MySQL表指数基数

Question

我有一个MySQL数据库表，它很大，有250万行，而且还在增长。为了加快查询速度，我向其中一列添加了一个索引。例如，当我通过PHPMyAdmin手动设置索引时，基数大约是1500，这似乎是正确的，而且我的查询运行时没有问题。

然后，在运行了一些查询(特别是在INSERT (但不限于))之后，该索引的基数下降到17或18，查询运行得非常慢。有时候，它似乎很有效，可以追溯到1500个左右，或者我必须再通过PHPMyAdmin来完成它。

有什么办法阻止这种基数下降的发生吗？

CREATE TABLE IF NOT EXISTS `probe_results` (
  `probe_result_id` int(11) NOT NULL AUTO_INCREMENT,
  `date` date NOT NULL,
  `month` int(11) NOT NULL,
  `year` int(11) NOT NULL,
  `time` time NOT NULL,
  `type` varchar(11) NOT NULL,
  `probe_id` varchar(50) NOT NULL,
  `status` varchar(11) NOT NULL,
  `temp_1` decimal(11,0) NOT NULL,
  `temp_2` decimal(11,0) NOT NULL,
  `crc` varchar(11) NOT NULL,
  `raw_data` text NOT NULL,
  `txt_file` text NOT NULL,
  PRIMARY KEY (`probe_result_id`),
  KEY `probe_id` (`probe_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=2527300 ;

“probe_result_id”列是主键，probe_id是有问题索引的列。

示例查询：

/*
* 提示：该行代码过长，系统自动注释不进行高亮。一键复制会移除系统注释 
* SELECT IF(b.reactive_total IS NULL, 0, b.reactive_total) AS reactive_total, a.* FROM (SELECT COUNT(CASE WHEN asset_testing_results.asset_testing_year = '2016' AND asset_testing_results.asset_testing_month = '7' AND asset_testing_results.asset_stopped = '0' AND asset_testing_results.asset_testing_completed = '0' THEN 1 END) AS due_total, (COUNT(CASE WHEN asset_testing_results.asset_testing_year = '2016' AND asset_testing_results.asset_stopped = '0' AND asset_testing_results.asset_testing_completed = '1' AND asset_testing_results.asset_testing_satisfactory = '1' AND asset_testing_results.asset_testing_actioned = '0' THEN 1 END)+(IF(probes_passed_total IS NULL, 0, probes_passed_total))) AS passed_total, (COUNT(CASE WHEN asset_testing_results.asset_testing_year = '2016' AND asset_testing_results.asset_stopped = '0' AND asset_testing_results.asset_testing_completed = '1' AND asset_testing_results.asset_testing_satisfactory = '0' AND asset_testing_results.asset_testing_actioned = '0' THEN 1 END)+(IF(probes_failed_total IS NULL, 0, probes_failed_total))) AS failed_total, COUNT(CASE WHEN asset_testing_results.asset_testing_year = '2016' AND asset_testing_results.asset_stopped = '0' AND asset_testing_results.asset_testing_completed = '1' AND asset_testing_results.asset_testing_actioned = '1' THEN 1 END) AS actioned_total, COUNT(CASE WHEN asset_testing_results.asset_testing_year = '2016' AND asset_testing_results.asset_testing_month < '7' AND asset_testing_results.asset_testing_completed = '0' AND asset_testing_results.asset_testing_satisfactory = '0' AND asset_testing_results.asset_stopped = '0' THEN 1 END) AS missed_total, site.site_key, site.site_name FROM site LEFT JOIN location ON location.site_key = site.site_key LEFT JOIN sub_location ON sub_location.location_key = location.location_key LEFT JOIN asset ON asset.sub_location_key = sub_location.sub_location_key AND asset.stopped = '0' LEFT JOIN asset_testing ON asset_testing.asset_type_key = asset.asset_type_key AND asset_testing.probe_assessed = '0' LEFT JOIN asset_testing_results ON asset_testing_results.asset_testing_key = asset_testing.asset_testing_key AND asset_testing_results.asset_key = asset.asset_key LEFT JOIN (SELECT site.site_key, COUNT(CASE WHEN p.probe_id IS NOT NULL AND p.asset_testing_key IS NOT NULL THEN 1 END) AS probes_passed_total, COUNT(CASE WHEN p.probe_id IS NOT NULL AND p.asset_testing_key IS NULL AND p.temp_1 IS NOT NULL THEN 1 END) AS probes_failed_total FROM assetsvs_probes LEFT JOIN (SELECT q.probe_id, q.month, q.year, IF(r.temp_1 IS NULL, q.temp_1, r.temp_1) as temp_1, r.asset_testing_key FROM (SELECT DISTINCT probe_results.probe_id, probe_results.month, probe_results.year, probe_results.temp_1 FROM probe_results LEFT JOIN assetsvs_probes ON assetsvs_probes.probe_id = probe_results.probe_id LEFT JOIN asset ON asset.asset_key = assetsvs_probes.asset_key LEFT JOIN sub_location ON sub_location.sub_location_key = asset.sub_location_key LEFT JOIN location ON location.location_key = sub_location.location_key LEFT JOIN site ON site.site_key = location.site_key WHERE site.client_key = '25')q LEFT JOIN (SELECT probe_results.month, probe_results.year, probe_results.probe_id, temp_1, asset_testing.asset_testing_key FROM probe_results LEFT JOIN assetsvs_probes ON assetsvs_probes.probe_id = probe_results.probe_id LEFT JOIN asset_testing ON asset_testing.asset_testing_key = assetsvs_probes.asset_testing_key LEFT JOIN asset ON asset.asset_key = assetsvs_probes.asset_key LEFT JOIN sub_location ON sub_location.sub_location_key = asset.sub_location_key LEFT JOIN location ON location.location_key = sub_location.location_key LEFT JOIN site ON site.site_key = location.site_key WHERE temp_1 != 'invalid' AND ((temp_1 >= test_min AND test_max = '') OR (temp_1 <= test_max AND test_min = '') OR (temp_1 >= test_min AND temp_1 <= test_max)) AND year = '2016' AND site.client_key = '25' GROUP BY probe_results.month, probe_results.year, probe_results.probe_id)r ON r.probe_id = q.probe_id AND r.month = q.month AND r.year = q.year WHERE q.year = '2016' GROUP BY probe_id, month, year) p ON p.probe_id = assetsvs_probes.probe_id LEFT JOIN asset_testing ON asset_testing.asset_testing_key = assetsvs_probes.asset_testing_key LEFT JOIN asset ON asset.asset_key = assetsvs_probes.asset_key LEFT JOIN sub_location ON sub_location.sub_location_key = asset.sub_location_key LEFT JOIN location ON location.location_key = sub_location.location_key LEFT JOIN site ON site.site_key = location.site_key GROUP BY site.site_key) probe_results ON probe_results.site_key = site.site_key WHERE site.client_key = '25' GROUP BY site.site_key)a LEFT JOIN (SELECT COUNT(CASE WHEN jobs.status = '3' THEN 1 END) AS reactive_total, site.site_key FROM jobs LEFT JOIN jobs_meta ON jobs_meta.job_id = jobs.job_id AND jobs_meta.meta_key = 'start_date' LEFT JOIN site ON site.site_key = jobs.site_key WHERE site.client_key = '25' AND jobs_meta.meta_value LIKE '%/2016 %' GROUP BY site.site_key)b ON b.site_key = a.site_key
*/

谢谢

Answer 1

基数(以及其他统计数据)是由MySQL自动计算和更新的，因此您没有直接的方法来防止它下降。

但是，您可以采取一些步骤来减少发生这种情况的可能性，或者纠正这种行为。

首先，如果运行MySQL命令，分析表将更新所有受支持的表引擎的索引统计信息。

对于innodb引擎，MySQL提供了一组可以影响采样行为的配置设置。MySQL文档中描述了这些设置及其效果：

• 非持久性统计
• 持久统计

主要设置为innodb_stats_transient_sample_pages：

*像1或2这样的小值可能导致基数的不准确估计。 ·增加innodb_stats_transient_sample_pages值可能需要更多磁盘读取。值远远大于8(例如，100)，会导致打开表或执行显示表状态所需的时间显著减少。 ·优化器可以根据不同的索引选择性估计值选择非常不同的查询计划

。

对于myisam，MySQL dos不提供这样的各种设置。myisam_stats_method设置用一般索引统计文档描述