This benchmark exists to test if ordering has any noticeable impact in
the test environment.

While these benchmarks run at roughly 1500 i/sec setting the threshold
to 1000 leaves some room for deviations (e.g. due to different DB
setups).

This query used to rely on a JOIN, effectively producing the following
SQL:

    SELECT users.*
    FROM users
    LEFT OUTER JOIN emails ON emails.user_id = users.id
    WHERE (users.email = X OR emails.email = X)
    LIMIT 1;

The use of a JOIN means having to scan over all Emails and users, join
them together and then filter out the rows that don't match the criteria
(though this step may be taken into account already when joining).

In the new setup this query instead uses a sub-query, producing the
following SQL:

    SELECT *
    FROM users
    WHERE id IN (select user_id FROM emails WHERE email = X)
    OR email = X
    LIMIT 1;

This query has the benefit that it:

1. Doesn't have to JOIN any rows
2. Only has to operate on a relatively small set of rows from the
   "emails" table.

Since most users will only have a handful of Emails associated
(certainly not hundreds or even thousands) the size of the set returned
by the sub-query is small enough that it should not become problematic.

Performance of the old versus new version can be measured using the
following benchmark:

    # Save this in ./bench.rb
    require 'benchmark/ips'

    email = 'yorick@gitlab.com'

    def User.find_by_any_email_old(email)
      user_table = arel_table
      email_table = Email.arel_table

      query = user_table.
        project(user_table[Arel.star]).
        join(email_table, Arel::Nodes::OuterJoin).
        on(user_table[:id].eq(email_table[:user_id])).
        where(user_table[:email].eq(email).or(email_table[:email].eq(email)))

      find_by_sql(query.to_sql).first
    end

    Benchmark.ips do |bench|
      bench.report 'original' do
        User.find_by_any_email_old(email)
      end

      bench.report 'optimized' do
        User.find_by_any_email(email)
      end

      bench.compare!
    end

Running this locally using "bundle exec rails r bench.rb" produces the
following output:

    Calculating -------------------------------------
                original     1.000  i/100ms
               optimized    93.000  i/100ms
    -------------------------------------------------
                original     11.103  (± 0.0%) i/s -     56.000
               optimized    948.713  (± 5.3%) i/s -      4.743k

    Comparison:
               optimized:      948.7 i/s
                original:       11.1 i/s - 85.45x slower

In other words, the new setup is 85x faster compared to the old setup,
at least when running this benchmark locally.

For GitLab.com these improvements result in User.find_by_any_email
taking only ~170 ms to run, instead of around 800 ms. While this is
"only" an improvement of about 4.5 times (instead of 85x) it's still
significantly better than before.

Fixes #3242

This cuts down the time it takes to sort issues of a milestone by about
10x. In the previous setup the code would run a SQL query for every
issue that had to be sorted. The new setup instead runs a single SQL
query to update all the given issues at once.

The attached benchmark used to run at around 60 iterations per second,
using the new setup this hovers around 600 iterations per second. Timing
wise a request to update a milestone with 40-something issues would take
about 760 ms, in the new setup this only takes about 130 ms.

Fixes #3066

Improve User.by_login performance

This greatly speeds up the performance of `User.by_login`. I adopted some changes from @haynes in this patch, the credits go to him for coming up with those originally.

Fixes #2341

See merge request !1545

By comparing objects in Ruby we can greatly improve the performance of
this method. In the worst case (should no data be eager loaded) this
will run the same amount of queries as before, in the best case (when
data _is_ eager loadeD) it requires no queries at all.

The added benchmark used to produce around 273 iterations per second.
With this commit this has been increased to almost 40 000 iterations per
second: a speedup of roughly 145 times.

Combined with eager loading Note associations this results in about 30
queries less when viewing a single issue, this in turn cuts down the
loading time by 30-40%.

Performance is improved in two steps:

1. On PostgreSQL an expression index is used for checking lower(email)
   and lower(username).
2. The check to determine if we're searching for a username or Email is
   moved to Ruby. Thanks to @haynes for suggesting and writing the
   initial implementation of this.

Moving the check to Ruby makes this method an additional 1.5 times
faster compared to doing the check in the SQL query.

With performance being improved I've now also tweaked the amount of
iterations required by the User.by_login benchmark. This method now runs
between 900 and 1000 iterations per second.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

By using a JOIN we can remove the need for using 2 separate queries to
find a project by its namespace. Combined with an index (only needed for
PostgreSQL) this reduces the query time from ~245 ms (~520 ms for the
first call) down to roughly 10 ms (~15 ms for the first call).