SELECT 
  pfv.feature_id, 
  pfv.product_id, 
  pfv.variant_id, 
  fv.position, 
  fvd.variant 
FROM 
  cscart_product_features_values AS pfv 
  INNER JOIN cscart_product_feature_variants AS fv ON pfv.feature_id = fv.feature_id 
  AND pfv.variant_id = fv.variant_id 
  INNER JOIN cscart_product_feature_variant_descriptions AS fvd ON pfv.variant_id = fvd.variant_id 
  AND fvd.lang_code = 'en' 
WHERE 
  pfv.feature_id IN (656, 648, 658, 643, 649, 766) 
  AND pfv.product_id IN (
    5750, 7013, 5739, 5727, 4432, 2574, 247, 
    944, 478, 985, 3754, 525, 2478, 10284, 
    6151, 1274, 908, 1930, 2527, 1134, 5696, 
    4022, 2490, 2698, 1406, 5721, 508, 3090, 
    5621, 4416, 1266, 5724, 3974, 851, 1126, 
    4067, 8483, 3605, 2509, 2752, 1428, 
    1263, 3883, 2471, 2576, 5944, 4143, 
    2395, 2620, 4399, 1143, 1256, 3776, 
    4951, 1319, 2310, 1273, 4526, 2502, 
    2473, 2508, 1532, 2565, 2300, 2580, 
    2638, 3155, 3834, 2301, 4294, 4316, 
    2737, 2550, 2559, 2363, 4126, 4796, 
    5892, 5625, 2845, 8499, 2599, 2822, 
    8502, 8501, 2640, 2646, 2503, 2602, 
    2459, 2431, 5899, 5934
  ) 
  AND pfv.lang_code = 'en'

Query time 0.00803

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "186.16"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "pfv",
          "access_type": "range",
          "possible_keys": [
            "PRIMARY",
            "fl",
            "variant_id",
            "lang_code",
            "product_id",
            "fpl",
            "idx_product_feature_variant_id"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "feature_id",
            "product_id"
          ],
          "key_length": "6",
          "rows_examined_per_scan": 558,
          "rows_produced_per_join": 181,
          "filtered": "32.52",
          "using_index": true,
          "cost_info": {
            "read_cost": "99.29",
            "eval_cost": "18.14",
            "prefix_cost": "117.43",
            "data_read_per_join": "140K"
          },
          "used_columns": [
            "feature_id",
            "product_id",
            "variant_id",
            "lang_code"
          ],
          "attached_condition": "((`softwarepirmam_hewadelivard_cscart_4`.`pfv`.`feature_id` in (656,648,658,643,649,766)) and (`softwarepirmam_hewadelivard_cscart_4`.`pfv`.`product_id` in (5750,7013,5739,5727,4432,2574,247,944,478,985,3754,525,2478,10284,6151,1274,908,1930,2527,1134,5696,4022,2490,2698,1406,5721,508,3090,5621,4416,1266,5724,3974,851,1126,4067,8483,3605,2509,2752,1428,1263,3883,2471,2576,5944,4143,2395,2620,4399,1143,1256,3776,4951,1319,2310,1273,4526,2502,2473,2508,1532,2565,2300,2580,2638,3155,3834,2301,4294,4316,2737,2550,2559,2363,4126,4796,5892,5625,2845,8499,2599,2822,8502,8501,2640,2646,2503,2602,2459,2431,5899,5934)) and (`softwarepirmam_hewadelivard_cscart_4`.`pfv`.`lang_code` = 'en'))"
        }
      },
      {
        "table": {
          "table_name": "fv",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY",
            "feature_id"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "variant_id"
          ],
          "key_length": "3",
          "ref": [
            "softwarepirmam_hewadelivard_cscart_4.pfv.variant_id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 14,
          "filtered": "8.23",
          "cost_info": {
            "read_cost": "45.36",
            "eval_cost": "1.49",
            "prefix_cost": "180.93",
            "data_read_per_join": "17K"
          },
          "used_columns": [
            "variant_id",
            "feature_id",
            "position"
          ],
          "attached_condition": "(`softwarepirmam_hewadelivard_cscart_4`.`fv`.`feature_id` = `softwarepirmam_hewadelivard_cscart_4`.`pfv`.`feature_id`)"
        }
      },
      {
        "table": {
          "table_name": "fvd",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "variant_id",
            "lang_code"
          ],
          "key_length": "9",
          "ref": [
            "softwarepirmam_hewadelivard_cscart_4.pfv.variant_id",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 14,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "3.73",
            "eval_cost": "1.49",
            "prefix_cost": "186.16",
            "data_read_per_join": "45K"
          },
          "used_columns": [
            "variant_id",
            "variant",
            "lang_code"
          ]
        }
      }
    ]
  }
}