pg_dump does not write "CREATE EXTENSION postgis;"

You can use this to dump schema and extensions (tested on 11.2):

pg_dump -U postgres -d mydb -s > dump.sql

Several possibilities:

1. This database has been upgraded from an old version where CREATE EXTENSION did not yet exist.

2. Somebody mistakenly installed PostGIS by executing the SQL script rather than running CREATE EXTENSION.

3. You don't dump the whole database, but restrict the dump to a schema using the -n option.