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From Matt Cheah <mch...@palantir.com>
Subject Re: [Discuss] Follow ANSI SQL on table insertion
Date Wed, 31 Jul 2019 17:13:45 GMT
Sorry I meant the current behavior for V2, which fails the query compilation if the cast is
not safe.

 

Agreed that a separate discussion about overflow might be warranted. I’m surprised we don’t
throw an error now, but it might be warranted to do so.

 

-Matt Cheah

 

From: Reynold Xin <rxin@databricks.com>
Date: Wednesday, July 31, 2019 at 9:58 AM
To: Matt Cheah <mcheah@palantir.com>
Cc: Russell Spitzer <russell.spitzer@gmail.com>, Takeshi Yamamuro <linguin.m.s@gmail.com>,
Gengliang Wang <gengliang.wang@databricks.com>, Ryan Blue <rblue@netflix.com>,
Spark dev list <dev@spark.apache.org>, Hyukjin Kwon <gurwls223@gmail.com>, Wenchen
Fan <cloud0fan@gmail.com>
Subject: Re: [Discuss] Follow ANSI SQL on table insertion

 

Matt what do you mean by maximizing 3, while allowing not throwing errors when any operations
overflow? Those two seem contradicting.

 

 

On Wed, Jul 31, 2019 at 9:55 AM, Matt Cheah <mcheah@palantir.com> wrote:

I’m -1, simply from disagreeing with the premise that we can afford to not be maximal on
standard 3. The correctness of the data is non-negotiable, and whatever solution we settle
on cannot silently adjust the user’s data under any circumstances.

 

I think the existing behavior is fine, or perhaps the behavior can be flagged by the destination
writer at write time.

 

-Matt Cheah

 

From: Hyukjin Kwon <gurwls223@gmail.com>
Date: Monday, July 29, 2019 at 11:33 PM
To: Wenchen Fan <cloud0fan@gmail.com>
Cc: Russell Spitzer <russell.spitzer@gmail.com>, Takeshi Yamamuro <linguin.m.s@gmail.com>,
Gengliang Wang <gengliang.wang@databricks.com>, Ryan Blue <rblue@netflix.com>,
Spark dev list <dev@spark.apache.org>
Subject: Re: [Discuss] Follow ANSI SQL on table insertion

 

>From my look, +1 on the proposal, considering ASCI and other DBMSes in general.

 

2019년 7월 30일 (화) 오후 3:21, Wenchen Fan <cloud0fan@gmail.com>님이 작성:

We can add a config for a certain behavior if it makes sense, but the most important thing
we want to reach an agreement here is: what should be the default behavior? 

 

Let's explore the solution space of table insertion behavior first:

At compile time,

1. always add cast

2. add cast following the ASNI SQL store assignment rule (e.g. string to int is forbidden
but long to int is allowed)

3. only add cast if it's 100% safe

At runtime,

1. return null for invalid operations

2. throw exceptions at runtime for invalid operations

 

The standards to evaluate a solution:

1. How robust the query execution is. For example, users usually don't want to see the query
fails midway.

2. how tolerant to user queries. For example, a user would like to write long values to an
int column as he knows all the long values won't exceed int range.

3. How clean the result is. For example, users usually don't want to see silently corrupted
data (null values).

 

The current Spark behavior for Data Source V1 tables: always add cast and return null for
invalid operations. This maximizes standard 1 and 2, but the result is least clean and users
are very likely to see silently corrupted data (null values).

 

The current Spark behavior for Data Source V2 tables (new in Spark 3.0): only add cast if
it's 100% safe. This maximizes standard 1 and 3, but many queries may fail to compile, even
if these queries can run on other SQL systems. Note that, people can still see silently corrupted
data because cast is not the only one that can return corrupted data. Simple operations like
ADD can also return corrected data if overflow happens. e.g. INSERT INTO t1 (intCol) SELECT
anotherIntCol + 100 FROM t2 

 

The proposal here: add cast following ANSI SQL store assignment rule, and return null for
invalid operations. This maximizes standard 1, and also fits standard 2 well: if a query can't
compile in Spark, it usually can't compile in other mainstream databases as well. I think
that's tolerant enough. For standard 3, this proposal doesn't maximize it but can avoid many
invalid operations already.

 

Technically we can't make the result 100% clean at compile-time, we have to handle things
like overflow at runtime. I think the new proposal makes more sense as the default behavior.

  

 

On Mon, Jul 29, 2019 at 8:31 PM Russell Spitzer <russell.spitzer@gmail.com> wrote:

I understand spark is making the decisions, i'm say the actual final effect of the null decision
would be different depending on the insertion target if the target has different behaviors
for null.

 

On Mon, Jul 29, 2019 at 5:26 AM Wenchen Fan <cloud0fan@gmail.com> wrote:

> I'm a big -1 on null values for invalid casts. 

 

This is why we want to introduce the ANSI mode, so that invalid cast fails at runtime. But
we have to keep the null behavior for a while, to keep backward compatibility. Spark returns
null for invalid cast since the first day of Spark SQL, we can't just change it without a
way to restore to the old behavior.

 

I'm OK with adding a strict mode for the upcast behavior in table insertion, but I don't agree
with making it the default. The default behavior should be either the ANSI SQL behavior or
the legacy Spark behavior.

 

> other modes should be allowed only with strict warning the behavior will be determined
by the underlying sink.

 

Seems there is some misunderstanding. The table insertion behavior is fully controlled by
Spark. Spark decides when to add cast and Spark decided whether invalid cast should return
null or fail. The sink is only responsible for writing data, not the type coercion/cast stuff.

 

On Sun, Jul 28, 2019 at 12:24 AM Russell Spitzer <russell.spitzer@gmail.com> wrote:

I'm a big -1 on null values for invalid casts. This can lead to a lot of even more unexpected
errors and runtime behavior since null is  

 

1. Not allowed in all schemas (Leading to a runtime error anyway)
2. Is the same as delete in some systems (leading to data loss)

And this would be dependent on the sink being used. Spark won't just be interacting with ANSI
compliant sinks so I think it makes much more sense to be strict. I think Upcast mode is a
sensible default and other modes should be allowed only with strict warning the behavior will
be determined by the underlying sink.

 

On Sat, Jul 27, 2019 at 8:05 AM Takeshi Yamamuro <linguin.m.s@gmail.com> wrote:

Hi, all 

 

+1 for implementing this new store cast mode.

>From a viewpoint of DBMS users, this cast is pretty common for INSERTs and I think this
functionality could

promote migrations from existing DBMSs to Spark. 

 

The most important thing for DBMS users is that they could optionally choose this mode when
inserting data.

Therefore, I think it might be okay that the two modes (the current upcast mode and the proposed
store cast mode)

co-exist for INSERTs. (There is a room to discuss which mode  is enabled by default though...)

 

IMHO we'll provide three behaviours below for INSERTs;

 - upcast mode

 - ANSI store cast mode and runtime exceptions thrown for invalid values

 - ANSI store cast mode and null filled for invalid values

 

 

On Sat, Jul 27, 2019 at 8:03 PM Gengliang Wang <gengliang.wang@databricks.com> wrote:

Hi Ryan, 

 

Thanks for the suggestions on the proposal and doc.

Currently, there is no data type validation in table insertion of V1. We are on the same page
that we should improve it. But using UpCast is from one extreme to another. It is possible
that many queries are broken after upgrading to Spark 3.0. 

The rules of UpCast are too strict. E.g. it doesn't allow assigning Timestamp type to Date
Type, as there will be "precision loss". To me, the type coercion is reasonable and the "precision
loss" is under expectation. This is very common in other SQL engines. 

As long as Spark is following the ANSI SQL store assignment rules, it is users' responsibility
to take good care of the type coercion in data writing. I think it's the right decision.

 

> But the new behavior is only applied in DataSourceV2, so it won’t affect existing jobs
until sources move to v2 and break other behavior anyway.

Eventually, most sources are supposed to be migrated to DataSourceV2 V2. I think we can discuss
and make a decision now.

 

> Fixing the silent corruption by adding a runtime exception is not a good option, either.


The new optional mode proposed in https://issues.apache.org/jira/browse/SPARK-28512 [issues.apache.org]
is disabled by default. This should be fine.

 

 

 

On Sat, Jul 27, 2019 at 10:23 AM Wenchen Fan <cloud0fan@gmail.com> wrote:

I don't agree with handling literal values specially. Although Postgres does it, I can't find
anything about it in the SQL standard. And it introduces inconsistent behaviors which may
be strange to users: 

* What about something like "INSERT INTO t SELECT float_col + 1.1"?
* The same insert with a decimal column as input will fail even when a decimal literal would
succeed
* Similar insert queries with "literal" inputs can be constructed through layers of indirection
via views, inline views, CTEs, unions, etc. Would those decimals be treated as columns and
fail or would we attempt to make them succeed as well? Would users find this behavior surprising?

 

Silently corrupt data is bad, but this is the decision we made at the beginning when design
Spark behaviors. Whenever an error occurs, Spark attempts to return null instead of runtime
exception. Recently we provide configs to make Spark fail at runtime for overflow, but that's
another story. Silently corrupt data is bad, runtime exception is bad, and forbidding all
the table insertions that may fail(even with very little possibility) is also bad. We have
to make trade-offs. The trade-offs we made in this proposal are:

* forbid table insertions that are very like to fail, at compile time. (things like writing
string values to int column)

* allow table insertions that are not that likely to fail. If the data is wrong, don't fail,
insert null.

* provide a config to fail the insertion at runtime if the data is wrong.

 

>  But the new behavior is only applied in DataSourceV2, so it won’t affect existing
jobs until sources move to v2 and break other behavior anyway.

When users write SQL queries, they don't care if a table is backed by Data Source V1 or V2.
We should make sure the table insertion behavior is consistent and reasonable. Furthermore,
users may even not care if the SQL queries are run in Spark or other RDBMS, it's better to
follow SQL standard instead of introducing a Spark-specific behavior.

 

We are not talking about a small use case like allowing writing decimal literal to float column,
we are talking about a big goal to make Spark compliant to SQL standard, w.r.t. https://issues.apache.org/jira/browse/SPARK-26217
[issues.apache.org] . This proposal is a sub-task of it, to make the table insertion behavior
follow SQL standard.

 

On Sat, Jul 27, 2019 at 1:35 AM Ryan Blue <rblue@netflix.com> wrote:

I don’t think this is a good idea. Following the ANSI standard is usually fine, but here
it would silently corrupt data.

>From your proposal doc, ANSI allows implicitly casting from long to int (any numeric type
to any other numeric type) and inserts NULL when a value overflows. That would drop data values
and is not safe.

Fixing the silent corruption by adding a runtime exception is not a good option, either. That
puts off the problem until much of the job has completed, instead of catching the error at
analysis time. It is better to catch this earlier during analysis than to run most of a job
and then fail.

In addition, part of the justification for using the ANSI standard is to avoid breaking existing
jobs. But the new behavior is only applied in DataSourceV2, so it won’t affect existing
jobs until sources move to v2 and break other behavior anyway.

I think that the correct solution is to go with the existing validation rules that require
explicit casts to truncate values.

That still leaves the use case that motivated this proposal, which is that floating point
literals are parsed as decimals and fail simple insert statements. We already came up with
two alternatives to fix that problem in the DSv2 sync and I think it is a better idea to go
with one of those instead of “fixing” Spark in a way that will corrupt data or cause runtime
failures.

 

On Thu, Jul 25, 2019 at 9:11 AM Wenchen Fan <cloud0fan@gmail.com> wrote:

I have heard about many complaints about the old table insertion behavior. Blindly casting
everything will leak the user mistake to a late stage of the data pipeline, and make it very
hard to debug. When a user writes string values to an int column, it's probably a mistake
and the columns are misordered in the INSERT statement. We should fail the query earlier and
ask users to fix the mistake. 

 

In the meanwhile, I agree that the new table insertion behavior we introduced for Data Source
V2 is too strict. It may fail valid queries unexpectedly.

 

In general, I support the direction of following the ANSI SQL standard. But I'd like to do
it with 2 steps:

1. only add cast when the assignment rule is satisfied. This should be the default behavior
and we should provide a legacy config to restore to the old behavior.

2. fail the cast operation at runtime if overflow happens. AFAIK Marco Gaido is working on
it already. This will have a config as well and by default we still return null.

 

After doing this, the default behavior will be slightly different from the SQL standard (cast
can return null), and users can turn on the ANSI mode to fully follow the SQL standard. This
is much better than before and should prevent a lot of user mistakes. It's also a reasonable
choice to me to not throw exceptions at runtime by default, as it's usually bad for long-running
jobs.

 

Thanks,

Wenchen 

 

On Thu, Jul 25, 2019 at 11:37 PM Gengliang Wang <gengliang.wang@databricks.com> wrote:

Hi everyone, 

 

I would like to discuss the table insertion behavior of Spark. In the current data source
V2, only UpCast is allowed for table insertion. I think following ANSI SQL is a better idea.

For more information, please read the Discuss: Follow ANSI SQL on table insertion [docs.google.com]

Please let me know if you have any thoughts on this.

 

Regards,

Gengliang


 

-- 

Ryan Blue 

Software Engineer

Netflix


 

-- 

---
Takeshi Yamamuro

 


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