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From "Reynold Xin" <r...@databricks.com>
Subject Re: FYI: The evolution on `CHAR` type behavior
Date Tue, 17 Mar 2020 00:35:36 GMT
BTW I'm not opposing us sticking to SQL standard (I'm in general for it). I was merely pointing
out that if we deviate away from SQL standard in any way we are considered "wrong" or "incorrect".
That argument itself is flawed when plenty of other popular database systems also deviate
away from the standard on this specific behavior.

On Mon, Mar 16, 2020 at 5:29 PM, Reynold Xin < rxin@databricks.com > wrote:

> 
> I looked up our usage logs (sorry I can't share this publicly) and trim
> has at least four orders of magnitude higher usage than char.
> 
> 
> 
> 
> On Mon, Mar 16, 2020 at 5:27 PM, Dongjoon Hyun < dongjoon. hyun@ gmail. com
> ( dongjoon.hyun@gmail.com ) > wrote:
> 
>> Thank you, Stephen and Reynold.
>> 
>> 
>> To Reynold.
>> 
>> 
>> The way I see the following is a little different.
>> 
>> 
>>       > CHAR is an undocumented data type without clearly defined
>> semantics.
>> 
>> Let me describe in Apache Spark User's View point.
>> 
>> 
>> Apache Spark started to claim `HiveContext` (and `hql/hiveql` function) at
>> Apache Spark 1.x without much documentation. In addition, there still
>> exists an effort which is trying to keep it in 3.0.0 age.
>> 
>>        https:/ / issues. apache. org/ jira/ browse/ SPARK-31088 (
>> https://issues.apache.org/jira/browse/SPARK-31088 )
>>        Add back HiveContext and createExternalTable
>> 
>> Historically, we tried to make many SQL-based customer migrate their
>> workloads from Apache Hive into Apache Spark through `HiveContext`.
>> 
>> Although Apache Spark didn't have a good document about the inconsistent
>> behavior among its data sources, Apache Hive has been providing its
>> documentation and many customers rely the behavior.
>> 
>>       - https:/ / cwiki. apache. org/ confluence/ display/ Hive/ LanguageManual+Types
>> ( https://cwiki.apache.org/confluence/display/Hive/LanguageManual+Types )
>> 
>> At that time, frequently in on-prem Hadoop clusters by well-known vendors,
>> many existing huge tables were created by Apache Hive, not Apache Spark.
>> And, Apache Spark is used for boosting SQL performance with its *caching*.
>> This was true because Apache Spark was added into the Hadoop-vendor
>> products later than Apache Hive.
>> 
>> 
>> Until the turning point at Apache Spark 2.0, we tried to catch up more
>> features to be consistent at least with Hive tables in Apache Hive and
>> Apache Spark because two SQL engines share the same tables.
>> 
>> For the following, technically, while Apache Hive doesn't changed its
>> existing behavior in this part, Apache Spark evolves inevitably by moving
>> away from the original Apache Spark old behaviors one-by-one.
>> 
>> 
>>       >  the value is already fucked up
>> 
>> 
>> The following is the change log.
>> 
>>       - When we switched the default value of `convertMetastoreParquet`.
>> (at Apache Spark 1.2)
>>       - When we switched the default value of `convertMetastoreOrc` (at
>> Apache Spark 2.4)
>>       - When we switched `CREATE TABLE` itself. (Change `TEXT` table to
>> `PARQUET` table at Apache Spark 3.0)
>> 
>> To sum up, this has been a well-known issue in the community and among the
>> customers.
>> 
>> Bests,
>> Dongjoon.
>> 
>> On Mon, Mar 16, 2020 at 5:24 PM Stephen Coy < scoy@ infomedia. com. au (
>> scoy@infomedia.com.au ) > wrote:
>> 
>> 
>>> Hi there,
>>> 
>>> 
>>> I’m kind of new around here, but I have had experience with all of all the
>>> so called “big iron” databases such as Oracle, IBM DB2 and Microsoft SQL
>>> Server as well as Postgresql.
>>> 
>>> 
>>> They all support the notion of “ANSI padding” for CHAR columns - which
>>> means that such columns are always space padded, and they default to
>>> having this enabled (for ANSI compliance).
>>> 
>>> 
>>> MySQL also supports it, but it defaults to leaving it disabled for
>>> historical reasons not unlike what we have here.
>>> 
>>> 
>>> In my opinion we should push toward standards compliance where possible
>>> and then document where it cannot work.
>>> 
>>> 
>>> If users don’t like the padding on CHAR columns then they should change to
>>> VARCHAR - I believe that was its purpose in the first place, and it does
>>> not dictate any sort of “padding".
>>> 
>>> 
>>> I can see why you might “ban” the use of CHAR columns where they cannot be
>>> consistently supported, but VARCHAR is a different animal and I would
>>> expect it to work consistently everywhere.
>>> 
>>> 
>>> 
>>> 
>>> Cheers,
>>> 
>>> 
>>> Steve C
>>> 
>>> 
>>>> On 17 Mar 2020, at 10:01 am, Dongjoon Hyun < dongjoon. hyun@ gmail. com
(
>>>> dongjoon.hyun@gmail.com ) > wrote:
>>>> 
>>>> Hi, Reynold.
>>>> (And +Michael Armbrust)
>>>> 
>>>> 
>>>> If you think so, do you think it's okay that we change the return value
>>>> silently? Then, I'm wondering why we reverted `TRIM` functions then?
>>>> 
>>>> 
>>>> > Are we sure "not padding" is "incorrect"?
>>>> 
>>>> 
>>>> 
>>>> Bests,
>>>> Dongjoon.
>>>> 
>>>> 
>>>> 
>>>> On Sun, Mar 15, 2020 at 11:15 PM Gourav Sengupta < gourav. sengupta@ gmail.
>>>> com ( gourav.sengupta@gmail.com ) > wrote:
>>>> 
>>>> 
>>>>> Hi,
>>>>> 
>>>>> 
>>>>> 100% agree with Reynold.
>>>>> 
>>>>> 
>>>>> 
>>>>> 
>>>>> Regards,
>>>>> Gourav Sengupta
>>>>> 
>>>>> 
>>>>> On Mon, Mar 16, 2020 at 3:31 AM Reynold Xin < rxin@ databricks. com
(
>>>>> rxin@databricks.com ) > wrote:
>>>>> 
>>>>> 
>>>>>> 
>>>>>> Are we sure "not padding" is "incorrect"?
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> I don't know whether ANSI SQL actually requires padding, but plenty
of
>>>>>> databases don't actually pad.
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> https:/ / docs. snowflake. net/ manuals/ sql-reference/ data-types-text.
html
>>>>>> (
>>>>>> https://aus01.safelinks.protection.outlook.com/?url=https:%2F%2Fdocs.snowflake.net%2Fmanuals%2Fsql-reference%2Fdata-types-text.html%23:~:text%3DCHAR%2520%252C%2520CHARACTER%2C(1)%2520is%2520the%2520default.%26text%3DSnowflake%2520currently%2520deviates%2520from%2520common%2Cspace-padded%2520at%2520the%2520end.&data=02%7C01%7Cscoy%40infomedia.com.au%7C5346c8d2675342008b5708d7c9fdff54%7C45d5407150f849caa59f9457123dc71c%7C0%7C0%7C637199965062044368&sdata=BvnZTTPTZBAi8oGWIvJk2fC%2FYSgdvq%2BAxtOj0nVzufk%3D&reserved=0
>>>>>> ) : "Snowflake currently deviates from common CHAR semantics in
that
>>>>>> strings shorter than the maximum length are not space-padded at the
end."
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> MySQL: https:/ / stackoverflow. com/ questions/ 53528645/ why-char-dont-have-padding-in-mysql
>>>>>> (
>>>>>> https://aus01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fstackoverflow.com%2Fquestions%2F53528645%2Fwhy-char-dont-have-padding-in-mysql&data=02%7C01%7Cscoy%40infomedia.com.au%7C5346c8d2675342008b5708d7c9fdff54%7C45d5407150f849caa59f9457123dc71c%7C0%7C0%7C637199965062044368&sdata=3OGLht%2Fa28GcKhAGwJPXIR%2BMODiIwXGVuNuResZqwXM%3D&reserved=0
>>>>>> )
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> On Sun, Mar 15, 2020 at 7:02 PM, Dongjoon Hyun < dongjoon. hyun@
gmail. com
>>>>>> ( dongjoon.hyun@gmail.com ) > wrote:
>>>>>> 
>>>>>>> Hi, Reynold.
>>>>>>> 
>>>>>>> 
>>>>>>> Please see the following for the context.
>>>>>>> 
>>>>>>> 
>>>>>>> https:/ / issues. apache. org/ jira/ browse/ SPARK-31136 (
>>>>>>> https://aus01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fissues.apache.org%2Fjira%2Fbrowse%2FSPARK-31136&data=02%7C01%7Cscoy%40infomedia.com.au%7C5346c8d2675342008b5708d7c9fdff54%7C45d5407150f849caa59f9457123dc71c%7C0%7C0%7C637199965062054364&sdata=pWQ9QhfVY4Uzyc8oIJ1QONQ0zOBAQ2DGSemyBj%2BvFeM%3D&reserved=0
>>>>>>> )
>>>>>>> "Revert SPARK-30098 Use default datasource as provider for CREATE
TABLE
>>>>>>> syntax"
>>>>>>> 
>>>>>>> 
>>>>>>> I raised the above issue according to the new rubric, and the
banning was
>>>>>>> the proposed alternative to reduce the potential issue.
>>>>>>> 
>>>>>>> 
>>>>>>> Please give us your opinion since it's still PR.
>>>>>>> 
>>>>>>> 
>>>>>>> Bests,
>>>>>>> Dongjoon.
>>>>>>> 
>>>>>>> On Sat, Mar 14, 2020 at 17:54 Reynold Xin < rxin@ databricks.
com (
>>>>>>> rxin@databricks.com ) > wrote:
>>>>>>> 
>>>>>>> 
>>>>>>>> I don’t understand this change. Wouldn’t this “ban”
confuse the hell out
>>>>>>>> of both new and old users?
>>>>>>>> 
>>>>>>>> 
>>>>>>>> For old users, their old code that was working for char(3)
would now stop
>>>>>>>> working. 
>>>>>>>> 
>>>>>>>> 
>>>>>>>> For new users, depending on whether the underlying metastore
char(3) is
>>>>>>>> either supported but different from ansi Sql (which is not
that big of a
>>>>>>>> deal if we explain it) or not supported. 
>>>>>>>> 
>>>>>>>> On Sat, Mar 14, 2020 at 3:51 PM Dongjoon Hyun < dongjoon.
hyun@ gmail. com
>>>>>>>> ( dongjoon.hyun@gmail.com ) > wrote:
>>>>>>>> 
>>>>>>>> 
>>>>>>>>> Hi, All.
>>>>>>>>> 
>>>>>>>>> Apache Spark has been suffered from a known consistency
issue on `CHAR`
>>>>>>>>> type behavior among its usages and configurations. However,
the evolution
>>>>>>>>> direction has been gradually moving forward to be consistent
inside Apache
>>>>>>>>> Spark because we don't have `CHAR` offically. The following
is the
>>>>>>>>> summary.
>>>>>>>>> 
>>>>>>>>> With 1.6.x ~ 2.3.x, `STORED PARQUET` has the following
different result.
>>>>>>>>> (`spark.sql.hive.convertMetastoreParquet=false` provides
a fallback to
>>>>>>>>> Hive behavior.)
>>>>>>>>> 
>>>>>>>>>     spark-sql> CREATE TABLE t1(a CHAR(3));
>>>>>>>>>     spark-sql> CREATE TABLE t2(a CHAR(3)) STORED
AS ORC;
>>>>>>>>>     spark-sql> CREATE TABLE t3(a CHAR(3)) STORED
AS PARQUET;
>>>>>>>>> 
>>>>>>>>>     spark-sql> INSERT INTO TABLE t1 SELECT 'a ';
>>>>>>>>>     spark-sql> INSERT INTO TABLE t2 SELECT 'a ';
>>>>>>>>>     spark-sql> INSERT INTO TABLE t3 SELECT 'a ';
>>>>>>>>> 
>>>>>>>>>     spark-sql> SELECT a, length(a) FROM t1;
>>>>>>>>>     a   3
>>>>>>>>>     spark-sql> SELECT a, length(a) FROM t2;
>>>>>>>>>     a   3
>>>>>>>>>     spark-sql> SELECT a, length(a) FROM t3;
>>>>>>>>>     a 2
>>>>>>>>> 
>>>>>>>>> Since 2.4.0, `STORED AS ORC` became consistent.
>>>>>>>>> (`spark.sql.hive.convertMetastoreOrc=false` provides
a fallback to Hive
>>>>>>>>> behavior.)
>>>>>>>>> 
>>>>>>>>>     spark-sql> SELECT a, length(a) FROM t1;
>>>>>>>>>     a   3
>>>>>>>>>     spark-sql> SELECT a, length(a) FROM t2;
>>>>>>>>>     a 2
>>>>>>>>>     spark-sql> SELECT a, length(a) FROM t3;
>>>>>>>>>     a 2
>>>>>>>>> 
>>>>>>>>> Since 3.0.0-preview2, `CREATE TABLE` (without `STORED
AS` clause) became
>>>>>>>>> consistent.
>>>>>>>>> (`spark.sql.legacy.createHiveTableByDefault.enabled=true`
provides a
>>>>>>>>> fallback to Hive behavior.)
>>>>>>>>> 
>>>>>>>>>     spark-sql> SELECT a, length(a) FROM t1;
>>>>>>>>>     a 2
>>>>>>>>>     spark-sql> SELECT a, length(a) FROM t2;
>>>>>>>>>     a 2
>>>>>>>>>     spark-sql> SELECT a, length(a) FROM t3;
>>>>>>>>>     a 2
>>>>>>>>> 
>>>>>>>>> In addition, in 3.0.0, SPARK-31147 aims to ban `CHAR/VARCHAR`
type in the
>>>>>>>>> following syntax to be safe.
>>>>>>>>> 
>>>>>>>>>     CREATE TABLE t(a CHAR(3));
>>>>>>>>>    https:/ / github. com/ apache/ spark/ pull/ 27902
(
>>>>>>>>> https://aus01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fapache%2Fspark%2Fpull%2F27902&data=02%7C01%7Cscoy%40infomedia.com.au%7C5346c8d2675342008b5708d7c9fdff54%7C45d5407150f849caa59f9457123dc71c%7C0%7C0%7C637199965062054364&sdata=lhwUP5TcTtaO%2BLUTmx%2BPTjT0ASXPrQ7oKLL0N6EG0Ug%3D&reserved=0
>>>>>>>>> )
>>>>>>>>> 
>>>>>>>>> This email is sent out to inform you based on the new
policy we voted.
>>>>>>>>> The recommendation is always using Apache Spark's native
type `String`.
>>>>>>>>> 
>>>>>>>>> Bests,
>>>>>>>>> Dongjoon.
>>>>>>>>> 
>>>>>>>>> References:
>>>>>>>>> 1. "CHAR implementation?", 2017/09/15
>>>>>>>>>      https:/ / lists. apache. org/ thread. html/ 96b004331d9762e356053b5c8c97e953e398e489d15e1b49e775702f%40%3Cdev.
>>>>>>>>> spark. apache. org%3E (
>>>>>>>>> https://aus01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.apache.org%2Fthread.html%2F96b004331d9762e356053b5c8c97e953e398e489d15e1b49e775702f%2540%253Cdev.spark.apache.org%253E&data=02%7C01%7Cscoy%40infomedia.com.au%7C5346c8d2675342008b5708d7c9fdff54%7C45d5407150f849caa59f9457123dc71c%7C0%7C0%7C637199965062064358&sdata=6hkno6zKTkcIrO%2FJo4hTYihsYvNynMuWcxhzL0fZR68%3D&reserved=0
>>>>>>>>> )
>>>>>>>>> 2. "FYI: SPARK-30098 Use default datasource as provider
for CREATE TABLE
>>>>>>>>> syntax", 2019/12/06
>>>>>>>>>    https:/ / lists. apache. org/ thread. html/ 493f88c10169680191791f9f6962fd16cd0ffa3b06726e92ed04cbe1%40%3Cdev.
>>>>>>>>> spark. apache. org%3E (
>>>>>>>>> https://aus01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.apache.org%2Fthread.html%2F493f88c10169680191791f9f6962fd16cd0ffa3b06726e92ed04cbe1%2540%253Cdev.spark.apache.org%253E&data=02%7C01%7Cscoy%40infomedia.com.au%7C5346c8d2675342008b5708d7c9fdff54%7C45d5407150f849caa59f9457123dc71c%7C0%7C0%7C637199965062064358&sdata=QJnEU3mvUJff53Gw8F%2FAbxzd%2F8ZA1hhuoQwicX4ZXyI%3D&reserved=0
>>>>>>>>> )
>>>>>>>>> 
>>>>>>>> 
>>>>>>>> 
>>>>>>> 
>>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>> 
>>>>> 
>>>> 
>>>> 
>>> 
>>> 
>>> 
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>> 
>> 
> 
>
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