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POSQL slowness when doing ActiveRecord find on a HABTM relationship
 

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  1. POSQL slowness when doing ActiveRecord find on a HABTM relationship
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    <p>I'm running Rails 2.3.5 with a MySQL database. I have a HABTM relationship between <code>Books</code> and <code>Users</code> and am trying to get all users who have a specified list of books (defined by an array of book names).</p> <p>I'm able to perform a <code>find</code> call that retrieves this list of users:</p> <pre><code>User.find( :all, :joins =&gt; :books, :conditions =&gt; { :books =&gt; { :name =&gt; book_names } } ) </code></pre> <p>However, this turns out to be extremely slow. After playing around in SQL, I found that the following call works much faster and retrieves the same results:</p> <pre><code>User.find_by_sql([ "SELECT users.* FROM users INNER JOIN books_users ON users.id = books_users.user_id WHERE books_users.book_id IN (SELECT id FROM books WHERE books.name IN (?))", book_names ]) </code></pre> <p>For the same query, the <code>find</code> call takes roughly 3000 ms on my computer whereas the <code>find_by_sql</code> call takes roughly 200 ms; this is an entire magnitude of speed difference. I suspect the culprit has something to do with the fact that the original <code>find</code> call is translated into a double <code>INNER JOIN</code> SQL query, equivalent to the following:</p> <pre><code>[ "SELECT users.* FROM users INNER JOIN books_users ON users.id = books_users.user_id INNER JOIN books ON books_users.book_id = books.id WHERE books.name IN (?)", book_names ] </code></pre> <p>My questions are:</p> <ul> <li>Does anyone know why this is the case? Why is the double <code>INNER JOIN</code> slower than my single <code>INNER JOIN</code> with a nested <code>SELECT</code> query?</li> <li>The <code>find_by_sql</code> call is not really taking advantage of the built-in support that Rails provides for HABTM relationships. In particular, it's surfacing the <code>books_users</code> join table that the Rails support typically abstracts away from the developer. Is there a way to specify the same query using a <code>find</code> call that hides this?</li> </ul>
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    1. USClaw
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    1. COHave you created indexes on the `book_id` and `user_id` fields for your `books_users` table?
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      1. POSQL slowness when doing ActiveRecord find on a HABTM relationship
      1. USDylan Markow
    2. COAck... I knew I forgot something. This was it! After adding the indexes, the find query now takes 15 ms.
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      1. USClaw
    3. CO@dmarkow: add it as an answer, so we can give you the points
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      1. USnathanvda
 

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