Add index snippets on project_id and title

This index will support the query we use for matching snippets during
the GitLab Migration feature.

Changelog: added

db/migrate/20211204010826_add_index_snippets_on_project_id_and_title.rb

+# frozen_string_literal: true
+
+class AddIndexSnippetsOnProjectIdAndTitle < Gitlab::Database::Migration[1.0]
+  disable_ddl_transaction!
+
+  INDEX_NAME = 'index_snippets_on_project_id_and_title'
+
+  def up
+    add_concurrent_index :snippets, [:project_id, :title], name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :snippets, name: INDEX_NAME
+  end
+end

db/schema_migrations/20211204010826

+6a3591e70ddd6573ad68360c1a8774ef61e7812ce831c75066baec5754e2bd76
\ No newline at end of file

db/structure.sql

@@ -27422,6 +27422,8 @@ CREATE INDEX index_snippets_on_id_and_created_at ON snippets USING btree (id, cr
 
 CREATE INDEX index_snippets_on_id_and_type ON snippets USING btree (id, type);
 
+CREATE INDEX index_snippets_on_project_id_and_title ON snippets USING btree (project_id, title);
+
 CREATE INDEX index_snippets_on_project_id_and_visibility_level ON snippets USING btree (project_id, visibility_level);
 
 CREATE INDEX index_snippets_on_title_trigram ON snippets USING gin (title gin_trgm_ops);