allow to set first indexing block

apps/indexer/config/config.exs

@@ -33,7 +33,8 @@ config :indexer,
   ecto_repos: [Explorer.Repo],
   metadata_updater_days_interval: 7,
   # bytes
-  memory_limit: 1 <<< 30
+  memory_limit: 1 <<< 30,
+  first_block: System.get_env("FIRST_BLOCK") || 0
 
 # config :indexer, Indexer.ReplacedTransaction.Supervisor, disabled?: true
 # config :indexer, Indexer.Block.Reward.Supervisor, disabled?: true

apps/indexer/lib/indexer/block/catchup/fetcher.ex

@@ -80,7 +80,7 @@ defmodule Indexer.Block.Catchup.Fetcher do
       _ ->
         # realtime indexer gets the current latest block
         first = latest_block_number - 1
-        last = 0
+        last = Application.get_env(:indexer, :first_block)
 
         Logger.metadata(first_block_number: first, last_block_number: last)