What causes "beforeFirst" in apache/shardingsphere?

Buffer the rows once (List) and do multiple passes in memory instead of calling beforeFirst().

apache/shardingsphere · warning · SQLFeatureNotSupportedException

beforeFirst

Error message

beforeFirst

What it means

beforeFirst() on a federation result set throws SQLFeatureNotSupportedException: federation results are forward-only streams over the Calcite execution, and AbstractUnsupportedOperationSQLFederationResultSet explicitly rejects repositioning methods. Calling beforeFirst() to restart iteration is not possible on this result set.

Source

Thrown at kernel/sql-federation/core/src/main/java/org/apache/shardingsphere/sqlfederation/resultset/AbstractUnsupportedOperationSQLFederationResultSet.java:60

    
    @Override
    public boolean isAfterLast() throws SQLException {
        throw new SQLFeatureNotSupportedException("isAfterLast");
    }
    
    @Override
    public boolean isFirst() throws SQLException {
        throw new SQLFeatureNotSupportedException("isFirst");
    }
    
    @Override
    public boolean isLast() throws SQLException {
        throw new SQLFeatureNotSupportedException("isLast");
    }
    
    @Override
    public void beforeFirst() throws SQLException {
        throw new SQLFeatureNotSupportedException("beforeFirst");
    }
    
    @Override
    public void afterLast() throws SQLException {
        throw new SQLFeatureNotSupportedException("afterLast");
    }
    
    @Override
    public boolean first() throws SQLException {
        throw new SQLFeatureNotSupportedException("first");
    }
    
    @Override
    public boolean last() throws SQLException {
        throw new SQLFeatureNotSupportedException("last");
    }
    
    @Override

View on GitHub (pinned to e952770a21)

Solutions

Buffer the rows once (List) and do multiple passes in memory instead of calling beforeFirst().
Use a scroll-insensitive result set at the application level by wrapping the fetched data, not the live ResultSet.
Disable SQL federation for this statement/rule so a driver result set supporting beforeFirst() is returned (subject to driver TYPE settings).

Example fix

// before
processMetadata(rs);
rs.beforeFirst();
processData(rs);
// after
List<Row> rows = fetchAll(rs);
processMetadata(rows);
processData(rows);

Defensive patterns

Strategy: fallback

Validate before calling

if (rs.getType() == ResultSet.TYPE_FORWARD_ONLY) {
    rows = fetchAll(rs); // single forward pass; re-iterate the list instead of beforeFirst()
}

Try / catch

try {
    rs.beforeFirst();
} catch (final SQLFeatureNotSupportedException ex) {
    // re-execute the query or use the buffered copy taken during the first pass
}

Prevention

Buffer federated results in a List for multi-pass consumers.
Avoid beforeFirst()/afterLast()/first() on ShardingSphere result sets.
Set reporting tools to streaming single-pass mode where possible.

When it happens

Trigger: Application/ORM code calls rs.beforeFirst() to re-iterate a result set returned by a federated query through ShardingSphere (e.g. some report engines call beforeFirst after a metadata pass).

Common situations: Two-pass processing (metadata then data) in reporting tools; ORMs emulating scroll-insensitive result sets; generic JDBC utility code that resets cursors; works fine against plain MySQL driver, fails when federation kicks in.

Related errors

AI-assisted analysis of apache/shardingsphere@e952770a21 (2026-08-14). Data as JSON: /api/errors/6e32225229aaab9d. Report an issue: GitHub.