GitHub user ooq opened a pull request:
https://github.com/apache/spark/pull/14337
[SPARK-16699][SQL] Fix performance bug in hash aggregate on long string keys
## What changes were proposed in this pull request?
In the following code in `VectorizedHashMapGenerator.scala`:
```
def hashBytes(b: String): String = {
val hash = ctx.freshName("hash")
s"""
|int $result = 0;
|for (int i = 0; i < $b.length; i++) {
| ${genComputeHash(ctx, s"$b[i]", ByteType, hash)}
| $result = ($result ^ (0x9e3779b9)) + $hash + ($result << 6) +
($result >>> 2);
|}
""".stripMargin
}
```
when b=input.getBytes(), the current 2.0 code results in getBytes() being
called n times, n being length of input. getBytes() involves memory copy is
thus expensive and causes a performance degradation.
Fix is to evaluate getBytes() before the for loop.
## How was this patch tested?
Performance bug, no additional test added.
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/ooq/spark SPARK-16699
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/spark/pull/14337.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 #14337
----
commit 9102d50c12bf12ea4c404c71743457d1406a8428
Author: Qifan Pu <[email protected]>
Date: 2016-07-24T22:38:48Z
Fix SPARK-16699
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