Correct avg time calculation

CHANGELOG.md

@@ -17,6 +17,7 @@
 
 
 ### Fixes
+- [#3449](https://github.com/poanetwork/blockscout/pull/3449) - Correct avg time calculation
 - [#3440](https://github.com/poanetwork/blockscout/pull/3440) - Rewrite missing blocks range query
 - [#3439](https://github.com/poanetwork/blockscout/pull/3439) - Dark mode color fixes (search, charts)
 - [#3437](https://github.com/poanetwork/blockscout/pull/3437) - Fix Postgres Docker container

apps/explorer/lib/explorer/counters/average_block_time.ex

@@ -5,7 +5,7 @@ defmodule Explorer.Counters.AverageBlockTime do
   Caches the number of token holders of a token.
   """
 
-  import Ecto.Query, only: [from: 2]
+  import Ecto.Query, only: [from: 2, where: 2]
 
   alias Explorer.Chain.Block
   alias Explorer.Repo
@@ -62,27 +62,28 @@ defmodule Explorer.Counters.AverageBlockTime do
   end
 
   defp refresh_timestamps do
+    base_query =
+      from(block in Block,
+        limit: 100,
+        offset: 100,
+        order_by: [desc: block.number],
+        select: {block.number, block.timestamp}
+      )
+
     timestamps_query =
       if Application.get_env(:explorer, :include_uncles_in_average_block_time) do
-        from(block in Block,
+        base_query
-          limit: 100,
-          offset: 100,
-          order_by: [desc: block.number],
-          select: {block.number, block.timestamp}
-        )
       else
-        from(block in Block,
+        base_query
-          limit: 100,
+        |> where(consensus: true)
-          offset: 100,
-          order_by: [desc: block.number],
-          where: block.consensus == true,
-          select: {block.number, block.timestamp}
-        )
       end
 
-    timestamps =
+    timestamps_row =
       timestamps_query
       |> Repo.all()
+
+    timestamps =
+      timestamps_row
       |> Enum.sort_by(fn {_, timestamp} -> timestamp end, &>=/2)
       |> Enum.map(fn {number, timestamp} ->
         {number, DateTime.to_unix(timestamp, :millisecond)}
@@ -111,12 +112,13 @@ defmodule Explorer.Counters.AverageBlockTime do
 
   defp durations(timestamps) do
     timestamps
-    |> Enum.reduce({[], nil}, fn {_, timestamp}, {durations, last_timestamp} ->
+    |> Enum.reduce({[], nil, nil}, fn {block_number, timestamp}, {durations, last_block_number, last_timestamp} ->
       if last_timestamp do
-        duration = last_timestamp - timestamp
+        block_numbers_range = last_block_number - block_number
-        {[duration | durations], timestamp}
+        duration = (last_timestamp - timestamp) / block_numbers_range
+        {[duration | durations], block_number, timestamp}
       else
-        {durations, timestamp}
+        {durations, block_number, timestamp}
       end
     end)
     |> elem(0)