SELECT 
  us.user_type, 
  us.timestamp, 
  pref.avatar, 
  pref.isExpert, 
  pref.expertDomains, 
  pref.order_success_cnt, 
  pref.prod_review_cnt, 
  JSON_EXTRACT(
    pref.new_user_preferences, '$.kindcrew'
  ) AS kindcrew, 
  pref.user_name, 
  pref.dob, 
  pref.blog_cnt, 
  pref.video_cnt, 
  pref.about, 
  pref.gender 
from 
  kl_user_pref_master pref 
  left join cscart_users us on pref.user_id = us.user_id 
WHERE 
  pref.user_id = 51

Query time 0.00036

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.00"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "pref",
          "access_type": "const",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "user_id"
          ],
          "key_length": "3",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "0.00",
            "eval_cost": "0.10",
            "prefix_cost": "0.00",
            "data_read_per_join": "18K"
          },
          "used_columns": [
            "user_id",
            "dob",
            "user_name",
            "isExpert",
            "gender",
            "avatar",
            "order_success_cnt",
            "prod_review_cnt",
            "blog_cnt",
            "video_cnt",
            "expertDomains",
            "about",
            "new_user_preferences"
          ]
        }
      },
      {
        "table": {
          "table_name": "us",
          "access_type": "const",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "user_id"
          ],
          "key_length": "3",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "0.00",
            "eval_cost": "0.10",
            "prefix_cost": "0.00",
            "data_read_per_join": "7K"
          },
          "used_columns": [
            "user_id",
            "user_type",
            "timestamp"
          ]
        }
      }
    ]
  }
}