diff --git a/mail_tracking/models/mail_message.py b/mail_tracking/models/mail_message.py
index 8cb742f9c..3d0e5f1f7 100644
--- a/mail_tracking/models/mail_message.py
+++ b/mail_tracking/models/mail_message.py
@@ -34,6 +34,16 @@ class MailMessage(models.Model):
         search="_search_is_failed_message",
     )
 
+    # get_failed_count runs on every web client load (res.users._init_messaging)
+    # and filters on this flag, which only a handful of messages carry. Without a
+    # partial index the planner walks the failed trackings and probes mail_message
+    # by primary key once per tracking. Keep "IS TRUE": it is what the ORM generates
+    # for ("mail_tracking_needs_action", "=", True), and PostgreSQL does not use a
+    # plain "WHERE mail_tracking_needs_action" index for it.
+    _mail_tracking_needs_action_index = models.Index(
+        "(id) WHERE mail_tracking_needs_action IS TRUE"
+    )
+
     @api.model
     def get_failed_states(self):
         """The 'failed' states of the message"""
diff --git a/mail_tracking/readme/INSTALL.md b/mail_tracking/readme/INSTALL.md
index f9e4eab48..bcaa7a320 100644
--- a/mail_tracking/readme/INSTALL.md
+++ b/mail_tracking/readme/INSTALL.md
@@ -3,3 +3,20 @@ option) where /web/databse/selector returns a list of more than one
 database, then you need to add `mail_tracking` addon to wide load addons
 list (by default, only `web` addon), setting `--load` option. For
 example, `--load=web,mail,mail_tracking`
+
+The first update of the module after this index was added creates a partial
+index on `mail_message` (`mail_message_mail_tracking_needs_action_index`). Later
+updates do not build it again: Odoo keeps it while its definition does not change.
+
+On a database with a very large `mail_message` table that first build blocks
+writes to the table while it runs (13 minutes on 122 million rows). To avoid the
+lock, create the index beforehand exactly as the ORM would, with the comment it
+stores on it; Odoo compares that comment with the index definition and keeps
+the index when they match:
+
+```sql
+CREATE INDEX CONCURRENTLY mail_message_mail_tracking_needs_action_index
+    ON mail_message (id) WHERE mail_tracking_needs_action IS TRUE;
+COMMENT ON INDEX mail_message_mail_tracking_needs_action_index
+    IS 'INDEX (id) WHERE mail_tracking_needs_action IS TRUE';
+```