Rails 5.2 fixes query caching in MySQL & PostgreSQL adapters

Prior to Rails 5.2, MySQL and PostgreSQL adapters had select_value, select_values & select_rows select_{value,values,rows} methods. They improve the performance by not instantiating ActiveRecord::Result.

However these methods broke query caching of ActiveRecord::FinderMethods#exists? method. Let's check the issue.

1>> User.cache do
2>>   2.times { User.exists?(1) }
3>> end
4
5User Exists (2.1ms)  SELECT  1 AS one FROM "users" WHERE "users"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]
6
7User Exists (2ms)  SELECT  1 AS one FROM "users" WHERE "users"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]
8
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As we can see, query was not cached and sql was executed second time.

From Rails 5.2, MySQL and PostgreSQL adapters are no longer override select_{value,values,rows} methods which fix this query caching issue.

Also, the performance improvement provided by these methods was marginal and not a hotspot in Active Record, so this change was accepted.

Let's check query caching of ActiveRecord::FinderMethods#exists? after the change.

1>> User.cache do
2>>   2.times { User.exists?(1) }
3>> end
4
5User Exists (2.1ms)  SELECT  1 AS one FROM "users" WHERE "users"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]
6
7CACHE User Exists (0.0ms)  SELECT  1 AS one FROM "users" WHERE "users"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]
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Now, query has been cached as expected.

This change has been backported in rails 5.1 from version 5.1.2 as well.