Am Montag, 21. Januar 2008 01:16 schrieb Morgan:
> Hi Guys,
>
> I have field that I want to put a unique identifier in. This unique Id i
> would like to be a composite key or simply a random number. What do you
> guys suggest for this, is there a particular method which works well for
> some of you?
That's a good question, I asked myself some weeks ago, here's how I solved
this:
In my case, I have database records that have sequential numbers as primary
keys. These keys can be calculated by the database and are unique by design
(as the primary index is unique).
This record should hold another field, which should be also unique and in a
form of a 8-digit number. However, I'd rather not want this number to be
sequential, it should look random. The first way would have been to simple
generate a number via "random.randint(99999999)", look into the database, if
it's already in and if not, insert it. However, to guarantee that the number
is unique, one should create a unique index on this column. In case the
number is already there, the database will raise an error, which has to be
catched by the application. Another way would be to lock the table after the
select, so that the rare case, that another application instance inserts the
same number after my select, is avoided. So, the algorithm could look like
this (in pseudo code):
# Variant 1 with exception handling
while 1:
num = random.randint(99999999)
try:
insert into db_table (col1, col2, col_num, col3, ) % num
except UniqueNum_IndexViolated:
continue
else:
break
# Variant 2 with locking
while 1:
num = random.randint(99999999)
lock db_table
result = select * from db_table where col_num = num
if result:
continue
else:
insert into db_table (col1, col2, col_num, col3, ....) % num
unlock db_table
continue
My problem with variant (1) was that I could not find out how to lock a whole
table with SQLAlchemy, moreover, each insert needs a table lock and a select,
which is bad performance-wise. The problem with (2) was that I did not know
how to catch this specific exception, as I can't simply except any database
error but this specific index violation (which may be different on different
databases).
My third idea, which I use now, is to calculate my "random" number out of my
sequential, unique primary index, which is generated by the database during
the insert. One helpful guy from #sqlalchemy helped me out with
the "randomization" of the sequential number with this algorithm:
def mk_arb_seq(id):
""" Return an arbitrary number. This number is calculated out of
the given id. For that, it is multiplied by the large prime number"A".
Then a modulo operation with prime "M" where M < A. If "A" is
chosen as a non-prime, the sequence is not very arbitrary,
therefore a prime is recommended. """
M = 99999989
A = 2760727302517
return str((A*id) % M).zfill(len(str(M)))
The last problem with this is that I have no real mathematical proof for that
algorithm, that "id" never maps to one number more than once. However, I
simply tested this with a little program and it seems to work.
If you use the ORM, don't forget to do a session.flush() after adding the
object to the session, as this will calculate the primary index. Then you can
simply set col_num = mk_arb_seq(primary_index).
Best Regards,
Hermann
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