SELECT 
  * 
FROM 
  cscart_product_feature_variants 
WHERE 
  variant_id IN (
    1219, 1257, 5902, 5908, 1425, 1199, 1241
  )

Query time 0.00056

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_feature_variants",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["variant_id"],
      "rows": 7,
      "filtered": 100,
      "index_condition": "cscart_product_feature_variants.variant_id in (1219,1257,5902,5908,1425,1199,1241)"
    }
  }
}