Rewrite SnippetsFinder to improve performance

This completely rewrites the SnippetsFinder class from the ground up in
order to improve its performance. The old code was beyond salvaging. It
was complex, included various Rails 5 workarounds, comments that
shouldn't be necessary, and most important of all: it produced a really
poorly performing database query.

As a result, I opted for rewriting the finder from scratch, instead of
trying to patch the existing code. Instead of trying to reuse as many
existing methods as possible, I opted for defining new methods
specifically meant for the SnippetsFinder. This requires some extra code
here and there, but allows us to have much more control over the
resulting SQL queries. It is these changes that then allow us to produce
a _much_ more efficient query.

To illustrate how bad the old query was, we will use my own snippets as
an example. Currently I have 52 snippets, most of which are global ones.
To retrieve these, you would run the following Ruby code:

    user = User.find_by(username: 'yorickpeterse')

    SnippetsFinder.new(user, author: user).execute

On GitLab.com the resulting query will take between 10 and 15 seconds to
run, producing the query plan found at
https://explain.depesz.com/s/Y5IX. Apart from the long execution time,
the total number of buffers (the sum of all shared hits) is around 185
GB, though the real number is probably (hopefully) much lower as I doubt
simply summing these numbers produces the true total number of buffers
used.

The new query's plan can be found at https://explain.depesz.com/s/wHdN,
and this query takes between 10 and 100-ish milliseconds to run. The
total number of buffers used is only about 30 MB.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/52639

1 files changed, 56 insertions, 21 deletions

diff --git a/app/models/snippet.rb b/app/models/snippet.rb
index 1c5846b4023..11856b55902 100644
--- a/app/models/snippet.rb
+++ b/app/models/snippet.rb
@@ -63,6 +63,62 @@ class Snippet < ActiveRecord::Base
   attr_spammable :title, spam_title: true
   attr_spammable :content, spam_description: true
 
+  def self.with_optional_visibility(value = nil)
+    if value
+      where(visibility_level: value)
+    else
+      all
+    end
+  end
+
+  def self.only_global_snippets
+    where(project_id: nil)
+  end
+
+  def self.only_include_projects_visible_to(current_user = nil)
+    levels = Gitlab::VisibilityLevel.levels_for_user(current_user)
+
+    joins(:project).where('projects.visibility_level IN (?)', levels)
+  end
+
+  def self.only_include_projects_with_snippets_enabled(include_private: false)
+    column = ProjectFeature.access_level_attribute(:snippets)
+    levels = [ProjectFeature::ENABLED, ProjectFeature::PUBLIC]
+
+    levels << ProjectFeature::PRIVATE if include_private
+
+    joins(project: :project_feature)
+      .where(project_features: { column => levels })
+  end
+
+  def self.only_include_authorized_projects(current_user)
+    where(
+      'EXISTS (?)',
+      ProjectAuthorization
+        .select(1)
+        .where('project_id = snippets.project_id')
+        .where(user_id: current_user.id)
+    )
+  end
+
+  def self.for_project_with_user(project, user = nil)
+    return none unless project.snippets_visible?(user)
+
+    if user && project.team.member?(user)
+      project.snippets
+    else
+      project.snippets.public_to_user(user)
+    end
+  end
+
+  def self.visible_to_or_authored_by(user)
+    where(
+      'snippets.visibility_level IN (?) OR snippets.author_id = ?',
+      Gitlab::VisibilityLevel.levels_for_user(user),
+      user.id
+    )
+  end
+
   def self.reference_prefix
     '$'
   end
@@ -81,27 +137,6 @@ class Snippet < ActiveRecord::Base
     @link_reference_pattern ||= super("snippets", /(?<snippet>\d+)/)
   end
 
-  # Returns a collection of snippets that are either public or visible to the
-  # logged in user.
-  #
-  # This method does not verify the user actually has the access to the project
-  # the snippet is in, so it should be only used on a relation that's already scoped
-  # for project access
-  def self.public_or_visible_to_user(user = nil)
-    if user
-      authorized = user
-        .project_authorizations
-        .select(1)
-        .where('project_authorizations.project_id = snippets.project_id')
-
-      levels = Gitlab::VisibilityLevel.levels_for_user(user)
-
-      where('EXISTS (?) OR snippets.visibility_level IN (?) or snippets.author_id = (?)', authorized, levels, user.id)
-    else
-      public_to_user
-    end
-  end
-
   def to_reference(from = nil, full: false)
     reference = "#{self.class.reference_prefix}#{id}"