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I set null=True on one of the ForeignKey fields in my Django model, and now when I query that model, it is about 10 times slower. (I'm using select_related())

Looking at my Postgres logs before and after the change gives clues to the reason:

  • Before setting null=True, the SQL that is generated is a single select statement with a couple of inner joins.
  • After setting null=True, the SQL that is generated leaves out one of the joins and instead is followed by thousands of identical select statements.

So it's the classic n+1 query issue, and until this is fixed, how can I set null=True on a ForeignKey field without taking performance hits?

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  • do you've bounded the select_related through the depth arg or a field name? Commented Jan 16, 2013 at 23:14
  • No bounds on it, would that help? Commented Jan 17, 2013 at 2:27
  • Maybe your breakdown isn't just for the null=True; Commented Jan 17, 2013 at 15:43

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You can solve this through a raw query. Take a look to the query that is generated before putting null=True and execute it via a raw query instead of using 'top-level django ORM'. The breakdown is due the ORM don't the Postgres server, so you can avoid useless code generation running the SQL code directly.

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This is a nice solution. I'll give it a try.
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From the official document, that is the limitation of the select_related() method, here is the details.

But you could have specified the related field in select_relate(), something like 

modelA.objects.select_related('ModelAForeignKeyName__RelatedModelFeildName')
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