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SELECT 
  cscart_vendor_plan_descriptions.*, 
  cscart_vendor_plans.* 
FROM 
  cscart_vendor_plans 
  LEFT JOIN cscart_vendor_plan_descriptions ON cscart_vendor_plan_descriptions.plan_id = cscart_vendor_plans.plan_id 
  AND cscart_vendor_plan_descriptions.lang_code = 'en' 
WHERE 
  (
    status IN('A')
  ) 
  AND (
    FIND_IN_SET(1, storefronts) 
    OR storefronts IS NULL 
    OR storefronts = ''
  ) 
ORDER BY 
  position asc

Query time 0.00077

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "5.20"
    },
    "ordering_operation": {
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "1.00"
      },
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_vendor_plans",
            "access_type": "ALL",
            "rows_examined_per_scan": 10,
            "rows_produced_per_join": 1,
            "filtered": "10.00",
            "cost_info": {
              "read_cost": "2.80",
              "eval_cost": "0.20",
              "prefix_cost": "3.00",
              "data_read_per_join": "80"
            },
            "used_columns": [
              "plan_id",
              "status",
              "position",
              "is_default",
              "price",
              "periodicity",
              "commission",
              "fixed_commission",
              "products_limit",
              "revenue_limit",
              "vendor_store",
              "categories",
              "storefronts",
              "usergroups",
              "cp_is_blog_avail",
              "offer_booking",
              "ga_vendors_tracking"
            ],
            "attached_condition": "((`webmarco`.`cscart_vendor_plans`.`status` = 'A') and (find_in_set(1,`webmarco`.`cscart_vendor_plans`.`storefronts`) or isnull(`webmarco`.`cscart_vendor_plans`.`storefronts`) or (`webmarco`.`cscart_vendor_plans`.`storefronts` = '')))"
          }
        },
        {
          "table": {
            "table_name": "cscart_vendor_plan_descriptions",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "plan_id",
              "lang_code"
            ],
            "key_length": "10",
            "ref": [
              "webmarco.cscart_vendor_plans.plan_id",
              "const"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 1,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "1.00",
              "eval_cost": "0.20",
              "prefix_cost": "4.20",
              "data_read_per_join": "792"
            },
            "used_columns": [
              "plan_id",
              "lang_code",
              "plan",
              "description"
            ]
          }
        }
      ]
    }
  }
}

Result

plan_id	lang_code	plan	status	position	is_default	price	periodicity	commission	products_limit	revenue_limit	vendor_store	usergroups	cp_is_blog_avail	ga_vendors_tracking
11	en	Stripe Connected Plan	A	0	0	0.00	month	0.00	0	0.00	1	0	N	N
12	en	Vat Included 10.00	A	0	0	0.00	month	10.00	0	0.00	1	0	N	N
13	en	Vat Excluded 12.20	A	0	0	0.00	month	12.20	0	0.00	1	0	N	N
14	en	Marco Travel	A	0	1	0.00	month	20.00	0	0.00	1	0	N	N
9	en	Free	A	10	0	0.00	month	8.00	100	150.00	0	0	N	N
1	en	Gold	A	20	0	10.00	month	4.00	50	500.00	0		N	N
2	en	Premium	A	30	0	20.00	month	2.00	200	1000.00	1		N	N
3	en	Unlimited	A	40	0	0.00	month	5.00	0	0.00	1		N	N
10	en	Marco 499€	A	41	0	0.00	month	6.10	0	0.00	1	0	N	N