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Re: Tables Locks Quetion or Strictlly subsequent numbers

From: "Andrew Hammond" <andrew(dot)george(dot)hammond(at)gmail(dot)com>
To: pgsql-novice(at)postgresql(dot)org
Subject: Re: Tables Locks Quetion or Strictlly subsequent numbers
Date: 2006-07-28 20:51:52
Message-ID: (view raw or whole thread)
Lists: pgsql-novice
Kaloyan Iliev wrote:
> Hello All,
> I have such a question.
> I want to receive from the database subsequent numbers and I can't
> afford to miss one. There must not be any missing numbers.
> So the sequence is not good for me because if transaction rollback the
> there will be gaps.
> So I make a table with one row and the row contains one int.
> Every time I update the row in Serializable transaction level:
> update foo set lastvalue = lastvalue+1;
> select lastvalue from foo;
> This is my decision of the problem. But here is my next question.
> If two apllications try to take next number at the same time one of both
> transactions will abort.
> The one way is to catch the error and try again,  but this is what I
> don't want to do.
> So is there a way to escape transaction error. I read about the locks
> and I think they can solve my problem.
> First I thick I must change my transaction Isolation Level to Read Commited.
> Then If I first lock (in ROW EXCLUSIVE mode) the table, then update and
> then read - will this solve my problem.
> And if two functions try to do this in the same time will the second
> transaction waith until it can lock the table and then without errors to
> take the next number?
> And my questions:
> 1. Should I change the transaction isolation level to Read Commited or
> Serializable transaction level is good enough (I prefer to work in
> Serializable transaction level)?

Going to Read Committed from Serializable would actually decrease the
level of isolation for your transaction. I'm not sure that's what you
want to do.

> 2. Is my algorithm correct and will it give me secure way to get
> subsequent numbers without gaps?
> 3. Can I use SELECT FOR UPDATE instead ot locks in this case?

Well, if the table has only one row, and that row is only for the
counter, you could use

UPDATE counter_tbl SET counter = counter + 1;
SELECT counter FROM counter_tbl;

Which would cause your transactions to queue up when dealing with the
counter. You don't get rollbacks that way. :)

> 4. Can I change the transaction level back to Serializable after I get
> the number I want, without commiting the transaction?

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