featzhang opened a new pull request, #27610:
URL: https://github.com/apache/flink/pull/27610
## What is the purpose of the change
This PR enhances the explain output of FlinkLogicalJoin to display join
conditions in a more readable and structured format. Instead of showing
low-level positional references like `=($0, $3)`, it now displays actual field
names in a clear format.
**Before:**
```
Join(condition=[=($0, $3)])
```
**After:**
```
Join:
Type: INNER
Condition: user_id = id
```
## Brief change log
- Add `explainTerms` override method in FlinkLogicalJoin class
- Implement `formatJoinCondition` method for recursive RexNode condition
parsing
- Implement `formatOperand` method for field reference resolution
- Support for EQUALS, AND, OR operators with proper field name resolution
- Use FlinkRexUtil.getExpressionString as fallback for complex expressions
## Verifying this change
This change is a display-only enhancement and can be verified by:
1. Build the flink-table-planner module:
```bash
./mvnw clean install -DskipTests -Pfast -pl
flink-table/flink-table-planner
```
2. Run EXPLAIN queries in Flink SQL Client with joins:
```sql
EXPLAIN SELECT * FROM orders o JOIN users u ON o.user_id = u.id;
```
3. Verify the join condition is displayed with field names instead of
positional references
## Does this pull request potentially affect one of the following parts:
- Dependencies (does it add or upgrade a dependency): **No**
- The public API, i.e., is any changed class annotated with
`@Public(Evolving)`: **No**
- The serializers: **No**
- The runtime per-record code paths (performance sensitive): **No**
- Anything that affects deployment or recovery: JobManager (and its
components), Checkpointing, Kubernetes/Yarn, ZooKeeper: **No**
- The S3 file system connector: **No**
## Documentation
- Does this pull request introduce a new feature? **No** (Enhancement only)
- If yes, how is the feature documented? **Not applicable**
This change only enhances the explain output format and does not modify any
RelNode structure, query optimization logic, or runtime behavior. It has zero
impact on performance and is 100% backward compatible.
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