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https://issues.apache.org/jira/browse/DRILL-5235?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15848701#comment-15848701
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Zelaine Fong commented on DRILL-5235:
-------------------------------------
If I do:
explain plan for select columns[0] col1 from `employee.json` order by col1;
this is what I get:
{code}
00-00 Screen
00-01 Project(col1=[$0])
00-02 SelectionVectorRemover
00-03 Sort(sort0=[$0], dir0=[ASC])
00-04 Project(col1=[ITEM($0, 0)])
00-05 Scan(groupscan=[EasyGroupScan
[selectionRoot=classpath:/employee.json, numFiles=1, columns=[`columns`[0]],
files=[classpath:/employee.json]]])
{code}
I.e., there's only a single column in the project, and a single column in the
sort.
> Column alias doubles sort data size when reading a text file
> ------------------------------------------------------------
>
> Key: DRILL-5235
> URL: https://issues.apache.org/jira/browse/DRILL-5235
> Project: Apache Drill
> Issue Type: Improvement
> Affects Versions: 1.9.0
> Reporter: Paul Rogers
> Priority: Minor
>
> Consider a simple query that reads data from a pipe-separated-value file and
> sorts it. The file has just one column. The query looks something like this:
> {code}
> SELECT columns[0] col1 FROM `dfs.data`.`input-file.tbl` ORDER BY col1
> {code}
> Looking at the query plan, we see that a project operator not just creates an
> alias {{col1}} for {{column\[0]}}, it also makes a *copy*.
> The particular input file is 20 GB in size and contains just one column. As a
> result of materializing the alias, data size to the sort doubles to 40 GB.
> This results in doubling query run time. If the sort must spill to disk, run
> times increases by a much larger factor.
> The fix is to treat the alias as an alias, not a materialized copy.
> {code}
> {
> "graph" : [ {
> "pop" : "fs-scan",
> "columns" : [ "`columns`[0]" ],
> }, {
> "pop" : "project",
> "@id" : 4,
> "exprs" : [ {
> "ref" : "`col1`",
> "expr" : "`columns`[0]"
> } ],
> }, {
> "pop" : "external-sort",
> "orderings" : [ {
> "order" : "ASC",
> "expr" : "`col1`",
> "nullDirection" : "UNSPECIFIED"
> } ],
> }, {
> "pop" : "selection-vector-remover",
> }, {
> "pop" : "project",
> }, {
> "pop" : "screen",
> } ]
> }
> {code}
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