How to drop automatically some databases?

[Andreas <maps.on@xxxxxxx>]

I've got a developer PC (Windows) that hosts copies of a production 
database so I can tinker with it without causing damage to the 
production database.

Every night a script fetches a dump of the production database.
Then it renames the test-db on my PC, creates a new test-db and imports 
the dump.

The current version is named "db_test".
The older copies are named "db_test_yymmdd_hhmm". The name is extended 
with the timestamp of the import.

Now I'd like to keep just 3 older copies in case I want to look up 
structural moifications I've made the day before.
Older copies should get dropped.


This would show the names to drop.

select  datname
from    (
            select      datname
            from        pg_database
            where       datname ilike 'db_test_%'
            order by    datname
        )   as  x

EXCEPT
select  datname
from    (
            select      datname
            from        pg_database
            where       datname ilike 'db_test_%'
            order by    datname     desc
            limit       3
        )   as  y;


I tried to run this

drop database ( ... the select above ... )

in the DB "postgres" but it didn't work.
After a couple of seconds I get a syntax error.

What now?   :}


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