Otimizando uma consulta de junção de 3 tabelas

Question

SELECT job.id_job, job.fk_company, job.fk_user, job.fk_job_category, 
job.fk_job_type, job.fk_place, job.job_identifier, job.external_job_id, job.title,
job.short_description, job.status, job.expires_at, job.nb_vacancies, 
job.featured_rank, job.created_at, job.updated_at, job.deleted_at, job.slug, 
COUNT(fk_job) AS `application_count` 
FROM `job` 
INNER JOIN `company` ON (job.fk_company=company.id_company) 
LEFT JOIN `job_application` ON (job.id_job=job_application.fk_job) 
WHERE job.deleted_at IS NULL  
GROUP BY job.id_job 
ORDER BY job.id_job DESC 
LIMIT 50

Esta consulta demora um pouco, embora todos os campos que usamos no JOIN, GROUP BY e ORDER BY declarações são indexadas ou declaradas como foreign key ou primary key.

+----+-------------+-----------------+-------+----------------------+----------------------+---------+----------------------------+------+----------------------------------------------+
| id | select_type | table           | type  | possible_keys        | key                  | key_len | ref                        | rows | Extra                                        |
+----+-------------+-----------------+-------+----------------------+----------------------+---------+----------------------------+------+----------------------------------------------+
|  1 | SIMPLE      | company         | index | PRIMARY              | company_I_2          | 4       | NULL                       | 3244 | Using index; Using temporary; Using filesort |
|  1 | SIMPLE      | job             | ref   | job_FI_1             | job_FI_1             | 4       | intjobs.company.id_company |    3 | Using where                                  |
|  1 | SIMPLE      | job_application | ref   | job_application_FI_2 | job_application_FI_2 | 4       | intjobs.job.id_job         |    1 | Using index                                  |
+----+-------------+-----------------+-------+----------------------+----------------------+---------+----------------------------+------+----------------------------------------------+
3 rows in set (0.00 sec)

SHOW CREATE TABLE job;

CREATE TABLE `job` (
  `id_job` int(11) NOT NULL AUTO_INCREMENT,
  `fk_company` int(11) NOT NULL,
  `fk_user` int(11) NOT NULL,
  `fk_job_category` int(11) NOT NULL,
  `fk_job_type` int(11) NOT NULL,
  `fk_place` int(11) DEFAULT NULL,
  `job_identifier` varchar(20) DEFAULT NULL,
  `external_job_id` varchar(50) DEFAULT NULL,
  `title` varchar(255) DEFAULT NULL,
  `short_description` text,
  `status` enum('new','active_expiration_reminded','active','inactive','expired') NOT NULL DEFAULT 'new',
  `expires_at` datetime DEFAULT NULL,
  `nb_vacancies` int(11) NOT NULL DEFAULT '1',
  `featured_rank` int(11) NOT NULL DEFAULT '0',
  `created_at` datetime DEFAULT NULL,
  `updated_at` datetime DEFAULT NULL,
  `deleted_at` datetime DEFAULT NULL,
  `slug` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id_job`),
  UNIQUE KEY `job_U_1` (`job_identifier`),
  UNIQUE KEY `job_slug` (`slug`),
  KEY `job_FI_1` (`fk_company`),
  KEY `job_FI_2` (`fk_user`),
  KEY `job_FI_3` (`fk_job_category`),
  KEY `job_FI_4` (`fk_job_type`),
  KEY `job_FI_6` (`fk_place`),
  KEY `job_FI_5` (`fk_place`),
  CONSTRAINT `job_FK_1` FOREIGN KEY (`fk_company`) REFERENCES `company` (`id_company`),
  CONSTRAINT `job_FK_2` FOREIGN KEY (`fk_user`) REFERENCES `user` (`id_user`),
  CONSTRAINT `job_FK_3` FOREIGN KEY (`fk_job_category`) REFERENCES `job_category` (`id_job_category`),
  CONSTRAINT `job_FK_4` FOREIGN KEY (`fk_job_type`) REFERENCES `job_type` (`id_job_type`),
  CONSTRAINT `job_FK_5` FOREIGN KEY (`fk_place`) REFERENCES `place` (`id_place`)
) ENGINE=InnoDB AUTO_INCREMENT=27630 DEFAULT CHARSET=utf8 |

SHOW CREATE TABLE company;

    CREATE TABLE `company` (
      `id_company` int(11) NOT NULL AUTO_INCREMENT,
      `fk_place` int(11) DEFAULT NULL,
      `fk_industry` int(11) DEFAULT NULL,
      `fk_modified_by_user` int(11) DEFAULT NULL,
      `name` varchar(255) NOT NULL,
      `featured_rank` int(11) NOT NULL DEFAULT '0',
      `created_at` datetime DEFAULT NULL,
      `updated_at` datetime DEFAULT NULL,
      `deleted_at` datetime DEFAULT NULL,
      `slug` varchar(255) DEFAULT NULL,
      PRIMARY KEY (`id_company`),
      UNIQUE KEY `company_slug` (`slug`),
      KEY `company_I_1` (`name`),

SHOW CREATE TABLE job_application;

CREATE TABLE `job_application` (
  `id_job_application` int(11) NOT NULL AUTO_INCREMENT,
  `fk_user` int(11) NOT NULL,
  `fk_job` int(11) NOT NULL,
  `status` enum('new','application_sent','approval_pending','in_call','invalid','jobseeker_notified','improvement_pending','read_pending','application_read','read_pending_reminded','manual_interaction_pending') NOT NULL DEFAULT 'new',
  `created_at` datetime DEFAULT NULL,
  `updated_at` datetime DEFAULT NULL,
  `deleted_at` datetime DEFAULT NULL,
  PRIMARY KEY (`id_job_application`),
  KEY `job_application_FI_1` (`fk_user`),
  KEY `job_application_FI_2` (`fk_job`),
  CONSTRAINT `job_application_FK_1` FOREIGN KEY (`fk_user`) REFERENCES `user` (`id_user`),
  CONSTRAINT `job_application_FK_2` FOREIGN KEY (`fk_job`) REFERENCES `job` (`id_job`)
) ENGINE=InnoDB AUTO_INCREMENT=29 DEFAULT CHARSET=utf

Versão MySQL:

+-------------------------+------------------------------+
| Variable_name           | Value                        |
+-------------------------+------------------------------+
| innodb_version          | 5.6.22                       |
| protocol_version        | 10                           |
| slave_type_conversions  |                              |
| version                 | 5.6.22-log                   |
| version_comment         | MySQL Community Server (GPL) |
| version_compile_machine | x86_64                       |
| version_compile_os      | Linux                        |
+-------------------------+------------------------------+

Answer 1

Três pontos que a consulta pode ser melhorada:

• remova a junção para company.A condição de união é através de uma chave estrangeira não anulável, portanto deve ser sempre verdadeira.
• mudar o GROUP BY x ORDER BY x DESC para: GROUP BY x DESC.Isso evitará classificações desnecessárias.Observe que a sintaxe está obsoleta, então você pode precisar alterá-la novamente em uma atualização futura do mysql.
• adicione um índice em (deleted_at, id_job).Isso é essencial para esta consulta.Em geral, um índice em uma coluna de sinalização (verdadeiro/falso) ou que tenha poucos valores distintos geralmente é inútil.Mas o deleted_at coluna é um carimbo de data/hora, portanto o índice também pode ser útil em outras consultas.Esta consulta tem o extra ORDER BY / LIMIT, para que o índice de 2 colunas possa ser usado de forma eficaz.

A consulta reescrita:

SELECT job.id_job, job.fk_company, job.fk_user, job.fk_job_category, 
  job.fk_job_type, job.fk_place, job.job_identifier, job.external_job_id, job.title,
  job.short_description, job.status, job.expires_at, job.nb_vacancies, 
  job.featured_rank, job.created_at, job.updated_at, job.deleted_at, job.slug, 
  COUNT(fk_job) AS application_count 
FROM job
  LEFT JOIN job_application ON job.id_job = job_application.fk_job 
WHERE job.deleted_at IS NULL  
GROUP BY job.id_job DESC 
LIMIT 50 ;

Você também pode usar esta versão, que força o uso do índice e evita o GROUP BY na consulta principal (agrupa na subconsulta in-line que será executada apenas para 50 valores):

SELECT job.id_job, job.fk_company, job.fk_user, job.fk_job_category, 
  job.fk_job_type, job.fk_place, job.job_identifier, job.external_job_id, job.title,
  job.short_description, job.status, job.expires_at, job.nb_vacancies, 
  job.featured_rank, job.created_at, job.updated_at, job.deleted_at, job.slug, 
  ( SELECT COUNT(*)
    FROM job_application AS ja
    WHERE j.id_job = ja.fk_job  
  ) AS application_count 
FROM 
  ( SELECT id_job
    FROM job
    WHERE deleted_at IS NULL  
    ORDER BY id_job DESC 
    LIMIT 50
  ) AS j
  JOIN job ON job.id_job = j.id_job  
ORDER BY j.id_job DESC  ;