Dane,
> So the mental model I've built based on the first four sentences of
> the first paragraph is that when a transaction starts in read
> committed mode a snapshot is taken of the (database) universe as it
> exists at the moment of its creation and that it's only updated by
> changes made by the transaction that created the snapshot.
This is (almost) true for the REPEATABLE READ or SERIALIZABLE modes.
(However, the snapshot is taken as the first non-transaction-control
statement in the transaction starts.)
For "READ COMMITTED", the docs read (emphasis added):
"In effect, a SELECT query sees a snapshot of the database as of the
instant the *query* begins to run. However, SELECT does see the effects
of previous updates executed within its own transaction, even though
they are not yet committed."
(
http://www.postgresql.org/docs/9.5/static/transaction-iso.html#XACT-READ-COMMITTED
)
So if you re-run the same query multiple times within a single
transaction, you might get different results depending.
best regards,
-hannes
