Add script to migrate block_number to token transfers

This script copy block_number from the transactions' table to token
transfers' table in batches.

apps/explorer/priv/repo/migrations/20181121170616_add_block_number_to_token_transfers.exs

@@ -1,4 +1,13 @@
 defmodule Explorer.Repo.Migrations.AddBlockNumberToTokenTransfers do
+  @moduledoc """
+  Use `priv/repo/migrations/scripts/20181121170616_token_transfers_update_block_number_in_batches.sql` to migrate data.
+
+  ```sh
+  mix ecto.migrate
+  psql -d $DATABASE -a -f priv/repo/migrations/scripts/20181121170616_token_transfers_update_block_number_in_batches.sql
+  ```
+  """
+
   use Ecto.Migration
 
   def change do

apps/explorer/priv/repo/migrations/scripts/20181121170616_token_transfers_update_block_number_in_batches.sql

@@ -0,0 +1,39 @@
+DO $$
+DECLARE
+   row_count integer;
+   batch_size  integer := 100000; -- HOW MANY ITEMS WILL BE UPDATED AT TIME
+   affected integer;
+BEGIN
+  RAISE NOTICE 'Counting items to be updated';
+
+  row_count := (SELECT COUNT(*) FROM token_transfers WHERE block_number IS NULL);
+
+  RAISE NOTICE '% items', row_count;
+
+  WHILE row_count > 0 LOOP
+    WITH cte AS (
+     SELECT
+       t.hash,
+       t.block_number
+     FROM token_transfers AS tt
+     INNER JOIN transactions AS t ON t.hash = tt.transaction_hash
+	   WHERE tt.block_number IS NULL
+     LIMIT batch_size
+    )
+    UPDATE token_transfers
+    SET
+      block_number = cte.block_number
+    FROM cte
+    WHERE token_transfers.transaction_hash = cte.hash;
+
+    GET DIAGNOSTICS affected = ROW_COUNT;
+    RAISE NOTICE '-> % token transfers updated!', affected;
+
+    -- UPDATES THE COUNTER SO IT DOESN'T TURN INTO AN INFINITE LOOP
+    row_count := row_count - batch_size;
+
+    RAISE NOTICE '-> % items missing to update', row_count;
+
+    CHECKPOINT; -- COMMITS THE BATCH UPDATES
+  END LOOP;
+END $$;