sql - randomly generating unique number between 1-999 for primary key in table -

i have problem i'm not sure how solve elegantly.

background information

i have table of widgets. each widget assigned id range of numbers, let's between 1-999. values of 1-999 saved in database "lower_range" , "upper_range" in table called "config". when user requests create new widget using web app, need able following:

• generate random number between 1 , 999 using lua's math.random function or maybe random number generator in sqlite (so far, in tests, lua's math.random returns same value...but that's different issue)
• do select statement see if there widget number assigned...
• if not, create new widget.
• otherwise repeat process until number not in use.

problem

the problem see above logic two-fold:

1. the algorithm can potentially take long time because have keep searching until find unique value.
2. how prevent simultaneous requests new widget numbers generating same value?

any suggestions appreciated. thanks

generate random numbers ahead of time , store them in table; make sure numbers unique. when need next number, check how many have been assigned , next number table. so, instead of

• generate number between 1-999
• check if it's assigned
• generate new number, , on.

do this:

• generate array of 999 elements have values 1-999 in random order
• your getnextid function becomes return ids[currentmaxid+1]

to manage simultaneous requests, need have resource generates proper sequence. easiest use key in widget table index in ids array. so, add record widgets table first, key , generate widget id using ids[key].