Re: Serialization questions

Before understanding how postgres implements the serializable isolation level (I have see many paper related to it), I have question about how it should be. 

I mainly read the ideas from https://www.postgresql.org/docs/11/transaction-iso.html. 

In fact, this isolation level works exactly the same as Repeatable Read except that it monitors for conditions which could make execution of a concurrent set of serializable transactions behave in a manner inconsistent with all possible serial (one at a time) executions of those transactions.  

in repeatable read,  every statement will use the transaction start timestamp,  so is it in serializable isolation level? 

When relying on Serializable transactions to prevent anomalies, it is important that any data read from a permanent user table not be considered valid until the transaction which read it has successfully committed. This is true even for read-only transactions ...

What does the "not be considered valid" mean?  and if it is a read-only transaction (assume T1),  I think it is ok to let other transaction do anything with the read set of T1, since it is invisible to T1(use the transaction start time as statement timestamp). 

Thanks