query/locking/efficiency question

I have an IP Address allocation system that uses a networks table like
so:

CREATE TABLE "networks" (
"netblock" cidr,
"router" integer,
"interface" character varying(256),
"dest_ip" inet,
"mis_token" character(16),
"assigned_date" date,
"assigned_by" character varying(256),
"justification_now" integer,
"justification_1yr" integer,
"cust_asn" integer,
"comments" character varying(2048),
"other_reference" character varying(256),
"parent_asn" integer,
"status" integer,
"purpose" integer,
"last_update_by" character varying(256),
"last_update_at" timestamp with time zone,
"customer_reference" integer
);

When I go looking for an available netblock, I do the following query:

BEGIN TRANSACTION;
SELECT host(netblock),masklen(netblock),netblock,netmask(netblock)
FROM networks
WHERE parent_asn=xxxx AND
status=get_status_code('available') AND
masklen(netblock) = xxx FOR UPDATE LIMIT 1;

(if this fails, we go looking for a /24 to bust up, and if we can find
that we add new available rows for that, and retry this query).

get_status_code is a function to look up a number based on text in
another table (not marked cacheable at the moment, but should it be? )

My questions are:

1) if this code is running twice for the same size block what will
happen ?

2) what can I do to make this more efficient?

the table will contain ~5000 rows to begin with.

LER

--
Larry Rosenman http://www.lerctr.org/~ler
Phone: +1 972-414-9812 E-Mail: ler(at)lerctr(dot)org
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