diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 8adc290..cc8b841 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -5,6 +5,23 @@ The issue tracker is at [github.com/CartoDB/dataservices-api](https://github.com
 
 We love pull requests from everyone, see [Contributing to Open Source on GitHub](https://guides.github.com/activities/contributing-to-open-source/#contributing).
 
+## PostgreSQL
+When adding or modifying PostgreSQL functions make sure that the
+[VOLATILITY](https://www.postgresql.org/docs/current/static/xfunc-volatility.html) and [PARALLEL](https://www.postgresql.org/docs/9.6/static/parallel-safety.html) categories are updated accordingly.
+
+Please mark public [client functions](client/sql) or [templates](client/renderer/templates)
+as STABLE even if the internals are VOLATILE to allow the planner to cache
+results inside a query. For example, in this query we need `cdb_geocode_admin1_polygon`
+to be STABLE so it gets called only once (instead of once per row):
+```sql
+SELECT * japank WHERE NOT (the_geom && cdb_geocode_admin1_polygon('Madrid', 'Spain'));
+```
+
+As PARALLEL labels need to be stripped for incompatible PostgreSQL versions,
+please use _PARALLEL SAFE/RESTRICTED/UNSAFE_ in uppercase so it's handled
+automatically.
+
+
 
 ## Submitting Contributions