为什么MySQL优化器不使用所有列索引？

Question

Percona MySQL 5.7

表scheeme：

CREATE TABLE Developer.Rate (
  ID bigint(20) UNSIGNED NOT NULL AUTO_INCREMENT,
  TIME datetime NOT NULL,
  BASE varchar(3) NOT NULL,
  QUOTE varchar(3) NOT NULL,
  BID double NOT NULL,
  ASK double NOT NULL,
  PRIMARY KEY (ID),
  INDEX IDX_TIME (TIME),
  UNIQUE INDEX IDX_UK (BASE, QUOTE, TIME)
)
ENGINE = INNODB
ROW_FORMAT = COMPRESSED;

我尝试在选定期间之前请求最新数据。 optimazer使用no-complete唯一键，只有2列3。

如果我以共同的方式提出要求：

EXPLAIN FORMAT=JSON
SELECT
  BID
FROM 
  Rate
WHERE 
  BASE = 'EUR' 
  AND QUOTE = 'USD' 
  AND `TIME` <= (NOW() - INTERVAL 1 MONTH) 
ORDER BY 
  `TIME` DESC 
LIMIT 1
;

“Explain”显示只使用了2个第一列索引：BASE，QUOTE

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "10231052.40"
    },
    "ordering_operation": {
      "using_filesort": false,
      "table": {
        "table_name": "Rate",
        "access_type": "ref",
        "possible_keys": [
          "IDX_UK",
          "IDX_TIME"
        ],
        "key": "IDX_UK",
        "used_key_parts": [
          "BASE",
          "QUOTE"
        ],
        "key_length": "22",
        "ref": [
          "const",
          "const"
        ],
        "rows_examined_per_scan": 45966462,
        "rows_produced_per_join": 22983231,
        "filtered": "50.00",
        "cost_info": {
          "read_cost": "1037760.00",
          "eval_cost": "4596646.20",
          "prefix_cost": "10231052.40",
          "data_read_per_join": "1G"
        },
        "used_columns": [
          "ID",
          "TIME",
          "BASE",
          "QUOTE",
          "BID"
        ],
        "attached_condition": "((`Developer`.`Rate`.`BASE` <=> 'EUR') and (`Developer`.`Rate`.`QUOTE` <=> 'USD') and (`Developer`.`Rate`.`TIME` <= <cache>((now() - interval 1 month))))"
      }
    }
  }
}

但是如果你强制优化器使用IDX_UK，MySQL会使用请求中的所有3列：

EXPLAIN FORMAT=JSON
SELECT
  BID
FROM 
  Rate FORCE INDEX(IDX_UK)
WHERE 
  BASE = 'EUR' 
  AND QUOTE = 'USD' 
  AND `TIME` <= (NOW() - INTERVAL 1 MONTH) 
ORDER BY 
  `TIME` DESC 
LIMIT 1

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "10231052.40"
    },
    "ordering_operation": {
      "using_filesort": false,
      "table": {
        "table_name": "Rate",
        "access_type": "range",
        "possible_keys": [
          "IDX_UK"
        ],
        "key": "IDX_UK",
        "used_key_parts": [
          "BASE",
          "QUOTE",
          "TIME"
        ],
        "key_length": "27",
        "rows_examined_per_scan": 45966462,
        "rows_produced_per_join": 15320621,
        "filtered": "100.00",
        "index_condition": "((`Developer`.`Rate`.`BASE` = 'EUR') and (`Developer`.`Rate`.`QUOTE` = 'USD') and (`Developer`.`Rate`.`TIME` <= <cache>((now() - interval 1 month))))",
        "cost_info": {
          "read_cost": "1037760.00",
          "eval_cost": "3064124.31",
          "prefix_cost": "10231052.40",
          "data_read_per_join": "818M"
        },
        "used_columns": [
          "ID",
          "TIME",
          "BASE",
          "QUOTE",
          "BID"
        ]
      }
    }
  }
}

为什么优化器在没有明确声明索引的情况下不使用所有3列？

添加：

我理解正确，我应该使用这样的请求吗？

Reuest示例：

EXPLAIN FORMAT=JSON
SELECT
  BID
FROM 
  Rate
WHERE 
  BASE = 'EUR' 
  AND QUOTE = 'USD' 
  AND `TIME` <= (NOW() - INTERVAL 1 MONTH) 
ORDER BY 
  BASE DESC, QUOTE DESC, TIME DESC
LIMIT 1

如果我理解正确，那么Explain的输出就不会更好。 仍然只有2列没有TIME使用

解释输出

{ "query_block": { "select_id": 1, "cost_info": { "query_cost": "10384642.20" }, "ordering_operation": { "using_filesort": false, "table": { "table_name": "Rate", "access_type": "ref", "possible_keys": [ "IDX_UK", "IDX_TIME" ], "key": "IDX_UK", "used_key_parts": [ "BASE", "QUOTE" ], "key_length": "22", "ref": [ "const", "const" ], "rows_examined_per_scan": 46734411, "rows_produced_per_join": 23367205, "filtered": "50.00", "index_condition": "(( Developer . Rate . BASE <=> 'EUR') and ( Developer . Rate . QUOTE <=> 'USD') and ( Developer . Rate . TIME <= ((now() - interval 1 month))))", "cost_info": { "read_cost": "1037760.00", "eval_cost": "4673441.10", "prefix_cost": "10384642.20", "data_read_per_join": "1G" }, "used_columns": [ "ID", "TIME", "BASE", "QUOTE", "BID" ] } } } }

新增2：

我做了这4个请求：

- 1 -


<code>FLUSH STATUS;
SELECT
  BID
FROM 
  Rate
WHERE 
  BASE = 'EUR' 
  AND QUOTE = 'USD' 
  AND `TIME` <= (NOW() - INTERVAL 1 MONTH) 
LIMIT 1;
SHOW SESSION STATUS LIKE 'Handler%';</code>

- 2 -

<code>FLUSH STATUS;
SELECT
  BID
FROM 
  Rate FORCE INDEX (IDX_UK)
WHERE 
  BASE = 'EUR' 
  AND QUOTE = 'USD' 
  AND `TIME` <= (NOW() - INTERVAL 1 MONTH) 
LIMIT 1;
SHOW SESSION STATUS LIKE 'Handler%';
</code>

- 3 -

<code>FLUSH STATUS;
SELECT
  BID
FROM 
  Rate
WHERE 
  BASE = 'EUR' 
  AND QUOTE = 'USD' 
  AND `TIME` <= (NOW() - INTERVAL 1 MONTH) 
ORDER BY 
  `TIME` DESC 
LIMIT 1;
SHOW SESSION STATUS LIKE 'Handler%';</code>

- 4 -

<code>
FLUSH STATUS;
SELECT
  BID
FROM 
  Rate FORCE INDEX (IDX_UK)
WHERE 
  BASE = 'EUR' 
  AND QUOTE = 'USD' 
  AND `TIME` <= (NOW() - INTERVAL 1 MONTH) 
ORDER BY 
  `TIME` DESC 
LIMIT 1;
SHOW SESSION STATUS LIKE 'Handler%';</code>

session_status的输出在除请求3之外的所有请求中都是相同的。在请求3的输出中：Handler_read_prev = 486474; 在所有ather请求的输出中：Handler_read_prev = 0;

补充3：

我制作了表的副本，删除了Id字段，将UNIQUE键提升为PRIMARY。

方案：

CREATE TABLE Developer.Rate2 (
  TIME datetime NOT NULL,
  BASE varchar(3) NOT NULL,
  QUOTE varchar(3) NOT NULL,
  BID double NOT NULL,
  ASK double NOT NULL,
  PRIMARY KEY (BASE, QUOTE, TIME),
  INDEX IDX_BID_ASK (BID, ASK)
)
ENGINE = INNODB
AVG_ROW_LENGTH = 26
CHARACTER SET utf8
COLLATE utf8_general_ci
ROW_FORMAT = COMPRESSED;

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "9673452.20"
    },
    "ordering_operation": {
      "using_filesort": false,
      "table": {
        "table_name": "Rate2",
        "access_type": "range",
        "possible_keys": [
          "PRIMARY"
        ],
        "key": "PRIMARY",
        "used_key_parts": [
          "BASE",
          "QUOTE",
          "TIME"
        ],
        "key_length": "27",
        "rows_examined_per_scan": 48023345,
        "rows_produced_per_join": 16006180,
        "filtered": "100.00",
        "cost_info": {
          "read_cost": "68783.20",
          "eval_cost": "3201236.12",
          "prefix_cost": "9673452.20",
          "data_read_per_join": "732M"
        },
        "used_columns": [
          "TIME",
          "BASE",
          "QUOTE",
          "BID"
        ],
        "attached_condition": "((`Developer`.`Rate2`.`BASE` = 'EUR') and (`Developer`.`Rate2`.`QUOTE` = 'USD') and (`Developer`.`Rate2`.`TIME` <= <cache>((now() - interval 1 month))))"
      }
    }
  }
}

现在请求确实有效，Explain显示所有3列都被使用。 这种变体有效。

Answer 1

摆脱ID ，没用。 将您的UNIQUE键提升为PRIMARY 。 现在，奇迹般地，查询会更快，你提出的问题将变得毫无意义。 （您可能还需要洛林建议的DESC技巧。）

这是另一种比较性能的技术：

FLUSH STATUS;
SELECT ...;
SHOW SESSION STATUS LIKE 'Handler%';

我有兴趣看看SHOW的输出是否带有DESC技巧。 有/没有您提到的FORCE INDEX 。

为什么更快？ 您的查询使用的是二级索引，但它需要bid ，而索引并未对其进行“覆盖”。 要获得bid ，需要在'数据'中钻取PRIMARY KEY 。 通过更改它以便使用PK，可以避免这种额外的向下钻取。

Answer 2

您描述的行为（ref访问而不是更多列的范围访问）让我想起了Bug＃81341和Bug＃87613 。 这些错误分别在MySQL 5.7.17和5.7.21中修复。 你使用的是哪个版本？