SELECT 
  cscart_discussion_posts.*, 
  cscart_discussion_messages.message 
FROM 
  cscart_discussion_posts 
  LEFT JOIN cscart_discussion_messages ON cscart_discussion_messages.post_id = cscart_discussion_posts.post_id 
WHERE 
  cscart_discussion_posts.thread_id = 3791 
  AND cscart_discussion_messages.message <> '' 
  AND cscart_discussion_posts.status = 'A' 
ORDER BY 
  cscart_discussion_posts.timestamp DESC 
LIMIT 
  0, 10

Query time 0.00326

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.79"
    },
    "ordering_operation": {
      "using_filesort": true,
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_discussion_posts",
            "access_type": "ref",
            "possible_keys": [
              "PRIMARY",
              "thread_id",
              "thread_id_2"
            ],
            "key": "thread_id",
            "used_key_parts": [
              "thread_id"
            ],
            "key_length": "3",
            "ref": [
              "const"
            ],
            "rows_examined_per_scan": 2,
            "rows_produced_per_join": 0,
            "filtered": "12.50",
            "cost_info": {
              "read_cost": "0.50",
              "eval_cost": "0.03",
              "prefix_cost": "0.70",
              "data_read_per_join": "112"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "name",
              "timestamp",
              "user_id",
              "ip_address",
              "status"
            ],
            "attached_condition": "(`s2cart`.`cscart_discussion_posts`.`status` = 'A')"
          }
        },
        {
          "table": {
            "table_name": "cscart_discussion_messages",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "post_id"
            ],
            "key_length": "3",
            "ref": [
              "s2cart.cscart_discussion_posts.post_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "87.50",
            "cost_info": {
              "read_cost": "0.06",
              "eval_cost": "0.02",
              "prefix_cost": "0.79",
              "data_read_per_join": "5"
            },
            "used_columns": [
              "message",
              "post_id"
            ],
            "attached_condition": "(`s2cart`.`cscart_discussion_messages`.`message` <> '')"
          }
        }
      ]
    }
  }
}