add pg_cron docs

GUIDE.md

@@ -671,10 +671,6 @@ TBD.
 
 TBD.
 
-### Testsuite
-
-TBD.
-
 # Unit testing
 
 Most Unit tests follows this pattern:
@@ -983,8 +979,72 @@ you can just commit files into git. Then, you are covered.
 Files are saved in the same directory as other golden files, under the
 directory of the test case, with suffix of `.var.golden`.
 
-## Known Issues
+## Known issues
 
 1. Cannot use auto-generated loader
    if the table has any `GENERATED ALWAYS AS IDENTITY` column.
    Unless required by business logics, it is recommended to use `GENERATED BY DEFAULT AS IDENTITY`.
+
+# FAQ
+
+## Refresh materialized view using pg_cron
+
+### Installation
+
+See [this doc](https://docs.amazonaws.cn/en_us/AmazonRDS/latest/UserGuide/PostgreSQL_pg_cron.html) of how to enable pg_cron for RDS PostgreSQL.
+
+### Refresh materialized views example
+
+```sql
+SELECT cron.schedule(
+  'hourly_refresh_all_materialized_views',
+  '0 * * * *',
+  $CRON$
+    REFRESH MATERIALIZED VIEW CONCURRENTLY book_metrics;
+    REFRESH MATERIALIZED VIEW CONCURRENTLY customer_metrics;
+  $CRON$
+);
+```
+
+### Setup log clean-up job
+
+```sql
+SELECT cron.schedule('0 0 * * *', $CRON$ DELETE
+    FROM cron.job_run_details
+    WHERE end_time < now() - interval '7 days' $CRON$);
+```
+
+### Other Commands
+
+List scheduled jobs:
+
+```sql
+select * from cron.job;
+```
+
+Turn ON/OFF jobs.
+
+```sql
+-- deactivate
+update cron.job set active = false where jobid = $1;
+-- activate
+update cron.job set active = true where jobid = $1;
+```
+
+Unschedule
+
+```sql
+SELECT cron.unschedule(@jobid);
+```
+
+Get job run logs:
+
+```sql
+SELECT * FROM cron.job_run_details;
+```
+
+setting up cross-db jobs, extra steps are required:
+
+```sql
+UPDATE cron.job SET database = 'otherDB' WHERE jobid = $1;
+```