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From All In A Days Work <allinadays...@gmail.com>
Subject Re: Example of creating expressions for SchemaRDD methods
Date Fri, 04 Apr 2014 04:12:01 GMT
Hi Michael,

The idea is to build a pipeline of operators on RDD, leveraging existing
operations already done.

E.g.

rdd1 = rdd.select(...).
rdd2 = rdd1.where(....).
rdd3 = rdd2.groupBy(....) etc.

In such construct, each operator builds on the previous one, including any
materialized results etc. If I use a SQL for each of them, I suspect the
later SQLs will not leverage the earlier SQLs by any means - hence these
will be inefficient to first approach. Let me know if this is not correct.

The reason for building expressions is that the use case needs these to be
created on the fly based on some case class at runtime.

I.e., I can't type these in REPL. The scala code will define some case
class A (a: ... , b: ..., c: ... ) where class name, member names and types
will be known before hand and the RDD will be defined on this. Then based
on user action, above pipeline needs to be constructed on fly. Thus the
expressions has to be constructed on fly from class members and other
predicates etc., most probably using expression constructors.

Could you please share how expressions could be constructed using the APIs
on expression (and not on REPL) ?

Regards,




On Thu, Apr 3, 2014 at 11:35 AM, Michael Armbrust <michael@databricks.com>wrote:

> I'll start off by saying that the DSL is pretty experimental, and we are
> still figuring out exactly how to expose all of it to end users.  Right now
> you are going to get more full featured functionality from SQL.  Under the
> covers, using these two representations results in the same execution plans.
>
> That said, you can create expressions using implicit conversions that are
> provided when you import a SQLContext.
>
> The leaves of these expression trees will be either `Attributes` (columns
> of the tables), or `Literals` (constant values).  You can represent an
> attribute by creating a Scala Symbol<http://daily-scala.blogspot.com/2010/01/symbols.html>
(an
> identifier that is prefixed with a ').  When you perform an operation on a
> Symbol it gets converted into an attribute, and an expression is returned.
> scala> 'a + 1
> res7: org.apache.spark.sql.catalyst.expressions.Add = ('a + 1)
>
> You can also construct more complex expressions:
> scala> 'b + 1 === 3 && 'c === 2
> res9: org.apache.spark.sql.catalyst.expressions.And = ((('b + 1) = 3) &&
> ('c = 2))
>
> Note that sometimes we have to use special operators to avoid falling back
> on the standard JVM implementation.  For example since symbols already have
> a method ==, doing the following does not give us what we want.
> scala> 'a == 'b
> res13: Boolean = false
>
> In constrast, since === is not defined on a symbol, the compiler is forced
> to use an implicit conversion from the SQLContext.  This gives us the
> desired result.
> scala> 'a === 'b
> res14: org.apache.spark.sql.catalyst.expressions.Equals = ('a = 'b)
>
> Let me know if you have further questions!
>
> Michael
>
>
> On Wed, Apr 2, 2014 at 9:52 PM, All In A Days Work <
> allinadayswrk@gmail.com> wrote:
>
>> For various schemaRDD functions like select, where, orderby, groupby etc.
>> I would like to create expression objects and pass these to the methods for
>> execution.
>>
>> Can someone show some examples of how to create expressions for case
>> class and execute ? E.g., how to create expressions for select, order by,
>> group by etc. and execute methods using the expressions ?
>>
>> Regards,
>>
>
>

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