Allow filtering labels by a single character

When we use Gitlab::SQL::Pattern, this typically relates to a trigram
index. As the 'tri' indicates, we need at least three characters to be
able to use that index.

Labels don't have a trigram index, because we never allow you to search
for them globally: it's always in the context of a project or a
group. In that context, it's just as fast to search for a single
character (in general) because there is already a pretty specific index
being used.

app/models/label.rb

@@ -126,6 +126,13 @@ class Label < ActiveRecord::Base
     fuzzy_search(query, [:title, :description])
   end
 
+  # Override Gitlab::SQL::Pattern.min_chars_for_partial_matching as
+  # label queries are never global, and so will not use a trigram
+  # index. That means we can have just one character in the LIKE.
+  def self.min_chars_for_partial_matching
+    1
+  end
+
   def open_issues_count(user = nil)
     issues_count(user, state: 'opened')
   end

changelogs/unreleased/allow-filtering-labels-by-a-single-character.yml

+---
+title: Allow filtering labels list by one or two characters
+merge_request: 26012
+author:
+type: changed

lib/gitlab/sql/pattern.rb

@@ -23,8 +23,12 @@ module Gitlab
           end
         end
 
+        def min_chars_for_partial_matching
+          MIN_CHARS_FOR_PARTIAL_MATCHING
+        end
+
         def partial_matching?(query)
-          query.length >= MIN_CHARS_FOR_PARTIAL_MATCHING
+          query.length >= min_chars_for_partial_matching
         end
 
         # column - The column name to search in.

spec/finders/labels_finder_spec.rb

@@ -209,6 +209,12 @@ describe LabelsFinder do
 
         expect(finder.execute).to eq [project_label_1]
       end
+
+      it 'returns labels matching a single character' do
+        finder = described_class.new(user, search: '(')
+
+        expect(finder.execute).to eq [group_label_1]
+      end
     end
 
     context 'filter by subscription' do