private Set<Symbol> extractUniqueSymbols(Expression expression) {
    return uniqueSymbolsCache.computeIfAbsent(
        expression, e -> ImmutableSet.copyOf(extractAllSymbols(expression)));
  }
Returns a canonical expression that is fully contained by the symbolScope and that is
equivalent to the specified expression. Returns null if unable to find a canonical.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/planner/EqualityInference.java