mysql中的索引列上的简单where条件导致SQL性能差

Question

我有一个非常简单的sql where语句

  SELECT    `genre_application`.`genre_id`
      FROM  `genre_application`
      WHERE `genre_application`.`application_id` = 310633997

即使在列上有索引，也需要2秒钟左右。

{
"query_block": {
"select_id": 1,
"cost_info": {
  "query_cost": "2098202.80"
},
"table": {
  "table_name": "genre_application",
  "access_type": "index",
  "key": "PRIMARY",
  "used_key_parts": [
    "genre_id",
    "application_id"
  ],
  "key_length": "8",
  "rows_examined_per_scan": 10363019,
  "rows_produced_per_join": 1036301,
  "filtered": "10.00",
  "using_index": true,
  "cost_info": {
    "read_cost": "1890942.42",
    "eval_cost": "207260.38",
    "prefix_cost": "2098202.80",
    "data_read_per_join": "23M"
  },
  "used_columns": [
    "genre_id",
    "application_id"
  ],
  "attached_condition": "(`genre_application`.`application_id` = 310633997)"
}
}

如何改善执行时间？

Answer 1

这是您的查询：

SELECT ga.`genre_id`
FROM `genre_application` ga
WHERE ga.`application_id` = 310633997

您需要一个索引，其中application_id是索引中的第一个键。 最好的索引是genre_application(application_id, genre_id)上的复合索引。 请注意，索引中键的顺序很重要。