Performance fix: index on blocks [miner_hash, number]

CHANGELOG.md

@@ -3,6 +3,7 @@
 ### Features
 
 ### Fixes
+- [#3114](https://github.com/poanetwork/blockscout/pull/3114) - Fix performance of "Blocks validated" page
 
 ### Chore
 

apps/explorer/priv/repo/migrations/20200518075748_create_index_blocks_miner_hash_number_index.exs

@@ -0,0 +1,7 @@
+defmodule Explorer.Repo.Migrations.CreateIndexBlocksMinerHashNumberIndex do
+  use Ecto.Migration
+
+  def change do
+    create_if_not_exists(index(:blocks, [:miner_hash, :number]))
+  end
+end