terrymanu commented on issue #37276:
URL:
https://github.com/apache/shardingsphere/issues/37276#issuecomment-3615020429
## Root Cause
MySQL no-FROM queries are routed to the admin branch
(MySQLAdminExecutorCreator → UnicastResourceShowExecutor). This executor builds
a QueryHeader with 3 columns, but the row data collected from the backend only
has 2 columns. The mismatch
causes DatabaseAdminQueryBackendHandler to read a third column and throw
an out-of-bounds exception.
## Analysis
- Trigger: the session has no selected database (no schema in
handshake/connection) and executes a no-FROM multi-expression query.
- In the admin branch, the column metadata shows 3 columns, but the row
data list has only 2 elements; AbstractMemoryQueryResult.getValue accessing the
3rd column overflows.
- If the connection specifies a logic database or runs USE <db> first, it
follows the normal query path where columns and data align, so the issue does
not appear.
- Official usage guidance: ShardingSphere-Proxy docs
https://shardingsphere.apache.org/document/current/en/user-manual/shardingsphere-proxy/
recommend specifying a logic database on connect to avoid the admin branch for
ordinary queries.
## Conclusion (Fix Suggestion)
This is an admin-branch defect in 5.5.2. Fix by aligning column count and
row data in UnicastResourceShowExecutor, or by skipping the admin branch for
normal multi-expression no-FROM selects.
Example fix (5.5.2 path
proxy/backend/type/mysql/src/main/java/org/apache/shardingsphere/proxy/backend/mysql/handler/admin/executor/UnicastResourceShowExecutor.java):
private QueryResult createQueryResult() throws SQLException {
List<MemoryQueryResultDataRow> rows = new LinkedList<>();
int columnCount = getQueryResultMetaData().getColumnCount();
while (databaseConnector.next()) {
List<Object> data = databaseConnector.getRowData().getData();
List<Object> aligned = new ArrayList<>(columnCount);
aligned.addAll(data.size() >= columnCount ? data.subList(0,
columnCount) : data);
while (aligned.size() < columnCount) {
aligned.add(null); // choose null or empty string per protocol
expectation
}
rows.add(new MemoryQueryResultDataRow(aligned));
}
return new RawMemoryQueryResult(getQueryResultMetaData(), rows);
}
Alternative (more conservative): in MySQLAdminExecutorCreator, for “no
FROM + multi-expression” that is not a special function/system variable, return
Optional.empty() so it goes through the normal query pipeline instead of the
admin executor.
Community contributors are warmly invited to submit a PR to implement and
test this fix—thanks for the report!
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