Add Generating UUIDs With pgcrypto as a postgres til

postgres/generating-uuids-with-pgcrypto.md

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+# Generating UUIDs With pgcrypto
+
+If you check out the docs for the [`uuid-ossp`
+extension](https://www.postgresql.org/docs/current/static/uuid-ossp.html),
+you'll come across the following message.
+
+> The OSSP UUID library... is not well maintained, and is becoming
+> increasingly difficult to port to newer platforms. 
+
+A little bit later, it says:
+
+> If you only need randomly-generated (version 4) UUIDs, consider using the
+> gen_random_uuid() function from the pgcrypto module instead.
+
+So, if we are using the UUID data type and only need to generate random
+UUIDs, we can rely on the [`pgcrypto`
+extension](https://www.postgresql.org/docs/current/static/pgcrypto.html). It
+comes with the `gen_random_uuid()` function which generates random v4 UUIDs.
+
+```sql
+> create extension "pgcrypto";
+CREATE EXTENSION
+
+> select gen_random_uuid();
+           gen_random_uuid
+--------------------------------------
+ 0a557c31-0632-4d3e-a349-e0adefb66a69
+
+> select gen_random_uuid();
+           gen_random_uuid
+--------------------------------------
+ 83cdd678-8198-4d56-935d-d052f2e9db37
+```