2.7.9. Database optimization

Using examples of errors encountered by hosting users, we will consider how you can optimize the database.

An example of a table that consists of more than 40,000 records and to which requests are sent, which create a considerable load on the server, since there are a lot of them:

CREATE TABLE `links` (
  `id` INT(11) UNSIGNED NOT NULL AUTO_INCREMENT,
  `source` INT(11) UNSIGNED NOT NULL,
  `category` INT(11) UNSIGNED NOT NULL,
  `title` text NOT NULL,
  `description` text NOT NULL,
  `text` text NOT NULL,
  `link` text NOT NULL,
  `publication` datetime NOT NULL,
  `scan` datetime NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `unique` (`id`) USING BTREE,
  KEY `normal` (`source`,`category`,`title`(100),`publication`,`description`(100),`text`(100),`scan`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=40000 DEFAULT CHARSET=utf8

An example of incoming requests:

SELECT COUNT(*)
FROM links
WHERE link = 'http://www.......com.ua/31-08-2014/trolleybook-podgotovil-prazdnichnuyu-programmu-k-limba-noastr-/n96349/'
AND UNIX_TIMESTAMP(publication) = 1409472019

By executing a request with the prefix EXPLAIN, we see that the entire table is scanned to get the results — about 40 thousand rows. To optimize the work of the table, add an index on the field publication. This operation will already be enough to increase the speed of work and reduce the load on the MySQL server processor.

Adding an index

An index can be added in any of the following ways:
  • Through SQLqueries:
    ALTER TABLE `table_name` ADD INDEX(`column_name`);

    In the request: table_name - the name of the table, column_name — the name of the field for which you want to add an index.

As a supplement, we recommend:

  1. Change field type title, description, link with text on varchar desired length.
  2. Remove index UNIQ on the field id... There is no need for it, since this function is already performed by the index. PRIMARY on the same field.
  3. Index with title normal even more doubtful, since the server has to work hard to create it. It is simply huge, and it is extremely unlikely that the search occurs in all the fields that are specified in it. Most likely it is "dead" an index that hinders rather than helps. Developers should be aware that the index will only be used if the request contains fields that are in it from left to right. So, for example, this index will be used if in the condition WHERE fields will be specified category, title, source... If the field source will not be specified in the request, then the index will not be used.

And finally, there is a question regarding the need to use the request itself COUNT(*). It is very likely that its author wants in this way to obtain information about the presence of a record in the table, and not the total number of records in the table with the specified URL for the specified date. In this case, it is correct to use the query without COUNT(*) with the condition LIMIT 1:

SELECT id
FROM links
WHERE
link =  'http://www.......com.ua/31-08-2014/trolleybook-podgotovil-prazdnichnuyu-programmu-k-limba-noastr-/n96349/'
AND UNIX_TIMESTAMP(publication) = 1409472019
LIMIT 1

In this case, the database will stop the search when the first record is found, and in the case of COUNT will scan the entire table.

PS Despite the fact that there is a search in two columns in the SQL query publication and link, the field was not added to the index link... This is due to the fact that for this table the field publication with a date is pretty unique and will contain at most a few lines with different link... Therefore, the overhead of forming an index for the field link significantly exceeds the cost of scanning multiple rows at a single index publicate.

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