Add index to addresses table

Creates a partial index using the fetched_coin_balance and hash columns

apps/explorer/priv/repo/migrations/20181126203826_add_index_to_addresses.exs

@@ -0,0 +1,13 @@
+defmodule Explorer.Repo.Migrations.AddIndexToAddresses do
+  use Ecto.Migration
+
+  def up do
+    execute(
+      "CREATE INDEX addresses_fetched_coin_balance_hash_index ON addresses (fetched_coin_balance DESC, hash ASC) WHERE fetched_coin_balance > 0"
+    )
+  end
+
+  def down do
+    execute("DROP INDEX addresses_fetched_coin_balance_hash_index")
+  end
+end