core/src/main/java/com/orientechnologies/orient/core/sql/OCommandExecutorSQLSelect.java
public int getTemporaryRIDCounter(final OCommandContext iContext) {
    final OTemporaryRidGenerator parentQuery = (OTemporaryRidGenerator) iContext.getVariable("parentQuery");
    return parentQuery != null && parentQuery != this ?
        parentQuery.getTemporaryRIDCounter(iContext) :
        serialTempRID.getAndIncrement();
  }
public int getTemporaryRIDCounter(final OCommandContext iContext) {
    final OTemporaryRidGenerator parentQuery = (OTemporaryRidGenerator) iContext.getVariable("parentQuery");
    return parentQuery != null && parentQuery != this ?
        parentQuery.getTemporaryRIDCounter(iContext) :
        serialTempRID.getAndIncrement();
  }
Returns the temporary RID counter assuring it's unique per query tree.

@return Serial as integer