GitHub user HyukjinKwon opened a pull request:
https://github.com/apache/spark/pull/14660
[SPARK-17071][SQL] Fetch Parquet schema without another Spark job when it
is a single file to touch
## What changes were proposed in this pull request?
It seems Spark executes another job to figure out schema always
([ParquetFileFormat#L739-L778](https://github.com/apache/spark/blob/abff92bfdc7d4c9d2308794f0350561fe0ceb4dd/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetFileFormat.scala#L739-L778)).
However, it seems it's a bit of overhead to touch only a single file. I ran
a bench mark with the code below:
```scala
test("Benchmark for JSON writer") {
withTempPath { path =>
Seq((1, 2D, 3L, "4")).toDF("a", "b", "c", "d")
.write.format("parquet").save(path.getAbsolutePath)
val benchmark = new Benchmark("Parquet - read schema", 1)
benchmark.addCase("Parquet - read schema", 10) { _ =>
spark.read.format("parquet").load(path.getCanonicalPath).schema
}
benchmark.run()
}
}
```
with the results as below:
- **Before**
```scala
Parquet - read schema: Best/Avg Time(ms) Rate(M/s)
Per Row(ns) Relative
------------------------------------------------------------------------------------------------
Parquet - read schema 47 / 49 0.0
46728419.0 1.0X
```
- **After**
```scala
Parquet - read schema: Best/Avg Time(ms) Rate(M/s)
Per Row(ns) Relative
------------------------------------------------------------------------------------------------
Parquet - read schema 2 / 3 0.0
1811673.0 1.0X
```
It seems it became 20X faster (although It is a small bit in total job
run-time).
As a reference, it seems ORC is doing this within driver-side
[OrcFileOperator.scala#L74-L83](https://github.com/apache/spark/blob/a95252823e09939b654dd425db38dadc4100bc87/sql/hive/src/main/scala/org/apache/spark/sql/hive/orc/OrcFileOperator.scala#L74-L83).
## How was this patch tested?
Existing tests should cover this
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/HyukjinKwon/spark SPARK-17071
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/spark/pull/14660.patch
To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:
This closes #14660
----
commit 614abbc6b7a03ff0d3e505697c0bbfec3b330c2b
Author: hyukjinkwon <gurwls...@gmail.com>
Date: 2016-08-16T05:42:29Z
Fetch Parquet schema within driver-side when there is single file to touch
without another Spark job
commit e1214d50035441fb96551683cf38ae3e49f07b7d
Author: hyukjinkwon <gurwls...@gmail.com>
Date: 2016-08-16T05:46:12Z
Fix modifier
----
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