Re: pl/pgsql question (functions)

From: "Kristoff Bonne" <kristoff.bonne@skypro.be>

> Greetings,
>
> I am new to pl/pgsqm (I did some SQL programming at school, now 8 years
> ago so that part is rusty too ;-))

Kristoff - I know exactly how you feel. If only I'd paid more attention when
they were explaining what 17th normal form was...

> Anycase, I have a question concerning creating your own functions in
> pl/pgsql.
>
> This is my situation:
>
> I have two tables:
> - 'hosts': containing 'hostname' (primary key) and 'ip_address' (unique
> and non null).
> - 'aliases': containing 'aliasname' (primary key) and 'hostname'(unique
> and non null).
> - aliasname.hostname is a foreign reference to hosts.hostname
>
> Further, I've set up a trigger function in both hosts and aliases; so that
> a name cannot be in both tables.

You could use a constraint (foreign key) and get PG to do the triggers for
you.

> So far, so good.
>
> Now, I would like to create a function 'get_ip_address' that ... euh ...
> retrieves an IP-address (one argument: hostname).
> - If the hostname is in the table 'hosts', return the corresponding
> IP-address.
> - If the hostname is in the table 'aliases', return the IP-address of the
> corresponding hostname in 'hosts'.
> - If the hostname is in neither table, return nothing.

Are you sure you want a function here? Have you considered a view which does
a UNION of the results from both tables?

SELECT ip_addr FROM host_to_ip_view WHERE host='any.box.bigcorp.com';

will be rewritten on the fly and should be at least as fast as using a
function. Not sure about ordering of IPs when there are multiple entries
though.

> (See program below):
>
> The 'problem' is in the case where the data in in neither field. How do I
> program this?
>
> - If there is no 'RETURN' statement for that case; I get an error
> 'function terminated without RETURN'.

Yep - if you say you return an IP address you need to return one.

> - Just <RETURN>, <RETURN ""> or <RETURN ''> all produce an syntax-error.

Not valid as IP addr.

> - When I do '<RETURN ret>' (ret being the result of the last query, being
> the query in the 'aliases' table; I do NOT get an error; but the function
> does return something (an empty row).
>
> This I don't like for two reasons:
> 1/ When you do 'select ... from ... where ...', and the query doesn't
> 'find' anything; you get NOTHING (no rows).
>
> When I do get_ip_addr('something_that_does_not_exist'); I do get
> SOMETHING: one row (containing an empty field).
>
> 2/ When another function uses the "get_ip_addr('some_host')" function; I
> cannot use  'IF NOT FOUND ...'; as -even when 'some_host' does not exist,
> the function returns something. (hence, the 'IF NOT FOUND' case is never
> followed).
>
> So, does anybody any idea how to 'fix' this?

I think you probably want "return NULL" for this case. That's what NULL was
invented for and it is easy to test for:

myip := get_ip_addr('some_host');
if myip is null then...

> For some reason, I get the feeling I have the wrong 'concept' of functions
> in pl/pgsql.
> I am using functions in the wrong 'way'?

No - I think you're on the right track. The difference (if I understand
this) is that NOT FOUND is checking for an empty set of records, whereas
plpgsql can't return a set of records (at the moment) - you need SQL
functions for that.

Your options would seem to be:

1. Return NULL and test for that
2. Build a view
3. Rewrite the function as SQL

Any of the above should be fine.

HTH

- Richard Huxton