Re: HowTo - create user defined sequence with proper serialization

From: <p.santos000_at_gmail.com>
Date: 29 Aug 2006 09:15:21 -0700
Message-ID: <1156868121.878784.94090@i3g2000cwc.googlegroups.com>

bdbafh_at_gmail.com wrote:
> p.santos..._at_gmail.com wrote:
> > Folks,
> > Here is a difficult question that I'm hoping you can help me solve.
> >
> > Requirements:
> > - Create a user defined like "sequence" that increments by
> > power of 2.
> > - This defined "sequence" must distribute values like
> > 1,2,4,16,256 etc ..
> > - If a value is deleted, then the sequence must re-use that
> > value.
> > - acess to this "sequence" must be serialized so that no two
> > users can
> > grab the same value
> >
> > Example: (GROUPS table)
> >
> > ACCOUNT_ID NAME SEQ
> > ======= ===== =====
> > 347164311 GROUP1 1
> > 347164311 GROUP2 2
> > 347164311 GROUP2 4
> >
> > - So if account id " 347164311" wanted to create a new group in the
> > GROUPS
> > table, it would obtain a new sequence with the value of 16 and it
> > would insert it.
> >
> > - If at some point GROUP2 was removed from the table, the custom
> > sequence
> > would distribute 2 as the "NEXTVAL" ..
> >
> > The key thing here is serialization and I'm not sure how to achive
> > this.
> > Here is the sample function that delivers the next available sequence,
> > but
> > doesn't yet serialize access to it.
> >
> > CREATE OR REPLACE FUNCTION GET_SEQ (ACCTID NUMBER)
> > RETURN NUMBER IS
> > i NUMBER := 0;
> > two_power_i NUMBER := power(2,i);
> > curr_mask groups.seq%TYPE;
> >
> > CURSOR masksCursor IS SELECT seq FROM groups WHERE account_id = acctid
> > ORDER BY seq;
> >
> > BEGIN
> > OPEN masksCursor;
> > LOOP
> > FETCH masksCursor INTO curr_mask;
> > EXIT WHEN curr_mask != two_power_i OR masksCursor %NOTFOUND;
> > i := i + 1;
> > two_power_i := power(2,i);
> > END LOOP;
> > CLOSE masksCursor;
> > RETURN two_power_i;
> > END;
> > /
> >
> > Any idea on how to make the above function serial so that if 2
> > concurrent users execute the function, they don't both get the same
> > value?
> >
> > -peter

>

> http://www.stanford.edu/dept/itss/docs/oracle/10g/server.101/b10759/functions102.htm#i78493

>

> Peter,

>

> If what you are indeed after is a sequence of numbers which are 2^^n,
> where n = 0,1,2, ... m
> One could just create a sequence (e.g. myseq) with a starting value of
> 0, increment 1, nocache.
> Create a function to return POWER(2,myseq.nextval) and use that in your
> insert statement. Return myseq.currval if needed.
>
> -bdbafh

That is an interesting way of doing this, but then I would have to have a sequence per
account_id because each value returned is by account_id. Also if my sequence distributed a nextval of "4" and that group later on gets removed from the table, there is no way to re-use that value anymore.

-peter Received on Tue Aug 29 2006 - 11:15:21 CDT