SELECT 
  feature_variant_design, 
  multi_colors_filter, 
  pfv.color 
FROM 
  jmj_product_features 
  INNER JOIN jmj_product_feature_variants pfv ON(
    jmj_product_features.feature_id = pfv.feature_id
  ) 
WHERE 
  jmj_product_features.feature_id = 378

Query time 0.01278

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "7.20"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "jmj_product_features",
          "access_type": "const",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "feature_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": "0.20",
            "prefix_cost": "0.00",
            "data_read_per_join": "440"
          },
          "used_columns": [
            "feature_id",
            "feature_variant_design",
            "multi_colors_filter"
          ]
        }
      },
      {
        "table": {
          "table_name": "pfv",
          "access_type": "ref",
          "possible_keys": [
            "feature_id"
          ],
          "key": "feature_id",
          "used_key_parts": [
            "feature_id"
          ],
          "key_length": "3",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 6,
          "rows_produced_per_join": 6,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "6.00",
            "eval_cost": "1.20",
            "prefix_cost": "7.20",
            "data_read_per_join": "9K"
          },
          "used_columns": [
            "feature_id",
            "color"
          ]
        }
      }
    ]
  }
}