fudianchn commented on PR #39613:
URL: https://github.com/apache/shardingsphere/pull/39613#issuecomment-5619777656
The coexistence semantics is defined and implemented in fec19243.
Decision: the expanded-projection view is used only when the count mismatch
is fully explained by the derived columns the rewrite appends, i.e. after
removing those derived columns (matched by the DerivedColumn patterns,
case-insensitively for lowercase-folding backends) the returned metadata still
names the expanded projections. A derived alias alone no longer proves the
absence of drift.
When drift and appended derived columns occur together, the returned
metadata is authoritative for the client-facing columns and the derived columns
are hidden from it: the newly added user column is delivered with its own label
and index, the derived alias stays invisible. The decision and the
hidden-column view are shared by JDBC ResultSetMetaData, the proxy query
headers and COM_STMT_PREPARE, which all go through
ShardingSphereResultSetMetaData / ShardingSphereResultSetUtils.
Tests: coexistence cases for the label map, the metadata and the prepare
resolver, all red on the previous head (the label map returned {col1=1, col2=2}
instead of {col1=1, added_col=2, col2=3}); a rename case where the non-derived
counts match but the names differ, still detected as drift; a guard that a
derived-looking genuine column stays visible when the statement has no derived
projections. jdbc 1237, proxy-backend-core 583, proxy-frontend-mysql 200,
spotless and checkstyle pass.
The alternative of rejecting this state and requiring a metadata refresh is
not implemented: it would turn a currently silently-wrong result into a query
failure, which looked like a larger behavior decision. Happy to rework in that
direction if you prefer it.
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