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
>>
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