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POHow to efficiently write DISTINCT query in Django with table having foreign keys
 

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  1. POHow to efficiently write DISTINCT query in Django with table having foreign keys
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    <p>I want to show distinct cities of Users in the front end dropdown. For that, i make a db query which fetches distinct <code>city_name</code> from table <code>City</code> but only those cities where users are present. </p> <p>Something like below works for a small size of <code>User</code> table, but takes a very long time if <code>User</code> table in of size 10 million. Distinct cities of these users are still ~100 though.</p> <pre><code>class City(models.Model): city_code = models.IntegerField(unique=True) city_name = models.CharField(max_length=256) class User(models.Model): city = models.ForeignKey('City', to_field='city_code') </code></pre> <p>Now i try to search distinct city names as:</p> <pre><code>City.objects.filter().values_list('city__city_name').distinct() </code></pre> <p>which translates to this on PostgreSQL:</p> <pre><code>SELECT DISTINCT "city"."city_name" FROM "user" LEFT OUTER JOIN "city" ON ("user"."city_id" = "city"."city_code"); </code></pre> <p>Time: 9760.302 ms</p> <p>That clearly showed that PostgreSQL was not making use of index on 'user'.'city_id'. I also read about a workaround solution <a href="http://zogovic.com/post/44856908222/optimizing-postgresql-query-for-distinct-values" rel="nofollow">here</a> which involved writing a custom SQL query which somehow utilizes index.</p> <p>I tried to find distinct 'user'.'city_id' using the above query, and that actually turned out to be pretty fast.</p> <pre><code>WITH RECURSIVE t(n) AS (SELECT min(city_id) FROM user UNION SELECT (SELECT city_id FROM user WHERE city_id &gt; n order by city_id limit 1) FROM t WHERE n is not null) SELECT n FROM t; </code></pre> <p>Time: 79.056 ms</p> <p>But now i am finding it hard to incorporate this in my Django code. I still think it is a kind of hack adding custom query in the code for this. But a bigger concern for me is that the column name can be totally dynamic, and i can not hardcode these column names (eg. city_id, etc.) in the code.</p> <pre><code>#original_fields could be a list from input, like ['area_code__district_code__name'] dataset_klass.objects.filter().values_list(*original_fields).distinct() </code></pre> <p>Using the custom query would need atleast splitting the field name with '__' as delimiter and process the first part. But it looks like a bad hack to me.</p> <p>How can i improve this? </p> <p>PS. The <code>City</code> <code>User</code> example is just shown to explain the scenario. The syntax might not be correct.</p>
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