snuyanzin commented on code in PR #25707:
URL: https://github.com/apache/flink/pull/25707#discussion_r1861852208
##########
flink-table/flink-table-common/src/main/java/org/apache/flink/table/annotation/FunctionHint.java:
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@@ -175,13 +178,40 @@
ArgumentHint[] arguments() default {};
/**
- * Explicitly defines the intermediate result type that a function uses as
accumulator.
+ * Explicitly defines the intermediate result type (i.e. state entry) that
an aggregating
+ * function uses as its accumulator. The entry is managed by the framework
(usually via Flink's
+ * managed state).
*
* <p>By default, an explicit accumulator type is undefined and the
reflection-based extraction
* is used.
+ *
+ * <p>This parameter is primarily intended for aggregating functions (i.e.
{@link
+ * AggregateFunction} and {@link TableAggregateFunction}). It is
recommended to use {@link
+ * #state()} for {@link ProcessTableFunction}.
*/
DataTypeHint accumulator() default @DataTypeHint();
+ /**
+ * Explicitly lists the intermediate results (i.e. state entries) of a
function that is managed
+ * by the framework (i.e. Flink managed state). Including their names and
data types.
+ *
+ * <p>State hints are primarily intended for {@link ProcessTableFunction}.
A PTF supports
+ * multiple state entries at the beginning of an eval()/onTimer() method
(after an optional
+ * context parameter).
+ *
+ * <p>Aggregating functions (i.e. {@link AggregateFunction} and {@link
TableAggregateFunction})
+ * support a single state entry at the beginning of an
accumulate()/retract() method (i.e. the
+ * accumulator).
+ *
+ * <p>By default, explicit state is undefined and the reflection-based
extraction is used where
+ * {@link StateHint} is present.
+ *
+ * <p>Using both {@link #accumulator()} and this parameter is not allowed.
Specifying the list
+ * of state entries manually disables the entire reflection-based
extraction around {@link
+ * StateHint} and accumulators for aggregating functions.
Review Comment:
Should it also be mentioned in doc? Somewhere here e.g.
https://github.com/apache/flink/blob/b9c92371dba07b6bfe4368d7b6d7f7c575b4c603/docs/content/docs/dev/table/functions/udfs.md?plain=1#L439
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