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https://issues.apache.org/jira/browse/ARROW-13926?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17413185#comment-17413185
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Ian Cook commented on ARROW-13926:
----------------------------------
FWIW, this is perhaps better construed as an implicit *window function* (i.e.
an OVER expression in SQL). When you do this type of operation with dbplyr, the
SQL it generates uses an OVER expression:
{code:java}
mtcars_db <- dbplyr::memdb_frame(mtcars)
mtcars_db %>%
group_by(cyl) %>%
transmute(x = hp - mean(hp)) %>%
show_query()
#> <SQL>
#> SELECT `hp` - AVG(`hp`) OVER (PARTITION BY `cyl`) AS `x`
#> FROM `dbplyr_002`{code}
> [R] Support mutate/summarize with implicit join
> -----------------------------------------------
>
> Key: ARROW-13926
> URL: https://issues.apache.org/jira/browse/ARROW-13926
> Project: Apache Arrow
> Issue Type: New Feature
> Components: R
> Reporter: Neal Richardson
> Assignee: Neal Richardson
> Priority: Major
> Fix For: 6.0.0
>
>
> {code}
> mtcars %>%
> group_by(cyl) %>%
> mutate(x = hp - mean(hp)
> {code}
> essentially means something like
> {code}
> mtcars %>%
> left_join(mtcars %>%
> group_by(cyl) %>%
> summarize(tmp = mean(hp))
> ) %>%
> mutate(x = hp - tmp) %>%
> select(-tmp)
> {code}
> Apparently you can do the same inside summarize() too (though IDK if that's
> behavior we want to encourage). Once we can do joins, we can support these
> queries.
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