Use table statistics estimate for total transaction

count to speed up page load.

apps/explorer/lib/explorer/chain.ex

@@ -6,6 +6,7 @@ defmodule Explorer.Chain do
   import Ecto.Query,
     only: [from: 2, join: 4, limit: 2, or_where: 3, order_by: 2, order_by: 3, preload: 2, where: 2, where: 3]
 
+  alias Ecto.Adapters.SQL
   alias Ecto.{Changeset, Multi}
 
   alias Explorer.Chain.{
@@ -1495,41 +1496,30 @@ defmodule Explorer.Chain do
   @doc """
   Count of `t:Explorer.Chain.Transaction.t/0`.
 
-  With no options or an explicit `pending: nil`, both collated and pending transactions will be counted.
+  With :all both collated and pending transactions will be counted using estimates from table statistics.
 
       iex> insert(:transaction)
       iex> :transaction |> insert() |> with_block()
-      iex> Explorer.Chain.transaction_count()
-      2
-      iex> Explorer.Chain.transaction_count(pending: nil)
-      2
-
-  To count only collated transactions, pass `pending: false`.
-
-      iex> 2 |> insert_list(:transaction)
-      iex> 3 |> insert_list(:transaction) |> with_block()
-      iex> Explorer.Chain.transaction_count(pending: false)
-      3
+      iex> Explorer.Chain.transaction_count(:pending)
+      1
 
-  To count only pending transactions, pass `pending: true`.
+  ## Arguments
 
-      iex> 2 |> insert_list(:transaction)
-      iex> 3 |> insert_list(:transaction) |> with_block()
-      iex> Explorer.Chain.transaction_count(pending: true)
-      2
+    * `:all` - returns an estimated count of all collated and pending transactions using table statistics
+    * `:pending` - returns a count of all pending transactions
 
-  ## Options
+  """
+  @spec transaction_count(:all | :pending) :: non_neg_integer()
+  def transaction_count(:all) do
+    %Postgrex.Result{rows: [[rows]]} =
+      SQL.query!(Repo, "SELECT reltuples::BIGINT AS estimate FROM pg_class WHERE relname='transactions'")
 
-    * `:pending`
-      * `nil` - count all transactions
-      * `true` - only count pending transactions
-      * `false` - only count collated transactions
+    rows
+  end
 
-  """
-  @spec transaction_count([{:pending, boolean()}]) :: non_neg_integer()
-  def transaction_count(options \\ []) when is_list(options) do
+  def transaction_count(:pending) do
     Transaction
-    |> where_pending(options)
+    |> where([transaction], is_nil(transaction.block_hash))
     |> Repo.aggregate(:count, :hash)
   end
 
@@ -2043,21 +2033,6 @@ defmodule Explorer.Chain do
     end)
   end
 
-  defp where_pending(query, options) when is_list(options) do
-    pending = Keyword.get(options, :pending)
-
-    case pending do
-      false ->
-        from(transaction in query, where: not is_nil(transaction.block_hash))
-
-      true ->
-        from(transaction in query, where: is_nil(transaction.block_hash))
-
-      nil ->
-        query
-    end
-  end
-
   defp where_transaction_has_multiple_internal_transactions(query) do
     where(
       query,

apps/explorer_web/lib/explorer_web/controllers/pending_transaction_controller.ex

@@ -16,7 +16,7 @@ defmodule ExplorerWeb.PendingTransactionController do
     full_options = Keyword.merge([necessity_by_association: %{from_address: :optional, to_address: :optional}], options)
     transactions = Chain.recent_pending_transactions(full_options)
     last_seen_pending_inserted_at = last_seen_pending_inserted_at(transactions.entries)
-    transaction_count = Chain.transaction_count(pending: true)
+    transaction_count = Chain.transaction_count(:pending)
 
     render(
       conn,

apps/explorer_web/lib/explorer_web/controllers/transaction_controller.ex

@@ -36,7 +36,7 @@ defmodule ExplorerWeb.TransactionController do
       )
 
     transactions = Chain.recent_collated_transactions(full_options)
-    transaction_count = Chain.transaction_count()
+    transaction_count = Chain.transaction_count(:all)
 
     render(
       conn,

apps/explorer_web/test/explorer_web/controllers/transaction_controller_test.exs

@@ -23,7 +23,7 @@ defmodule ExplorerWeb.TransactionControllerTest do
 
       conn = get(conn, "/en/transactions")
 
-      assert conn.assigns.transaction_count == 1
+      assert is_integer(conn.assigns.transaction_count)
     end
 
     test "excludes pending transactions", %{conn: conn} do