GitHub user liancheng opened a pull request:
https://github.com/apache/spark/pull/12088
[SPARK-14295][SPARK-14274][SQL] Implements buildReader() for LibSVM
## What changes were proposed in this pull request?
This PR implements `FileFormat.buildReader()` for the LibSVM data source.
Besides that, a new interface method `prepareRead()` is added to `FileFormat`:
```scala
def prepareRead(
sqlContext: SQLContext,
options: Map[String, String],
files: Seq[FileStatus]): Map[String, String] = options
```
After migrating from `buildInternalScan()` to `buildReader()`, we lost the
opportunity to collect necessary global information, since `buildReader()`
works in a per-partition manner. For example, LibSVM needs to infer the total
number of features if the `numFeatures` data source option is not set. Any
necessary collected global information should be returned using the data source
options map. By default, this method just returns the original options
untouched.
An alternative approach is to combine absorb `inferSchema()` into
`prepareRead()`, since schema inference is also some kind of global information
gathering. However, this approach wasn't chosen because schema inference is
optional, while `prepareRead()` must be called whenever a `HadoopFsRelation`
based data source relation is instantiated.
## How was this patch tested?
Tested using existing test suites.
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/liancheng/spark
spark-14295-libsvm-build-reader
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/spark/pull/12088.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 #12088
----
commit 64c3efba2bb76f461466ba95109b48428d5f3698
Author: Cheng Lian <[email protected]>
Date: 2016-03-31T14:19:41Z
Implements buildReader() for LibSVM
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