Small corrections/improvements to DB optimization docs.

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Django's database layer provides various ways to help developers get the most

out of their databases. This documents gathers together links to the relevant

out of their databases. This document gathers together links to the relevant

documentation, and adds various tips, organized under an number of headings that

outline the steps to take when attempting to optimize your database usage.

For instance:

* At the most basic level, use :ref:`filter and exclude <queryset-api>` to

  filtering in the database to avoid loading data into your Python process, only

  to throw much of it away.

* At the most basic level, use :ref:`filter and exclude <queryset-api>` to do

  filtering in the database.

* Use :ref:`F() object query expressions <query-expressions>` to do filtering

  against other fields within the same model.

added for these methods will not be executed, including anything driven from the

normal database object :ref:`signals <ref-signals>`.

Don't retrieve things you already have

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Use foreign key values directly

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