Eric Ridge <eebbrr(at)gmail(dot)com> writes:
> http://www.postgresql.org/docs/8.4/static/transaction-iso.html says:
> "Also note that two successive SELECT commands can see different data,
> even though they are within a single transaction, if other
> transactions commit changes during execution of the first SELECT."
> I get that what that means in normal cases, but what about a single
> query comprised of one or more unions:
> SELECT ... FROM foo WHERE ...
> UNION
> SELECT ... FROM foo WHERE ...
That's just one SELECT command. Sub-SELECTs inside a query don't
count as separate commands for this purpose; the use of SELECT in
that way is just an artifact of the SQL grammar.
regards, tom lane