SQL expressions?
On Wed, May 30, 2018 at 11:09 AM Jacek Laskowski <ja...@japila.pl> wrote:
> Hi,
>
> I've been exploring RuntimeReplaceable expressions [1] and have been
> wondering what their purpose is.
>
> Quoting the scaladoc [2]:
>
> > An expression that gets replaced at runtime (currently by the optimizer)
> into a different expression for evaluation. This is mainly used to provide
> compatibility with other databases.
>
> For example, ParseToTimestamp expression is a RuntimeReplaceable
> expression and it is replaced by Cast(left, TimestampType)
> or Cast(UnixTimestamp(left, format), TimestampType) per to_timestamp
> function (there are two variants).
>
> My question is why is this RuntimeReplaceable better than simply using the
> Casts as the implementation of to_timestamp functions?
>
> def to_timestamp(s: Column, fmt: String): Column = withExpr {
> // pseudocode
> Cast(UnixTimestamp(left, format), TimestampType)
> }
>
> What's wrong with the above implementation compared to the current one?
>
> [1]
> https://github.com/apache/spark/blob/master/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala#L275
>
> [2]
> https://github.com/apache/spark/blob/master/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala#L266-L267
>
> Pozdrawiam,
> Jacek Laskowski
> ----
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