begin transaction;

\set autocommit off

\\set autocommit off

psql db1
psql (9.5.3)
Type "help" for help.

db1=# create table xxx(x int);
CREATE TABLE
db1=# select * from xxx;
 x 
---
(0 rows)

db1=# rollback;
WARNING:  there is no transaction in progress
ROLLBACK
db1=# select * from xxx;
 x 
---
(0 rows)

db1=# \q
MacBook-Air-Nick:p6 skelet$ cat ~/.psqlrc 
\set autocommit off
MacBook-Air-Nick:p6 skelet$ vim ~/.psqlrc 
MacBook-Air-Nick:p6 skelet$ 
MacBook-Air-Nick:p6 skelet$ psql db1
psql:/Users/skelet/.psqlrc:1: invalid command \
psql (9.5.3)
Type "help" for help.

db1=# 

alter system set transactional_mode=ON; 

autocommit off   

begin transaction;
select... 
insert ...
update ...
select ... 
commit; 
-- и вот тут пересылка изменений инстансу? 

begin transaction;
select... 
insert ...
update ...
select ... 
commit; 
-- и вот тут пересылка изменений инстансу? 

db1=# select * from t1;
 x  
----
  1
 -1
(2 rows)

db1=# select * from t1 where x>0 and sqrt(x) <1;
 x 
---
(0 rows)

db1=# select * from t1 where x>0 and sqrt(x) <2 ;
 x 
---
 1
(1 row)

db1=# select * from t1 where sqrt(x) <2 and x>0;
 x 
---
 1
(1 row)

user_tab_columns

( x > 0 ) and ( sqrt(x) < 2 )

int x=some_function();
if (x>0 && sqrt(x) < 2.0 ) { 
   ....
}