xuyangzhong commented on code in PR #27734:
URL: https://github.com/apache/flink/pull/27734#discussion_r2893529634
##########
flink-table/flink-table-planner/src/main/java/org/apache/flink/table/planner/plan/utils/DeltaJoinUtil.java:
##########
@@ -270,10 +279,543 @@ public static boolean isJoinTypeSupported(FlinkJoinType
flinkJoinType) {
}
/**
- * get the lookup key from the join keys.
+ * Try to build lookup chain for delta join to do lookup.
+ *
+ * <p>Take the follow join tree as example.
+ *
+ * <pre>{@code
+ * Top
+ * (Set1.a1 = Set2.c1 and Set1.b2 = Set2.d2)
Review Comment:
Considering that joins at different levels use different columns as join
keys, using the actual column names would be harder to understand. So here I
instead specify a simplified convention: each leaf table has columns named with
its lowercase letter plus a number, e.g., A(a0, a1), B(b0, b1, b2), C(c0, c1),
D(d0, d1, d2).
Do you think it would be easier to understand if I change it to the
following?
```
/**
* Try to build lookup chain for delta join to do lookup.
*
* <p>Take the following join tree as example. Each leaf table has
columns named with its
* lowercase letter and a number, e.g., A(a0, a1), B(b0, b1, b2), C(c0,
c1), D(d0, d1, d2).
*
* <pre>{@code
* Top
* (a1 = c1 and b2 = d2)
* / \
* Bottom1 Bottom2
* (a0 = b0) (c0 = d0)
* / \ / \
* A(a0,a1) B(b0,b1,b2) C(c0,c1) D(d0,d1,d2)
*
* }</pre>
*
* <p>If Bottom1 is treated as stream side and Bottom2 is treated as
lookup side, the lookup
* chain will be like this:
*
* <p>use A + B to lookup C with (a1 = c1) -> use C to lookup D with (c0
= d0).
*/
```
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