terrymanu commented on issue #30939:
URL:
https://github.com/apache/shardingsphere/issues/30939#issuecomment-3641614132
## Problem Understanding
- In 5.4.1 (JDBC) with check-table-metadata-enabled on, aggregating
multiple physical tables for a logic table throws a metadata consistency error
even though the physical table structures look identical. Master still
misjudges this scenario.
## Root Cause
- Consistency check still relies on TableMetaData toString/reference
comparison, but TableMetaData and its members have no value-based
equals/hashCode. When identical structures are loaded as different instances or
use different collection implementations/orders (e.g.,
WrappedSet), comparison returns false and triggers a false positive.
## Analysis
- Check entry: SchemaTableMetaDataAggregator.checkUniformed (master) uses
sample.toString().equals(each.toString()). Lacking value comparison means
multi-shard identical tables are deemed inconsistent.
- Config reference (default is off): check-table-metadata-enabled
(https://shardingsphere.apache.org/document/current/en/user-manual/common-config/props/#check-table-metadata-enabled).
With it enabled, the above comparison path runs and currently misfires.
- Usage note: keep the property off in daily use (default) and only enable
briefly during schema-change windows to reduce false positives.
## Conclusion
- This remains a real defect on master caused by missing content-based
comparison.
- Temporary workaround: keep check-table-metadata-enabled: false, or
enable only briefly during change windows.
- Fix suggestion: add value-semantics comparison for metadata models or
switch the check to field-by-field comparison instead of toString/reference.
Example approach:
```java
// Illustrative idea: use field-level comparison instead of
toString/reference
private boolean isSame(TableMetaData a, TableMetaData b) {
return a.getName().equals(b.getName())
&& columnsEqual(a.getColumns(), b.getColumns())
&& indexesEqual(a.getIndexes(), b.getIndexes())
&& constraintsEqual(a.getConstraints(), b.getConstraints())
&& a.getType() == b.getType();
}
```
- We warmly invite community contributors to submit a PR implementing
value-based comparison plus unit tests to ensure no false positives when the
check is enabled.
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