Hi Dawid,
thanks for your design document.
LIKE vs. INHERITS:
I would also not start creating transitive dependencies for table
metadata. This is very complicated to maintain in a long-term, esp. when
we ALTER or DELETE a table. Instead the new table metadata should be
materialized immediately before storing it in a catalog. We can consider
INHERITS in the future, once we see a need for it.
LIKE in schema vs. out of schema:
I'm fine with following DB2 syntax here. A LIKE after the schema but
with more options what should be inherited. That would be richer than
Hive and closer to the standard (even though it comes after the schema
part). I also think that inheriting columns from multiple tables is not
a very common case, except for dimension tables maybe.
Regards,
Timo
On 24.03.20 14:02, Jark Wu wrote:
+1 to use LIKE and put after schema part.
I also prefer the keyword LIKE than INHERITS, because it's easier to type
and understand, for a non-native English user :)
But I would like to limit a single LIKE clause in the DDL in the first
version. We can allow multiple LIKE clause in the future if needed.
Best,
Jark
On Tue, 24 Mar 2020 at 19:03, Dawid Wysakowicz <dwysakow...@apache.org>
wrote:
Sorry for a late reply, but I was on vacation.
As for putting the LIKE after the schema part. You're right, sql
standard lets it be only in the schema part. I was mislead by examples
for DB2 and MYSQL, which differ from the standard in that respect. My
bad, sorry.
Nevertheless I'd still be in favour of using the LIKE clause for that
purpose rather than INHERITS. I'm fine with putting it after the schema
part. The argument that it applies to the options part make sense to me.
I must admit I am not a fan of the INHERITS clause. @Jar I'd not
redefine the semantics of the INHERITS clause entirely. I am sure it
will pose unnecessary confusion if it differs significantly from what
was implemented for, let's be true, more popular vendors such as
PostgreSQL. My biggest concern is that the INHERITS clause in PostgreSQL
allows constructs such as SELECT * FROM ONLY B (where e.g. A INHERITS
B). My understanding of the purpose of the INHERITS clause is that it
really emulates inheritance that let's you create "nested" data sets. I
think what we are more interested in is a way to adjust only the
metadata of an already existing table.
Moreover I prefer the LIKE clause as it is more widespread. In some way
it is supported by PostgreSQL, DB2, SnowflakeDB, MySQL.
Lastly @Jingsong, I am not sure about the "link" part. I know at first
glance having a link and reflecting changes might seem appealing, but I
am afraid it would pose more threads than it would give benefits. First
of all it would make the LIKE/INHERITS clause unusable for creating e.g.
hive tables or jdbc tables that could be used from other systems, as the
link would not be understandable by those systems.
Best,
Dawid
On 05/03/2020 07:46, Jark Wu wrote:
Hi Dawid,
INHERITS creates a new table with a "link" to the original table.
Yes, INHERITS is a "link" to the original table in PostgreSQL.
But INHERITS is not SQL standard, I think it's fine for vendors to define
theire semantics.
Standard also allows declaring the clause after the schema part. We can
also do it.
Is that true? I didn't find it in SQL standard. If this is true, I prefer
to put LIKE after the schema part.
====================================
Hi Jingsong,
The concern you mentioned in (2) is exactly my concern too. That's why I
suggested INHERITS, or put LIKE after schema part.
Best,
Jark
On Thu, 5 Mar 2020 at 12:05, Jingsong Li <jingsongl...@gmail.com> wrote:
Thanks Dawid for starting this discussion.
I like the "LIKE".
1.For "INHERITS", I think this is a good feature too, yes, ALTER TABLE
will
propagate any changes in column data definitions and check constraints
down
the inheritance hierarchy. A inherits B, A and B share every things,
they
have the same kafka topic. If modify schema of B, this means underlying
kafka topic schema changed, so I think it is good to modify A too. If
this
for "ConfluentSchemaRegistryCatalog" mention by Jark, I think sometimes
this is just we want.
But "LIKE" also very useful for many cases.
2.For LIKE statement in schema, I know two kinds of like syntax, one is
MySQL/hive/sqlserver, the other is PostgreSQL. I prefer former:
- In the FLIP, there is "OVERWRITING OPTIONS", this will overwrite
properties in "with"? This looks weird, because "LIKE" is in schema,
but it
can affect outside properties.
Best,
Jingsong Lee
On Wed, Mar 4, 2020 at 2:05 PM Dawid Wysakowicz <dwysakow...@apache.org
wrote:
Hi Jark,
I did investigate the INHERITS clause, but it has a semantic that in my
opinion we definitely don't want to support. INHERITS creates a new
table
with a "link" to the original table. Therefore if you e.g change the
schema
of the original table it's also reflected in the child table. It's also
possible for tables like A inherits B query them like Select * from
only
A,
by default it returns results from both tables. I am pretty sure it's
not
what we're looking for.
PostgreSQL implements both the LIKE clause and INHERITS. I am open for
discussion if we should support multiple LIKE statements or not.
Standard
also allows declaring the clause after the schema part. We can also do
it.
Nevertheless I think including multiple tables might be useful, e.g.
when
you want to union two tables and output to the same Kafka cluster and
just
change the target topic. I know it's not a very common use case but
it's
not a big effort to support it.
Let me know what you think.
Best,
Dawid
On Wed, 4 Mar 2020, 04:55 Jark Wu, <imj...@gmail.com> wrote:
Hi Dawid,
Thanks for starting this discussion. I like the idea.
Once we support more intergrated catalogs,
e.g. ConfluentSchemaRegistryCatalog, this problem will be more urgent.
Because it's very common to adjust existing tables in catalog
slightly.
My initial thought was introducing INHERITS keyword, which is also
supported in PostgreSQL [1].
This is also similar to the functionality of Hive CREATE TABLE LIKE
[2].
CREATE TEMPORARY TABLE MyTable (WATERMARK FOR ts) INHERITS
cat.db.KafkoTopic
CREATE TEMPORARY TABLE MyTable (WATERMARK FOR ts) INHERITS
cat.db.KafkoTopic WITH ('k' = 'v')
The INHERITS can inherit an existing table with all columns,
watermark,
and
properties, but the properties and watermark and be overwrited
explicitly.
The reason I prefer INHERITS rather than LIKE is the keyword position.
We
are copying an existing table definition including the properties.
However, LIKE appears in the schema part, it sounds like copying
properties
into schema part of DDL.
Besides of that, I'm not sure whether the use case stands "merging two
tables into a single one with a different connector".
From my understanding, most use cases are just slightly adjusting on
an
existing catalog table with new properties or watermarks.
Do we really need to merge two table definitions into a single one?
For
example, is it possible to merge a Kafka table definition and
a Filesystem table definition into a new Kafka table, and the new
Kafka
table exactly matches the underlying physical data format?
Best,
Jark
[1]: https://www.postgresql.org/docs/9.5/sql-createtable.html
[2]:
https://cwiki.apache.org/confluence/display/Hive/LanguageManual+DDL#LanguageManualDDL-CreateTableLike
On Tue, 3 Mar 2020 at 21:12, Dawid Wysakowicz <dwysakow...@apache.org
wrote:
Hi devs,
I wanted to bring another improvement proposal up for a discussion.
Often
users need to adjust existing tables slightly. This is especially
useful
when users need to enhance a table created from an external tool
(e.g.
HIVE) with Flink's specific information such as e.g watermarks. It
can
also
be a useful tool for ETL processes, e.g. merging two tables into a
single
one with a different connector. My suggestion would be to support an
optional *Feature T171, “LIKE clause in table definition” *of SQL
standard 2008.
You can see the description of the proposal here:
https://cwiki.apache.org/confluence/display/FLINK/FLIP-110%3A+Support+LIKE+clause+in+CREATE+TABLE
Looking forward for your comments.
Best,
Dawid
--
Best, Jingsong Lee
