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From Wenchen Fan <cloud0...@gmail.com>
Subject Re: [SparkSql] Casting of Predicate Literals
Date Wed, 19 Aug 2020 14:07:55 GMT
Currently we can't. This is something we should improve, by either pushing
down the cast to the data source, or simplifying the predicates to
eliminate the cast.

On Wed, Aug 19, 2020 at 5:09 PM Bart Samwel <bart.samwel@databricks.com>
wrote:

> And how are we doing here on integer pushdowns? If someone does e.g.
> CAST(short_col AS LONG) < 1000, can we still push down "short_col < 1000"
> without the cast?
>
> On Tue, Aug 4, 2020 at 6:55 PM Russell Spitzer <russell.spitzer@gmail.com>
> wrote:
>
>> Thanks! That's exactly what I was hoping for! Thanks for finding the Jira
>> for me!
>>
>> On Tue, Aug 4, 2020 at 11:46 AM Wenchen Fan <cloud0fan@gmail.com> wrote:
>>
>>> I think this is not a problem in 3.0 anymore, see
>>> https://issues.apache.org/jira/browse/SPARK-27638
>>>
>>> On Wed, Aug 5, 2020 at 12:08 AM Russell Spitzer <
>>> russell.spitzer@gmail.com> wrote:
>>>
>>>> I've just run into this issue again with another user and I feel like
>>>> most folks here have seen some flavor of this at some point.
>>>>
>>>> The user registers a Datasource with a column of type Date (or some non
>>>> string) then performs a query that looks like.
>>>>
>>>> *SELECT * from Source WHERE date_col > '2020-08-03'*
>>>>
>>>> Seeing that the predicate literal here is a String, Spark needs to make
>>>> a change so that the DataSource column will be of the same type (Date),
>>>> so it places a "Cast" on the Datasource column so our plan ends up
>>>> looking like.
>>>>
>>>> Cast(date_col as String) > '2020-08-03'
>>>>
>>>> Since the Datasource Strategies can't handle a push down of the "Cast"
>>>> function we lose the predicate pushdown we could
>>>> have had. This can change a Job from a single partition lookup into a
>>>> full scan leading to a very confusing situation for
>>>> the end user. I also wonder about the relative cost here since we could
>>>> be avoiding doing X casts and instead just do a single
>>>> one on the predicate, in addition we could be doing the cast at the
>>>> Analysis phase and cut the run short before any work even
>>>> starts rather than doing a perhaps meaningless comparison between a
>>>> date and a non-date string.
>>>>
>>>> I think we should seriously consider whether in cases like this we
>>>> should attempt to cast the literal rather than casting the
>>>> source column.
>>>>
>>>> Please let me know if anyone has thoughts on this, or has some previous
>>>> Jiras I could dig into if it's been discussed before,
>>>> Russ
>>>>
>>>
>
> --
> Bart Samwel
> bart.samwel@databricks.com
>
>
>

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