@Override
    protected Scope visitShowDB(ShowDB node, Optional<Scope> context) {
      throw new SemanticException("Show Database statement is not supported yet.");
    }
Visitor context represents local query scope (if exists). The invariant is that the local query
scopes hierarchy should always have outer query scope (if provided) as ancestor.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/analyzer/StatementAnalyzer.java