Re: Postgresql "FIFO" Tables, How-To ?

On Wed, 16 Jul 2003, Shridhar Daithankar wrote:

> On 16 Jul 2003 at 17:59, Kirill Ponazdyr wrote:
> 
> > Hello,
> > 
> > We are currently working on a project where we need to limit number of
> > records in a table to a certain number. As soon as the number has been
> > reached, for each new row the oldest row should be deleted (Kinda FIFO),
> > thus keeping a total number of rows at predefined number.
> > 
> > The actual limits would be anywhere from 250k to 10mil rows per table.
> > 
> > It would be great if this could be achieved by RDBMS engine itself, does
> > Postgres supports this kind of tables ? And if not, what would be the most
> > elegant soluion to achieve our goal in your oppinion ?
> 
> It is practically impossible due to concurrency limitation unless you 
> explicitly serialize everything which might be a bad idea.
> 
> I think it is doable. Create a normal table 't' and write a before insert 
> trigger. Create another table 'control' which contains the limit value and oid 
> of last row deleted. In the before insert trigger, do a select for update on 
> table 'control' so that no other transaction can update it. Proceed to 
> insertion/deletion in  table 't'.
> 
> It would be a bad idea to update the control table itself. You need to release 
> the lock with transaction commit.( I hope it gets released with the commit) If 
> you update control table, you would generate a dead row for every insertion in 
> main table which could be a major performance penalty for sizes you are talking 
> about.
> 
> Frankly I would like to know fist why do you want to do this. Unless there are 
> good enough practical reasons, I would not recommend this approach at all. Can 
> you tell us why do you want to do this?

If he only needs an approximate number of rows (i.e. having max +/- 100 
rows is ok...)  then maybe just use a sequence and delete any rows that 
are current_max_seq - max_records???