Optimize rolling 28 days snippets counter

Adding an index to eliminate filters in for the postgres optimizer

changelogs/unreleased/221184-rolling-28-day-snippets-optimization.yml

+---
+title: Optimize rolling 28 days snippets counter
+merge_request: 34918
+author:
+type: performance

db/migrate/20200618105638_add_index_on_id_and_created_at_to_snippets.rb

+# frozen_string_literal: true
+
+class AddIndexOnIdAndCreatedAtToSnippets < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :snippets, [:id, :created_at]
+  end
+
+  def down
+    remove_concurrent_index :snippets, [:id, :created_at]
+  end
+end

db/structure.sql

@@ -10953,6 +10953,8 @@ CREATE INDEX index_snippets_on_description_trigram ON public.snippets USING gin
 
 CREATE INDEX index_snippets_on_file_name_trigram ON public.snippets USING gin (file_name public.gin_trgm_ops);
 
+CREATE INDEX index_snippets_on_id_and_created_at ON public.snippets USING btree (id, created_at);
+
 CREATE INDEX index_snippets_on_id_and_type ON public.snippets USING btree (id, type);
 
 CREATE INDEX index_snippets_on_project_id_and_visibility_level ON public.snippets USING btree (project_id, visibility_level);
@@ -14075,6 +14077,7 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200617001637
 20200617001848
 20200617002030
+20200618105638
 20200618134223
 20200618134723
 \.