Make set_transaction_id use the PK index for its lookup

This patch fixes a longstanding (yet undiscovered) bug in the `set_transaction_id`
procedure that caused it to not use the provided index on `id` spanning `xact_id`.
Reason was the use of `CURRVAL` in the `WHERE` clause which caused the query to
evaluate every row against a new function call, turning the should-be-fast lookup
into a seq scan of the `transactions` table. Consequently, `INSERT`s scaled linearly
with the size of the `transactions` table, instead of logarithmically.

Huge 🤦

CHANGELOG.md

@@ -1,3 +1,11 @@
+## Unreleased
+
+**New migration patches:** 9
+
+### Fixed
+
+- Fix `set_transaction_id` trigger not using the PK index on `transactions` due to being dependent on `CURRVAL` for every row. Replaced with a CTE the `EXIST` query is now an index-only scan as it was intended to be. As a consequence, scalability of inserts into the `transactions` table has been greatly improved.
+
 ## [0.12.1] - 2024-05-21
 
 ### Fixed

lib/carbonite/migrations.ex

@@ -18,7 +18,7 @@ defmodule Carbonite.Migrations do
   # --------------------------------- patch levels ---------------------------------
 
   @initial_patch 1
-  @current_patch 8
+  @current_patch 9
 
   @doc false
   @spec initial_patch :: non_neg_integer()

lib/carbonite/migrations/v4.ex

@@ -125,6 +125,40 @@ defmodule Carbonite.Migrations.V4 do
     :ok
   end
 
+  @spec create_set_transaction_id_procedure(prefix()) :: :ok
+  def create_set_transaction_id_procedure(prefix) do
+    """
+    CREATE OR REPLACE FUNCTION #{prefix}.set_transaction_id() RETURNS TRIGGER AS
+    $body$
+    BEGIN
+      BEGIN
+        /* verify that no previous INSERT within current transaction (with same id) */
+        IF
+          EXISTS(
+            SELECT 1 FROM #{prefix}.transactions
+            WHERE id = COALESCE(NEW.id, CURRVAL('#{prefix}.transactions_id_seq'))
+            AND xact_id = COALESCE(NEW.xact_id, pg_current_xact_id())
+          )
+        THEN
+          NEW.id = COALESCE(NEW.id, CURRVAL('#{prefix}.transactions_id_seq'));
+        END IF;
+      EXCEPTION WHEN object_not_in_prerequisite_state THEN
+        /* when NEXTVAL has never been called within session, we're good */
+      END;
+
+      NEW.id = COALESCE(NEW.id, NEXTVAL('#{prefix}.transactions_id_seq'));
+      NEW.xact_id = COALESCE(NEW.xact_id, pg_current_xact_id());
+
+      RETURN NEW;
+    END
+    $body$
+    LANGUAGE plpgsql;
+    """
+    |> squish_and_execute()
+
+    :ok
+  end
+
   @impl true
   @spec up([up_option()]) :: :ok
   def up(opts) do
@@ -161,34 +195,7 @@ defmodule Carbonite.Migrations.V4 do
     """
     |> squish_and_execute()
 
-    """
-    CREATE OR REPLACE FUNCTION #{prefix}.set_transaction_id() RETURNS TRIGGER AS
-    $body$
-    BEGIN
-      BEGIN
-        /* verify that no previous INSERT within current transaction (with same id) */
-        IF
-          EXISTS(
-            SELECT 1 FROM #{prefix}.transactions
-            WHERE id = COALESCE(NEW.id, CURRVAL('#{prefix}.transactions_id_seq'))
-            AND xact_id = COALESCE(NEW.xact_id, pg_current_xact_id())
-          )
-        THEN
-          NEW.id = COALESCE(NEW.id, CURRVAL('#{prefix}.transactions_id_seq'));
-        END IF;
-      EXCEPTION WHEN object_not_in_prerequisite_state THEN
-        /* when NEXTVAL has never been called within session, we're good */
-      END;
-
-      NEW.id = COALESCE(NEW.id, NEXTVAL('#{prefix}.transactions_id_seq'));
-      NEW.xact_id = COALESCE(NEW.xact_id, pg_current_xact_id());
-
-      RETURN NEW;
-    END
-    $body$
-    LANGUAGE plpgsql;
-    """
-    |> squish_and_execute()
+    create_set_transaction_id_procedure(prefix)
 
     # ------------- override_xact_id -------------
 

lib/carbonite/migrations/v9.ex

@@ -0,0 +1,75 @@
+# SPDX-License-Identifier: Apache-2.0
+
+defmodule Carbonite.Migrations.V9 do
+  @moduledoc false
+
+  use Ecto.Migration
+  use Carbonite.Migrations.Version
+  alias Carbonite.Migrations.V4
+
+  @type prefix :: binary()
+
+  @spec create_set_transaction_id_procedure(prefix()) :: :ok
+  def create_set_transaction_id_procedure(prefix) do
+    """
+    CREATE OR REPLACE FUNCTION #{prefix}.set_transaction_id() RETURNS TRIGGER AS
+    $body$
+    BEGIN
+      BEGIN
+        /* verify that no previous INSERT within current transaction (with same id) */
+        IF
+          EXISTS(
+            WITH constants AS (
+              SELECT
+                COALESCE(NEW.id, CURRVAL('#{prefix}.transactions_id_seq')) AS id,
+                COALESCE(NEW.xact_id, pg_current_xact_id()) AS xact_id
+            )
+            SELECT 1 FROM #{prefix}.transactions
+            JOIN constants
+            ON constants.id = transactions.id
+            AND constants.xact_id = transactions.xact_id
+          )
+        THEN
+          NEW.id = COALESCE(NEW.id, CURRVAL('#{prefix}.transactions_id_seq'));
+        END IF;
+      EXCEPTION WHEN object_not_in_prerequisite_state THEN
+        /* when NEXTVAL has never been called within session, we're good */
+      END;
+
+      NEW.id = COALESCE(NEW.id, NEXTVAL('#{prefix}.transactions_id_seq'));
+      NEW.xact_id = COALESCE(NEW.xact_id, pg_current_xact_id());
+
+      RETURN NEW;
+    END
+    $body$
+    LANGUAGE plpgsql;
+    """
+    |> squish_and_execute()
+
+    :ok
+  end
+
+  @type up_option :: {:carbonite_prefix, prefix()}
+
+  @impl true
+  @spec up([up_option()]) :: :ok
+  def up(opts) do
+    prefix = Keyword.get(opts, :carbonite_prefix, default_prefix())
+
+    create_set_transaction_id_procedure(prefix)
+
+    :ok
+  end
+
+  @type down_option :: {:carbonite_prefix, prefix()}
+
+  @impl true
+  @spec down([down_option()]) :: :ok
+  def down(opts) do
+    prefix = Keyword.get(opts, :carbonite_prefix, default_prefix())
+
+    V4.create_set_transaction_id_procedure(prefix)
+
+    :ok
+  end
+end