requesting lots of packages with a subset of fields still causes changelogs, filelists, requires, etc.. to be loaded, resulting in slow requests

Author: @jlsherrill (jsherril@redhat.com)

Redmine Issue: 9655, https://pulp.plan.io/issues/9655

When we make a request like this:

/pulp/api/v3/content/rpm/packages/?fields=pulp_href%2Cname%2Cversion%2Crelease%2Carch%2Cepoch%2Csummary%2Cis_modular%2Crpm_sourcerpm%2Clocation_href%2CpkgId&limit=1&offset=1000&repository_version=%2Fpulp%2Fapi%2Fv3%2Frepositories%2Frpm%2Frpm%2Fc6b93206-22bb-4c58-ba07-828c32326330%2Fversions%2F1%2F

where some subset of fields is requested, we're still seeing all of the fields being loaded from the database. The result is that the query takes much much longer than it should

Testing on the rhel7 repo, it took about ~100 seconds on my machine to fetch all 32K packages specifying the fields above. If I comment out the filelists, changelogs, provides/requires from the model and serializers, that drops down to ~33 seconds. This is a huge improvement.

Reading this: https://stackoverflow.com/questions/53319787/how-can-i-select-specific-fields-in-django-rest-framework

it looks like its may be possible to exclude certain fields from the query itself fairly easily.

Start by reproducing the packages endpoint request with the listed fields and inspect the Django REST Framework serializer and database-query path. Done means the query loads only requested fields while preserving the response, and the request completes substantially faster than the reported baseline.