Jark Wu created FLINK-4469:
------------------------------
Summary: Add support for user defined table function in Table API
& SQL
Key: FLINK-4469
URL: https://issues.apache.org/jira/browse/FLINK-4469
Project: Flink
Issue Type: New Feature
Components: Table API & SQL
Reporter: Jark Wu
Assignee: Jark Wu
Normal user-defined functions, such as concat(), take in a single input row and
output a single output row. In contrast, table-generating functions transform a
single input row to multiple output rows. It is very useful in some cases, such
as look up in HBase by rowkey and return one or more rows.
Adding a user defined table function should:
1. inherit from UDTF class with specific generic type T
2. define one or more evel function.
NOTE:
1. the eval method must be public and non-static.
2. eval should always return java.lang.Iterable or scala.collection.Iterable
with the generic type T.
3. the generic type T is the row type returned by table function. Because of
Java type erasure, we can’t extract T from the Iterable.
4. eval method can be overload. Blink will choose the best match eval method to
call according to parameter types and number.
{code}
public class Word {
public String word;
public Integer length;
}
public class SplitStringUDTF extends UDTF<Word> {
public Iterable<Word> eval(String str) {
if (str == null) {
return new ArrayList<>();
} else {
List<Word> list = new ArrayList<>();
for (String s : str.split(",")) {
Word word = new Word(s, s.length());
list.add(word);
}
return list;
}
}
}
// in SQL
tableEnv.registerFunction("split", new SplitStringUDTF())
tableEnv.sql("SELECT a, b, t.* FROM MyTable CROSS APPLY split(c) AS t(w,l)")
// in Java Table API
tableEnv.registerFunction("split", new SplitStringUDTF())
// rename split table columns to “w” and “l”
table.crossApply("split(c)", "w, l")
.select("a, b, w, l")
// without renaming, we will use the origin field names in the POJO/case/...
table.crossApply("split(c)")
.select("a, b, word, length")
// in Scala Table API
val split = new SplitStringUDTF()
table.crossApply(split('c), 'w, 'l)
.select('a, 'b, 'w, 'l)
// outerApply for outer join to a UDTF
table.outerApply(split('c))
.select('a, 'b, 'word, 'length)
{code}
Here we introduce CROSS/OUTER APPLY keywords to join table functions , which is
used in SQL Server. We can discuss the API in the comment.
Maybe the {{UDTF}} class should be replaced by {{TableFunction}} or something
others, because we have introduced {{ScalarFunction}} for custom functions, we
need to keep consistent. Although, I prefer {{UDTF}} rather than
{{TableFunction}} as the former is more SQL-like and the latter maybe confused
with DataStream functions.
**This issue is blocked by CALCITE-1309, so we need to wait Calcite fix this
and release.**
See [1] for more information about UDTF design.
[1]
https://docs.google.com/document/d/15iVc1781dxYWm3loVQlESYvMAxEzbbuVFPZWBYuY1Ek/edit#
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