How can I optimise COUNT with GROUPBY mysql query?

问题

I have 5M records in eus table and 121 records in es table. I am doing a left join but the COUNT query is making my query very slow. How can I optimize this?

public static function getAllActiveEvaluationSymptomsWithNameForDataTable(){
    $queryBuilder = new Builder();

    $queryBuilder
        ->from(array('es' =>  static::class))
        ->leftJoin('EvaluationUserSymptom',  'es.id = eus.eb_evaluation_symptom_id','eus')
        ->columns('es.id, es.title, COUNT(eus.eb_evaluation_symptom_id) AS counts')
        ->groupBy('eus.eb_evaluation_symptom_id')
        ->where('es.is_active = 1');

    return  $queryBuilder;
}

Raw query with Explain:

EXPLAIN SELECT es.id AS id, es.title AS title, COUNT(eus.eb_evaluation_symptom_id) AS counts, eus.date_created AS date_created FROM eb_evaluation_symptom AS es LEFT JOIN eb_evaluation_user_symptom AS eus ON es.id = eus.eb_evaluation_symptom_id WHERE es.is_active = 1 GROUP BY eus.eb_evaluation_symptom_id;

Output of Explain:

Explain Visual View:

This full table scan of count is making the problem.

Note: All JOINs and necessary columns fields are having proper indexes.

回答1:

A correlated subquery can be a fast method:

SELECT es.id, es.title,
      (select count(*)
       from eb_evaluation_user_symptom eus
       where es.id = eus.eb_evaluation_symptom_id
      ) as cnt
FROM eb_evaluation_symptom es  
WHERE es.is_active = 1 ;

For performance, you want an index on eb_evaluation_user_symptom(eb_evaluation_symptom_id).

An index on eb_evaluation_symptom won't be of much help, because that table is so small.

回答2:

Try to aggregate before the join:

SELECT es.id AS id, es.title AS title
  , coalesce(eus.counts, 0) as counts
  , eus.date_created
FROM eb_evaluation_symptom AS es 
LEFT JOIN 
 ( select eb_evaluation_symptom_id  
     , COUNT(*) AS counts
     , min(date_created) AS date_created -- or MAX?
   from eb_evaluation_user_symptom 
   GROUP BY eb_evaluation_symptom_id
 ) AS eus 
ON es.id = eus.eb_evaluation_symptom_id 
WHERE es.is_active = 1 ;

来源：https://stackoverflow.com/questions/58990863/how-can-i-optimise-count-with-groupby-mysql-query

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mysql

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performance

count

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