+1
Thanks for bringing up this discussion one more time Marios.
I strongly support enabling the new behavior in 1.15. It definitely has
implications on existing users, but as Seth said, thinking about the
upcoming upgrade story we need to make sure that at least the core/basic
operations are correct. Otherwise we will have to maintain multiple
versions of functions with broken semantics.
I since we also try to fix various issues around configuration, maybe it
might still be possible to configure the legacy cast behavior globally
via flink-conf.yaml. This should make the transitioning period easier in
production.
Regards,
Timo
Am 28.02.22 um 19:04 schrieb Seth Wiesman:
+1
Especially as SQL upgrades are right around the corner, it makes sense to
get our defaults right.
Seth
On Mon, Feb 28, 2022 at 7:14 AM Martijn Visser <mart...@ververica.com>
wrote:
+1 for setting this option to disabled by default. I believe failures
should be brought forward as soon as possible, so they can be fixed as fast
as possible. It will also be less confusing for new users. Last but not
least, I believe the impact on existing users will be minimal (since it can
be changed by changing one flag).
Best regards,
Martijn
On Tue, 22 Feb 2022 at 17:55, Marios Trivyzas <mat...@gmail.com> wrote:
Thanks Francesco,
The two arguments you posted, further strengthen the need to make it
DISABLED by default.
On Tue, Feb 22, 2022 at 12:10 PM Francesco Guardiani <
france...@ververica.com> wrote:
Hi all,
I'm +1 with what everything you said Marios.
I'm gonna add another argument on top of that: the
"legacy-cast-behavior"
has also a broken type inference, leading to incorrect results or
further
errors down in the pipeline[1]. For example, take this:
SELECT COALESCE(CAST('a' AS INT), 0) ...
With the legacy cast behavior ENABLED, this is going to lead to the
wrong
result, as 'a' is inferred as VARCHAR NOT NULL, the CAST return value
is
inferred as INT NOT NULL, so the planner will drop COALESCE, and will
never
return 0. Essentially, CAST will infer the wrong nullability leading to
assigning its result to a NOT NULL type, when its value can effectively
be
NULL.
You introduce a deprecated flag to help users
using old versions of the software to smoothly transition to the new
version, while the new users experience the new features/behavior,
without the need to set a flag.
This is IMO the major point on why we should disable the legacy cast
behavior by default. This is even more relevant with 1.15 and the
upgrade
story, as per the problem described above, because users will now end
up
generating plans with wrong type inference, which will be hard to
migrate
in the next flink versions.
FG
[1] In case you're wondering why it wasn't fixed, the reason is that
fixing
it means creating a breaking change, for details
https://github.com/apache/flink/pull/18611#issuecomment-1028174877
On Tue, Feb 22, 2022 at 10:07 AM Marios Trivyzas <mat...@gmail.com>
wrote:
Hello all!
I would like to bring back the discussion regarding the
*table.exec.legacy-cast-behaviour*
configuration option which we are introducing with Flink *1.15*. This
option provides the users
with the flexibility to continue using the old (incorrect, according
to
SQL
standards) behaviour
of *CAST.*
With Flink *1.15* we have introduced a bunch of fixes, improvements
and
new
casting functionality
between types, see https://issues.apache.org/jira/browse/FLINK-24403
,
and
some of them are
guarded behind the legacy behaviour option:
- Trimming and padding when casting to CHAR/VARCHAR types to
respect
the
specified length
- Changes for casting various types to CHAR/VARCHAR/STRING
- Runtime errors for CAST no longer emit *null *as result but
exceptions
are thrown with a meaningful message for the cause, that fail the
pipeline. *TRY_CAST
*is introduced instead, which emits *null* results instead of
throwing
exceptions.
Those changes become active if users set the
*table.exec.legacy-cast-behaviour
*option to *DISABLED*, otherwise
they will continue to experience the old, *erroneous*, behaviour of
*CAST*.
Currently, we have set the *table.exec.legacy-cast-behaviour *option
to be *ENABLED
*by default, so if users want
to get the new correct behaviour, they are required to set explicitly
the
option to *DISABLED*. Moreover, the option
itself is marked as deprecated, since the plan is to be removed in
the
future, so that the old, erroneous behaviour
won't be an option, and the *TRY_CAST* would be the way to go if
users
don't want to have errors and failed pipelines,
but have *null*s emitted in case of runtime errors when casting.
I would like to start a discussion and maybe ask for voting, so that
we
set
the *table.exec.legacy-cast-behaviour* option
to *DISABLED *by default, so that users that want to keep their old
pipelines working the same way, without changing their
SQL/TableAPI code, would need to explicitly set it to *ENABLED.*
My main argument for changing the default value for the option, is
that
the
*DISABLED* value is the one that enables
the *correct* behaviour for CAST which should be the default for all
new
users. This way, new FLINK users, or users which
build new pipelines, from now on would get the correct behaviour by
default
without the need of changing some flag in their
configuration. It feels weird to me, especially for people very
familiar
with standard SQL, to be obliged to set some config
flag, to be able to get the correct behaviour for CAST. On top, users
that
won't read about this option in our docs, will,
"blindly", experience the old incorrect behaviour for their new
pipelines,
and issues that could cause the CAST to fail
will remain hidden from them, since *nulls* would be emitted. IMHO,
this
last part is also very important during the development
stages of an application/pipeline. Normally, developers would want to
see
all possible errors/scenarios during development
stages , in order to build a robust production system. If errors, are
hidden then, they can easily end up with those errors
in the production system which would be even harder to discover and
debug,
since no exception will ever be thrown.
Imagine that there is a CAST which generates nulls because of runtime
errors, and its result is used in an aggregate function:
SELECT AVG(CAST(col1 AS FLOAT)), ... FROM t GROUP BY ....
If nulls are emitted by the CAST (let's say because some records
have a
quoted string value for col1, then simply the AVG result
would be wrong and in would be extremely difficult to realise and fix
the
issue by simply wrapping col1 first with, for example,
a REGEXP_REPLACE, to get rid of the quotes.
My second argument is that, since we have marked
*table.exec.legacy-cast-behaviour* as deprecated, and we want to
completely
remove it in the future, if the default value is *DISABLED*, when
it's
removed we also make a breaking change, by changing the
default behaviour for all users, which is against the common software
practices. You introduce a deprecated flag to help users
using old versions of the software to smoothly transition to the new
version, while the new users experience the new features/behaviour,
without the need to set a flag. So, when in the future this flag is
completely removed, for those "new" users it would be completely
transparent. If we continue with having the default value *ENABLED*,
those
new user would experience an "unnatural" breaking change
when this option is completely removed.
Best,
Marios
--
Marios
