diff --git a/backend/app/services/post_tag_search.rb b/backend/app/services/post_tag_search.rb
index 461400d..1e4b9dd 100644
--- a/backend/app/services/post_tag_search.rb
+++ b/backend/app/services/post_tag_search.rb
@@ -3,8 +3,8 @@ class PostTagSearch
   def self.literals(locale, raw)
     tokens = raw.split.reject { _1.downcase == 'type:or' }
     names = tokens.map { _1.sub(/\Anot:/i, '') }
-    registered = TagName.where(language_code: locale.language_code, name: names)
-        .pluck(:name, :tag_id).to_h
+    registered = matching_names(TagName.where(language_code: locale.language_code),
+        names, 'tag_names.name', :tag_id).to_h
     primary = TagName.where(language_code: locale.language_code,
         tag_id: registered.values, primary_flg: true).pluck(:tag_id, :name).to_h
     tokens.zip(names).map do |token, name|
@@ -19,12 +19,25 @@ class PostTagSearch
 
   def self.references_for(locale, literals)
     names = literals.map { _1[:name] }.uniq
-    internal = TagName.where(language_code: locale.language_code, name: names)
-        .pluck(:name, :tag_id).group_by(&:first)
-    external = names.empty? ? {} : ExternalTag.where(
-        "CONCAT(platform, ':', name) IN (?)", names)
-        .pluck(Arel.sql("CONCAT(platform, ':', name)"), :id).group_by(&:first)
+    internal = matching_names(TagName.where(language_code: locale.language_code),
+        names, 'tag_names.name', :tag_id).group_by(&:first)
+    external = matching_names(ExternalTag.all, names,
+        "CONCAT(external_tags.platform, ':', external_tags.name)", :id).group_by(&:first)
     names.to_h { |name| [name, [internal.fetch(name, []).map(&:last).uniq,
                               external.fetch(name, []).map(&:last).uniq]] }
   end
+
+  def self.matching_names(scope, names, expression, identifier)
+    return [] if names.empty?
+
+    queries = names.uniq.map do |name|
+      scope.klass.sanitize_sql_array(['SELECT ? AS name', name])
+    end
+    # Return the input spelling so Ruby does not recheck database equivalence.
+    scope.joins(<<~SQL).pluck(Arel.sql('search_names.name'), identifier)
+      INNER JOIN (#{ queries.join(' UNION ALL ') }) AS search_names
+        ON #{ expression } = search_names.name COLLATE utf8mb4_0900_ai_ci
+    SQL
+  end
+  private_class_method :matching_names
 end