Fix: Fetch last_seen beforehand and use it as the first fallback (#7812)

lib/mentat/services/eventstorage.py

@@ -1569,6 +1569,12 @@ class EventStorageService:
         :rtype: list
         """
         enum_table = "enum_{}".format(column)
+
+        # Get last_seen first.
+        self.cursor.execute(psycopg2.sql.SQL("SELECT max(last_seen) FROM {}").format(psycopg2.sql.Identifier(enum_table)))
+        last_seen = self.cursor.fetchone()[0]
+        self.commit()
+
         # Build and execute query for updating enumeration table.
         enum_query = psycopg2.sql.SQL("INSERT INTO {} (SELECT * FROM (").format(psycopg2.sql.Identifier(enum_table))
         if column not in ('eventclass', 'targetclass', 'eventseverity', 'targetseverity'):
@@ -1577,10 +1583,13 @@ class EventStorageService:
             enum_query += psycopg2.sql.SQL("SELECT {}").format(psycopg2.sql.Identifier(column))
         enum_query += psycopg2.sql.SQL(
             " AS data, max(storagetime) AS last_seen FROM events "
-            "WHERE storagetime >= COALESCE((SELECT max(last_seen) FROM {}), %s, (SELECT min(storagetime) FROM events)) "
+            "WHERE storagetime >= COALESCE(%s, %s, (SELECT min(storagetime) FROM events)) "
             "GROUP BY data) AS enum WHERE data IS NOT NULL) ON CONFLICT (data) DO UPDATE SET last_seen = excluded.last_seen"
-        ).format(psycopg2.sql.Identifier(enum_table))
-        self.cursor.execute(enum_query, (last_run_ts,))
+        )
+        # Use the time of the last run before the value of last_seen from the database.
+        # Some values may be quite rare and it doesn't make sense to always search all events
+        # since the time when this rare value appeared the last time.
+        self.cursor.execute(enum_query, (last_run_ts, last_seen,))
         self.commit()
 
         # Return all entries from recently updated enumeration table.