The experimental results show that in fact, here's the first such read refers to: first select statement to establish a consistent reading of the transaction snapshot on the same table or different tables of other update, delete, insert statements and establish consistency read snapshot does not matter. Data submitted after snapshot build, consistency can not read attending, previously submitted data can be read.
In fact, the starting point of a transaction is the first statement executed as a starting point, rather than begin a transaction as a starting point.
Open a point in time snapshot of all the catalogs in all the tables:
start transaction with consistent snapshot;