Add notes on pg_stat_statements

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>

docs/datanommer_datagrepper/index.rst

@@ -20,3 +20,8 @@ Ideas box
 ---------
 
 Here is the list of ideas/things we looked at:
+
+.. toctree::
+    :maxdepth: 1
+
+    pg_stat_statements

docs/datanommer_datagrepper/pg_stat_statements.rst

@@ -0,0 +1,14 @@
+Postgresql's pg_stat_statements
+===============================
+
+This is a postgresql module allowing to track planning and execution statistics
+of all SQL statements executed by a server.
+
+Using this, we can monitor/figure out what the slowest queries executed
+on the server are.
+
+Resources:
+
+* Postgresql doc: https://www.postgresql.org/docs/13/pgstatstatements.html
+* How to enable it: https://www.virtual-dba.com/postgresql-performance-enabling-pg-stat-statements/
+* How to use it: https://www.virtual-dba.com/postgresql-performance-identifying-hot-and-slow-queries/