Github user gatorsmile commented on a diff in the pull request:
https://github.com/apache/spark/pull/12975#discussion_r62451866
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/command/functions.scala
---
@@ -157,6 +157,9 @@ case class DropFunction(
throw new AnalysisException(s"Specifying a database in DROP
TEMPORARY FUNCTION " +
s"is not allowed: '${databaseName.get}'")
}
+ if (FunctionRegistry.builtin.functionExists(functionName)) {
--- End diff --
Yeah, you can overwrite it by creating a temporary function, but you are
unable to drop it.
For example,
```
hive> drop function lower;
FAILED: SemanticException [Error 10301]: Cannot drop native function lower
hive> CREATE TEMPORARY FUNCTION lower AS
'org.apache.hadoop.hive.ql.udf.generic.GenericUDFUpper';
OK
Time taken: 0.005 seconds
hive> select lower('a');
OK
A
Time taken: 0.068 seconds, Fetched: 1 row(s)
hive> drop temporary function lower;
OK
Time taken: 0.01 seconds
hive> select lower('a');
OK
a
Time taken: 0.057 seconds, Fetched: 1 row(s)
hive> drop function lower;
FAILED: SemanticException [Error 10301]: Cannot drop native function lower
```
In this example, I overwrite the built-in function `lower` by creating the
same name temporary function by the implementation of `upper`. Obviously, when
I called `lower`, the actual logic is `upper`. That means, we can overwrite the
built-in functions. After I dropping the temporary function, the built-in
function becomes active again.
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