Paul Rogers created DRILL-5235:
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Summary: 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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