SAVEPOINT — define a new savepoint within the current transaction
SAVEPOINT savepoint_name
   SAVEPOINT establishes a new savepoint within
   the current transaction.
  
A savepoint is a special mark inside a transaction that allows all commands that are executed after it was established to be rolled back, restoring the transaction state to what it was at the time of the savepoint.
savepoint_nameThe name to give to the new savepoint. If savepoints with the same name already exist, they will be inaccessible until newer identically-named savepoints are released.
   Use ROLLBACK TO to
   rollback to a savepoint.  Use RELEASE SAVEPOINT
   to destroy a savepoint, keeping
   the effects of commands executed after it was established.
  
Savepoints can only be established when inside a transaction block. There can be multiple savepoints defined within a transaction.
To establish a savepoint and later undo the effects of all commands executed after it was established:
BEGIN;
    INSERT INTO table1 VALUES (1);
    SAVEPOINT my_savepoint;
    INSERT INTO table1 VALUES (2);
    ROLLBACK TO SAVEPOINT my_savepoint;
    INSERT INTO table1 VALUES (3);
COMMIT;
The above transaction will insert the values 1 and 3, but not 2.
To establish and later destroy a savepoint:
BEGIN;
    INSERT INTO table1 VALUES (3);
    SAVEPOINT my_savepoint;
    INSERT INTO table1 VALUES (4);
    RELEASE SAVEPOINT my_savepoint;
COMMIT;
The above transaction will insert both 3 and 4.
To use a single savepoint name:
BEGIN;
    INSERT INTO table1 VALUES (1);
    SAVEPOINT my_savepoint;
    INSERT INTO table1 VALUES (2);
    SAVEPOINT my_savepoint;
    INSERT INTO table1 VALUES (3);
    -- rollback to the second savepoint
    ROLLBACK TO SAVEPOINT my_savepoint;
    SELECT * FROM table1;               -- shows rows 1 and 2
    -- release the second savepoint
    RELEASE SAVEPOINT my_savepoint;
    -- rollback to the first savepoint
    ROLLBACK TO SAVEPOINT my_savepoint;
    SELECT * FROM table1;               -- shows only row 1
COMMIT;
The above transaction shows row 3 being rolled back first, then row 2.
   SQL requires a savepoint to be destroyed automatically when another
   savepoint with the same name is established.  In
   PostgreSQL, the old savepoint is kept, though only the more
   recent one will be used when rolling back or releasing.  (Releasing the
   newer savepoint with RELEASE SAVEPOINT will cause the older one
   to again become accessible to ROLLBACK TO SAVEPOINT and
   RELEASE SAVEPOINT.) Otherwise, SAVEPOINT is
   fully SQL conforming.