---
title: "Rails 5.2 fixes query caching in MySQL & PostgreSQL adapters"
description: "Rails 5.2 fixes query caching in MySQL and PostgreSQL adapters"
canonical_url: "https://www.bigbinary.com/blog/rails-5-2-fixes-query-caching-in-mysql-and-postgresql-adapters"
markdown_url: "https://www.bigbinary.com/blog/rails-5-2-fixes-query-caching-in-mysql-and-postgresql-adapters.md"
---

# Rails 5.2 fixes query caching in MySQL & PostgreSQL adapters

Rails 5.2 fixes query caching in MySQL and PostgreSQL adapters

- Author: Sushant Mittal
- Published: May 16, 2018
- Categories: Rails 5.2, Rails

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.

```ruby
>> User.cache do
>>   2.times { User.exists?(1) }
>> end

User Exists (2.1ms)  SELECT  1 AS one FROM "users" WHERE "users"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]

User Exists (2ms)  SELECT  1 AS one FROM "users" WHERE "users"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]

```

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](https://github.com/rails/rails/pull/29454)
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.

```ruby
>> User.cache do
>>   2.times { User.exists?(1) }
>> end

User Exists (2.1ms)  SELECT  1 AS one FROM "users" WHERE "users"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]

CACHE User Exists (0.0ms)  SELECT  1 AS one FROM "users" WHERE "users"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]
```

Now, query has been cached as expected.

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

## Links

- [Human page](https://www.bigbinary.com/blog/rails-5-2-fixes-query-caching-in-mysql-and-postgresql-adapters)