> - Why is PostgreSQL not using the functional index I created and why is it
> not being ordered correctly?
Your example works for me:
> hasegeli=# CREATE TABLE device_port (port text);
> CREATE TABLE
>
> hasegeli=# CREATE INDEX idx_device_port_port_proper ON device_port (cast_to_port(port) port_ops DESC);
> CREATE INDEX
>
> hasegeli=# INSERT INTO device_port VALUES ('a'), ('b'), ('c');
> INSERT 0 3
>
> hasegeli=# SELECT port FROM device_port ORDER BY port;
> port
> ------
> c
> b
> a
> (3 rows)
>
> hasegeli=# SET enable_seqscan = 0;
> SET
>
> hasegeli=# EXPLAIN ANALYZE SELECT port FROM device_port ORDER BY cast_to_port(port);
> QUERY PLAN
>
--------------------------------------------------------------------------------------------------------------------------------------------------------
> Index Scan Backward using idx_device_port_port_proper on device_port (cost=0.15..408.55 rows=1360 width=32) (actual
time=0.042..0.053rows=3 loops=1)
> Planning time: 0.079 ms
> Execution time: 0.079 ms
> (3 rows)
> - Is creating a separate data type and using a functional index on the
> casts to this data type the right approach to a custom ordering?
You don't need to create a type for this. You can just create a
non-default operator class and use it with your text type by specify
the operator with ORDER BY ... USING clause.
> Creating the index:
> CREATE INDEX idx_device_port_port_proper on device_port (cast_to_port(port) port_ops desc);
The operator class is not necessary in here as it is the default for
the "port" type. DESC also wouldn't make any difference.