.x/search-fields.html#source-filtering>`_::
+
+    sh$ curl -sS -H 'Content-Type: application/json' \
+    ... -X POST localhost:9200/_opendistro/_sql/_explain \
+    ... -d '{"query" : "SELECT age FROM accounts"}'
+    {
+      "root": {
+        "name": "ProjectOperator",
+        "description": {
+          "fields": "[age]"
+        },
+        "children": [
+          {
+            "name": "ElasticsearchIndexScan",
+            "description": {
+              "request": "ElasticsearchQueryRequest(indexName=accounts, sourceBuilder={\"from\":0,\"size\":200,\"timeout\":\"1m\",\"_source\":{\"includes\":[\"age\"],\"excludes\":[]}}, searchDone=false)"
+            },
+            "children": []
+          }
+        ]
+      }
+    }
+
 Filter Merge Into Query DSL
 ---------------------------