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https://issues.apache.org/jira/browse/ARROW-15726?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17494586#comment-17494586
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Neal Richardson commented on ARROW-15726:
-----------------------------------------
We pass filter and columns for predicate pushdown to create the scan node:
https://github.com/apache/arrow/blob/master/r/R/query-engine.R#L92
https://github.com/apache/arrow/blob/master/r/src/compute-exec.cpp#L142-L158
So what's not working correctly here?
> [R] Support push-down projection/filtering in datasets / dplyr
> --------------------------------------------------------------
>
> Key: ARROW-15726
> URL: https://issues.apache.org/jira/browse/ARROW-15726
> Project: Apache Arrow
> Issue Type: Improvement
> Components: R
> Reporter: Weston Pace
> Priority: Major
>
> The following query should read a single column from the target parquet file.
> {noformat}
> open_dataset("lineitem.parquet") %>% select(l_tax) %>% filter(l_tax < 0.01)
> %>% collect()
> {noformat}
> Furthermore, it should apply a pushdown filter to the source node allowing
> parquet row groups to potentially filter out target data.
> At the moment it creates the following exec plan:
> {noformat}
> 3:SinkNode{}
> 2:ProjectNode{projection=[l_tax]}
> 1:FilterNode{filter=(l_tax < 0.01)}
> 0:SourceNode{}
> {noformat}
> There is no projection or filter in the source node. As a result we end up
> reading much more data from disk (the entire file) than we need to (at most a
> single column).
> This _could_ be fixed via heuristics in the dplyr code. However, it may
> quickly get complex (for example, the project comes after the filter, so you
> need to make sure you push down a projection that includes both the columns
> accessed by the filter and the columns accessed by the projection OR can you
> push down the projection through a join [yes you can], how do you know which
> columns to apply to which source node).
> A more complete fix would be to call into some kind of 3rd party optimizer
> (e.g. calcite) after the plan has been created by dplyr but before it is
> passed to the execution engine.
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