hequn8128 commented on a change in pull request #9886:
[FLINK-14027][python][doc] Add documentation for Python user-defined functions.
URL: https://github.com/apache/flink/pull/9886#discussion_r334367792
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File path: docs/dev/table/udfs.md
##########
@@ -74,9 +74,29 @@ myTable.select("string, string.hashCode(),
hashCode(string)");
// use the function in SQL API
tableEnv.sqlQuery("SELECT string, hashCode(string) FROM MyTable");
{% endhighlight %}
+
+By default the result type of an evaluation method is determined by Flink's
type extraction facilities. This is sufficient for basic types or simple POJOs
but might be wrong for more complex, custom, or composite types. In these cases
`TypeInformation` of the result type can be manually defined by overriding
`ScalarFunction#getResultType()`.
+
+The following example shows an advanced example which takes the internal
timestamp representation and also returns the internal timestamp representation
as a long value. By overriding `ScalarFunction#getResultType()` we define that
the returned long value should be interpreted as a `Types.TIMESTAMP` by the
code generation.
+
+{% highlight java %}
+public static class TimestampModifier extends ScalarFunction {
+ public long eval(long t) {
+ return t % 1000;
+ }
+
+ public TypeInformation<?> getResultType(Class<?>[] signature) {
+ return Types.SQL_TIMESTAMP;
+ }
+}
+{% endhighlight %}
</div>
<div data-lang="scala" markdown="1">
+In order to define a scalar function, one has to extend the base class
`ScalarFunction` in `org.apache.flink.table.functions` and implement (one or
more) evaluation methods. The behavior of a scalar function is determined by
the evaluation method. An evaluation method must be declared publicly and named
`eval`. The parameter types and return type of the evaluation method also
determine the parameter and return types of the scalar function. Evaluation
methods can also be overloaded by implementing multiple methods named `eval`.
Evaluation methods can also support variable arguments, such as `eval(String...
strs)`.
Review comment:
such as \`eval(String... strs)\` => such as \`eval(str: String*)\` with
@varargs annotation.
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