Re: Continuous inserts...

On Thu, 17 Aug 2000, Joerg Hessdoerfer wrote:

> Hi!
>
> I have an application, where I have to insert data into a table at several
> rows per second, 24 hours a day, 365 days a year.
>
> After some period (a week, maybe a month) the data will be reducted to some
> degree and deleted from the table.
>
> As far as I understood, I would have to use VACUUM to really free the table
> from deleted rows - but VACUUM (esp. on a table with several million rows)
> takes some time and prevents me from inserting new data.
>
> Now, I thought I could just rename the table, inserting into a temp table, and
> switch the tables back after VACUUMing. Ideally, this should work unnoticed
> (and thus without prog. effort) on the client (inserter) side.
>
> Question: would it work to use a transaction to perform the rename?
>
> i.e.: continuous insert into table 'main' from client.
>
> From somewhere else, execute:
>
> begin;
> alter table main rename to vac_main;
> create table main (...);
> end;
>
> would the inserter notice this? Read: would ALL inserts AT ANY TIME succeed?

Unfortunately -- no. Also, bad things can happen if the transaction
errors since the rename happens immediately. There's been talk on
-hackers about this subject in the past.

However, you might be able to do something like this, but
I'm not sure it'll work and it's rather wierd:

Have three tables you work with, a and b and c

Set up rule on a to change insert to insert on b.
Insert into a.
When you want to vacuum, change the rule to insert to c.
Vacuum b
Change rule back
move rows from a and c into b
vacuum c

[you will slowly lose space in a, but it should only
be an occasional row since you should only insert into
a while you've deleted the insert rule to b, but
haven't yet added the insert rule to c -- not too many
rows here]