terrymanu commented on issue #33292:
URL:
https://github.com/apache/shardingsphere/issues/33292#issuecomment-3640525784
## Issue Understanding
- Sharding-Proxy 5.5.0 + MySQL 5.7, sharding algorithm ds_${ monthtime >=
'2024-10' ? 2:0 }. A plain SELECT ... WHERE monthtime='2024-10' routes to ds_2,
but adding EXPLAIN returns ds_0. You want the proxy to show the correct shard.
## Root Cause
- In the proxy, EXPLAIN is parsed as a DAL statement and routed via
unicast (no sharding condition evaluation), typically choosing the first or
current connection’s data source. This is by design and not a sharding-aware
preview.
## Analysis
- To inspect sharding routing and rewritten SQL at the proxy layer, use
DistSQL PREVIEW <SQL> instead of EXPLAIN. Official doc:
docs/document/content/user-manual/shardingsphere-proxy/distsql/syntax/rul/preview-sql.en.md.
- PREVIEW returns data_source_name and actual_sql, so you can see ds_2 and
the rewritten SQL. For the physical execution plan, run EXPLAIN directly on the
target shard.
- The master branch still treats EXPLAIN as DAL unicast; no change there
yet.
## Conclusion
- This is expected behavior, not a fixed bug. Use DistSQL PREVIEW SELECT
... to view routing in the proxy, and run EXPLAIN on the target shard to see
the execution plan.
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