badges: fetch branch head and associated commit in same query

[matt-codecov]

fetching a badge uses 4+ queries:

• select * from owners where owners.name = ${request.username} and owners.service = ${request.service}
• select * from repos where repos.ownerid = ${owner.ownerid} and repos.name = ${request.repo_name}
• select * from branches where branches.name = ${request.branch_name} and branches.repo_id = ${repo.repoid}
• select * from commits where commits.commitid = ${branch.head} and commits.repoid = ${repo.repoid}

(side note: the first two queries actually have a LIMIT 21 which suggests they were logged while in queryset form. i am guessing a subsequent .first() call happens but doesn't need to re-run the query?)

in principle they could be folded into one:

select
  commits.*
from
  owners
inner join
  repos
on
  owners.ownerid = repos.ownerid
inner join
  branches
on
  branches.repo_id = repos.repoid
inner join
  commits
on
  commits.repoid = repos.repoid
where
  owners.username = ${request.username}
  and owners.service = ${request.service}
  and repos.name = ${request.repo_name}
  and branches.name = ${request.branch_name}
  and branches.head = commits.commitid

doing so might improve the performance of the badges endpoint. don't know

this PR doesn't actually do that:

• i'm not actually sure where the owners and repos queries happen, so those are left out
• because branches and commits don't actually have a FK relationship, Django won't perform a join between them

instead, this PR would result in SQL that looks something like:

select * from commits
where
  commits.repoid = ${repo.repoid}
  and commits.commit in (
    select * from branches
    where
      branches.name = ${request.branch_name}
      and branches.repoid = ${repo.repoid}
  )

maybe adding LIMIT = 1 to the subquery would help. not sure

i don't know whether the performance would actually improve, so i am closing this. just created a PR to document the result of this little exploration.

[seer-by-sentry]

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