Re: [Bizgres-general] WAL bypass for INSERT, UPDATE and - Mailing list pgsql-hackers

Greg Stark wrote:
> "Andrew Dunstan" <andrew@dunslane.net> writes:
> 
> > Bruce Momjian said:
> > > DROP would drop the table on a restart
> > > after a non-clean shutdown.  It would do _no_ logging on the table and
> > > allow concurrent access, plus index access.  DELETE is the same as
> > > DROP, but it just truncates the table (perhaps TRUNCATE is a better
> > > word).
> > >
> > > EXCLUSIVE would allow only a single session to modify the table, and
> > > would do all changes by appending to the table, similar to COPY LOCK.
> > > EXCLUSIVE would also not allow indexes because those can not be
> > > isolated like appending to the heap.  EXCLUSIVE would write all dirty
> > > shared buffers for the table and fsync them before committing.  SHARE
> > > is the functionality we have now, with full logging.
> > 
> > I an horribly scared that this will be used as a "performance boost" for
> > normal use. I would at least like to see some restrictions that make it
> > harder to mis-use. Perhaps restrict to superuser?
> 
> Well that's its whole purpose. At least you can hardly argue that you didn't
> realize the consequences of "DELETE ROWS ON RECOVERY"... :)

True.  I think we are worried about non-owners using it, but the owner
had to grant permissions for others to modify it, so we might be OK.

> Some thoughts:
> 
> a) I'm not sure I understand the purpose of EXCLUSIVE. When would I ever want to
>    use it instead of DELETE ROWS?

Good question.  The use case is doing COPY into a table that already had
data.  EXCLUSIVE allows additions to the table but preserves the
existing data on a crash.

> b) It seems like the other feature people were talking about of not logging
>    for a table created within the same transaction should be handled by
>    having this flag implicitly set for any such newly created table.
>    Ie, the test for whether to log would look like:
> 
>    if (!table->logged && table->xid != myxid) ...

Yes, the question is whether we want to limit users to having this
optimization _only_ when they have created the table in the same
transaction, and the short answer is we don't.

> c) Every option in ALTER TABLE should be in CREATE TABLE as well.

I looked into that and see that things like:
   ALTER [ COLUMN ] column SET STATISTICS integer   ALTER [ COLUMN ] column SET STORAGE { PLAIN | EXTERNAL | EXTENDED |
MAIN}
 

are not supported by CREATE TABLE, and probably shouldn't be because the
value can be changed after the table is created.  I think the only
things we usually support in CREATE TABLE are those that cannot be
altered.

> d) Yes as someone else mentioned, this should only be allowable on a table
>    with no foreign keys referencing it. 

Right, and EXCLUSIVE can not have an index either.

--  Bruce Momjian                        |  http://candle.pha.pa.us pgman@candle.pha.pa.us               |  (610)
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