Updated SQL for analyzing Synapse PostgreSQL database stats (markdown)

SQL-for-analyzing-Synapse-PostgreSQL-database-stats.md

@@ -39,6 +39,7 @@ SELECT relname, n_live_tup as rows
   ORDER BY n_live_tup DESC
   LIMIT 20;
 ```
+This query is quick, but may be very approximate, for exact number of rows use `SELECT COUNT(*) FROM <table_name>`.
 ### Result example:
 ```
 state_groups_state - 161687170