Обсуждение: successive select statements

In postgresql.org/docs/9.1/static/transaction-iso.html I read

13.2.1. Read Committed Isolation Level
. . . two successive SELECT commands can see different data, even though they are within a single transaction . . .

Please consider this code being executed by postgres:
= = = = = = = = = =
select some_int from tableX
union all
select another_int from tableY
;
= = = = = = = = = =
Are these two select statements considered "successive" when isolation == Read Committed ?
Or are they instead guaranteed to see the same snapshot of the database ?


Also, please consider this code being executed by postgres:
= = = = = = = = = =
CREATE Or Replace FUNCTION fx_one() RETURNS  setof integer AS
$$
select some_int from tableX;
$$ LANGUAGE 'sql';

CREATE Or Replace FUNCTION fx_two() RETURNS  setof integer AS
$$
select another_int from tableY;
$$ LANGUAGE 'sql';

select * from fx_one()
union all
select * from fx_two()
;
= = = = = = = = = =
Are the two select statements inside fx_one and fx_two considered "successive" when isolation == Read Committed ?
Or are they instead guaranteed to see the same snapshot of the database ?


Thanks,
-dvs-

<david.sahagian@emc.com> writes:
> In postgresql.org/docs/9.1/static/transaction-iso.html I read
> 13.2.1. Read Committed Isolation Level
> . . . two successive SELECT commands can see different data, even though they are within a single transaction . . .

> Please consider this code being executed by postgres:
> = = = = = = = = = =
> select some_int from tableX
> union all
> select another_int from tableY
> ;
> = = = = = = = = = =
> Are these two select statements considered "successive" when isolation == Read Committed ?

No, the UNION is a single command.  The fact that the word "SELECT"
occurs twice within it is a SQL syntactic artifact --- it doesn't
convert it into two commands.

            regards, tom lane