public Set<RewriteAlgorithm> getSupportedRewriteAlgorithms() {
    return supportedRewriteAlgorithms;
  }
Returns an error message if this plan can not be a definition of a Materialized view which is an input of
Calcite based materialized view query rewrite.
Null or empty string otherwise.
@return String contains error message or null.
ql/src/java/org/apache/hadoop/hive/ql/parse/CBOPlan.java