spark-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Wenchen Fan <cloud0...@gmail.com>
Subject Re: [VOTE][SPARK-28885] Follow ANSI store assignment rules in table insertion by default
Date Thu, 05 Sep 2019 07:27:40 GMT
+1

To be honest I don't like the legacy policy. It's too loose and easy for
users to make mistakes, especially when Spark returns null if a function
hit errors like overflow.

The strict policy is not good either. It's too strict and stops valid use
cases like writing timestamp values to a date type column. Users do expect
truncation to happen without adding cast manually in this case. It's also
weird to use a spark specific policy that no other database is using.

The ANSI policy is better. It stops invalid use cases like writing string
values to an int type column, while keeping valid use cases like timestamp
-> date.

I think it's no doubt that we should use ANSI policy instead of legacy
policy for v1 tables. Except for backward compatibility, ANSI policy is
literally better than the legacy policy.

The v2 table is arguable here. Although the ANSI policy is better than
strict policy to me, this is just the store assignment policy, which only
partially controls the table insertion behavior. With Spark's "return null
on error" behavior, the table insertion is more likely to insert invalid
null values with the ANSI policy compared to the strict policy.

I think we should use ANSI policy by default for both v1 and v2 tables,
because
1. End-users don't care how the table is implemented. Spark should provide
consistent table insertion behavior between v1 and v2 tables.
2. Data Source V2 is unstable in Spark 2.x so there is no backward
compatibility issue. That said, the baseline to judge which policy is
better should be the table insertion behavior in Spark 2.x, which is the
legacy policy + "return null on error". ANSI policy is better than the
baseline.
3. We expect more and more uses to migrate their data sources to the V2
API. The strict policy can be a stopper as it's a too big breaking change,
which may break many existing queries.

Thanks,
Wenchen


On Wed, Sep 4, 2019 at 1:59 PM Gengliang Wang <gengliang.wang@databricks.com>
wrote:

> Hi everyone,
>
> I'd like to call for a vote on SPARK-28885 <https://issues.apache.org/jira/browse/SPARK-28885>
"Follow ANSI store assignment rules in table insertion by default".
> When inserting a value into a column with the different data type, Spark performs type
coercion. Currently, we support 3 policies for the type coercion rules: ANSI, legacy and strict,
which can be set via the option "spark.sql.storeAssignmentPolicy":
> 1. ANSI: Spark performs the type coercion as per ANSI SQL. In practice, the behavior
is mostly the same as PostgreSQL. It disallows certain unreasonable type conversions such
as converting `string` to `int` and `double` to `boolean`.
> 2. Legacy: Spark allows the type coercion as long as it is a valid `Cast`, which is very
loose. E.g., converting either `string` to `int` or `double` to `boolean` is allowed. It is
the current behavior in Spark 2.x for compatibility with Hive.
> 3. Strict: Spark doesn't allow any possible precision loss or data truncation in type
coercion, e.g., converting either `double` to `int` or `decimal` to `double` is allowed. The
rules are originally for Dataset encoder. As far as I know, no maintainstream DBMS is using
this policy by default.
>
> Currently, the V1 data source uses "Legacy" policy by default, while V2 uses "Strict".
This proposal is to use "ANSI" policy by default for both V1 and V2 in Spark 3.0.
>
> There was also a DISCUSS thread "Follow ANSI SQL on table insertion" in the dev mailing
list.
>
> This vote is open until next Thurs (Sept. 12nd).
>
> [ ] +1: Accept the proposal
> [ ] +0
> [ ] -1: I don't think this is a good idea because ...
>
> Thank you!
>
> Gengliang
>
>

Mime
View raw message