My thoughts, ideas, questions and concerns on technology, sports, music and life tag:paulbarry.com,2007:Paulbarry.com PaulBarry.com 2009-01-05T21:07:06-05:00 Paul Barry mail@paulbarry.com urn:uuid:f4cc9a01-2727-40f6-b281-0fb770de696b 2008-01-06T20:58:56-05:00 2008-01-06T20:58:56-05:00

<p>The Rails' ORM ActiveRecord has support for counter caches. Say you have a blog, like this one, which has articles, and each article has many comments. If you are going to show a list of articles with the number of comments each article has, without a counter cache, you will end up with N+1 SQL queries. One query to get the articles, and one <code>select count(*) from comments where article_id = ?</code> query for each article. </p> <p>To avoid the N+1 select, you can add a column in the articles table that contains the number of comments the article has. With some ORM frameworks, this becomes somewhat of a pain, because you have to keep that count column updated, but ActiveRecord makes it easy. If you just create a table called <code>&lt;table_name&gt;_count</code> on a parent table, it will keep the total of the records that belong to that record in that column. So then when you call <code>article.comments_count</code>, there is no SQL query executed.</p> <p>This is nice, but what happens if you have a site in production and you want to add a counter cache column? You've got to create the column and then get the count updated. You can do this in a migration, but you can also do it in SQL using a <a href="http://dev.mysql.com/doc/refman/4.1/en/correlated-subqueries.html">correlated subquery</a>. Using MySQL, you can do it like this:</p> <pre><code>alter table articles add column comments_count int(11) default 0; update articles set articles.comments_count = (select count(comments.id) from comments where articles.id = comments.article_id); </code></pre>

<p>The Rails' ORM ActiveRecord has support for counter caches. Say you have a blog, like this one, which has articles, and each article has many comments. If you are going to show a list of articles with the number of comments each article has, without a counter cache, you will end up with N+1 SQL queries. One query to get the articles, and one <code>select count(*) from comments where article_id = ?</code> query for each article. </p> <p>To avoid the N+1 select, you can add a column in the articles table that contains the number of comments the article has. With some ORM frameworks, this becomes somewhat of a pain, because you have to keep that count column updated, but ActiveRecord makes it easy. If you just create a table called <code>&lt;table_name&gt;_count</code> on a parent table, it will keep the total of the records that belong to that record in that column. So then when you call <code>article.comments_count</code>, there is no SQL query executed.</p> <p>This is nice, but what happens if you have a site in production and you want to add a counter cache column? You've got to create the column and then get the count updated. You can do this in a migration, but you can also do it in SQL using a <a href="http://dev.mysql.com/doc/refman/4.1/en/correlated-subqueries.html">correlated subquery</a>. Using MySQL, you can do it like this:</p> <pre><code>alter table articles add column comments_count int(11) default 0; update articles set articles.comments_count = (select count(comments.id) from comments where articles.id = comments.article_id); </code></pre>