[FLINK-12295][table] Fix comments in MinAggFunction and MaxAggFunction

This closes apache#15940
derekm · May 18, 2021 · 057f86b · 057f86b
1 parent 4d33e85
commit 057f86b
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Showing 2 changed files with 7 additions and 6 deletions.
diff --git a/...k/src/main/java/org/apache/flink/table/planner/functions/aggfunctions/MaxAggFunction.java b/...k/src/main/java/org/apache/flink/table/planner/functions/aggfunctions/MaxAggFunction.java
@@ -72,9 +72,12 @@ public Expression[] accumulateExpressions() {
 
  @Override
  public Expression[] retractExpressions() {
- // TODO FLINK-12295, ignore exception now
- // throw new TableException("This function does not support retraction, Please choose
- // MaxWithRetractAggFunction.");
+ // See optimization in FlinkRelMdModifiedMonotonicity.
+ // This function can ignore retraction message:
+ // SQL: SELECT MAX(cnt), SUM(cnt) FROM (SELECT count(a) as cnt FROM T GROUP BY b)
+ // The cnt is modified increasing, so the MAX(cnt) can ignore retraction message. But this
+ // doesn't mean that the node won't receive the retraction message, because there are other
+ // aggregate operators that need retraction message, such as SUM(cnt).
  return new Expression[0];
  }
 

diff --git a/...k/src/main/java/org/apache/flink/table/planner/functions/aggfunctions/MinAggFunction.java b/...k/src/main/java/org/apache/flink/table/planner/functions/aggfunctions/MinAggFunction.java
@@ -72,9 +72,7 @@ public Expression[] accumulateExpressions() {
 
  @Override
  public Expression[] retractExpressions() {
- // TODO FLINK-12295, ignore exception now
- // throw new TableException("This function does not support retraction, Please choose
- // MinWithRetractAggFunction.");
+ // See MaxAggFunction.retractExpressions
  return new Expression[0];
  }