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Re: 0608c273598: MDEV-15966: Behavior for TRUNCATE versioned table is not documented and not covered by tests


Hi, Nikita!

On Dec 28, Nikita Malyavin wrote:
> Hi, Sergei!
> In short, I am on the side of standard conformance.

Our TRUNCATE is not exactly a standard TRUNCATE. it's defined as
DROP+REPLACE. But generally I agree, standard conformance.

> Next two statements are generally out of scope of this bug, but: I
> like to see extra2 reading in separate function, and i believe that
> `TABLE_SHARE::init_from_binary_frm_image` needs more decomposiiton.

May be... not sure...

> Further:
> > I don't quite like that a small and simple function dd_frm_type(),
> > which used to just read few first bytes of the frm file, gets more
> > and more complex, growing into a complete frm-open method.
> >
> > So, perhaps, I'd consider changing TRUNCATE to open the table
> > properly without all that just-read-few-bytes shortcuts.
> Wasn't the there some performance issue as the reason to introduce
> such read-few-bytes thing? If it's not the case, then I'm okay with
> that, but feels like TRUNCATE code written in manner to make IO as
> fast as possible there.

I don't know if there was... but now it's certainly used in some cases
to avoid full table open. Mainly in ha_table_exists(), which is used in
many places in the server. And there we don't care whether a table is
versioned or not.

TRUNCATE uses it too, but it's not a performance critical statement, so
I think it's ok to do a full table open.

> > Or, may be, simply document that TRUNCATE works on versioning tables
> > just as DROP+CREATE does. There is no logical reason why it
> > shouldn't - if one can do SHOW CREATE TABLE and CREATE OR REPLACE,
> > then TRUNCATE won't add any new functionality on top of that. That's
> > the easiest and most logical "fix" to this bug. Agree?
> Consider we'll have VTMD someday at last. Then the TRUNCATE behavior
> will differ for different `vers_alter_history` modes. It is better to
> extend the command (or create new one) to make something related to
> versioning: either truncate only actual data, or only history (we
> already have delete history for this one), or both.

Okay, then just do a full table open.

I'd try to remove HTON_CAN_RECREATE flag completely and implement
recreate logic inside handler::delete_all_rows()

Chief Architect MariaDB
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