Read Committed isolation level

Create a table:

α=> CREATE TABLE t(n integer);

CREATE TABLE

α=> INSERT INTO t VALUES (42);

INSERT 0 1

Query the table from the first transaction:

α=> BEGIN;

BEGIN

α=> SELECT * FROM t;

 n  
----
 42
(1 row)

Delete the row in the second transaction and commit the change:

α=> DELETE FROM t;

DELETE 1

Query the table:

α=> SELECT * FROM t;

 n 
---
(0 rows)

The first transaction sees the committed changes.

α=> COMMIT;

COMMIT

Repeatable Read isolation level

Return the row back to the table:

α=> INSERT INTO t VALUES (42);

INSERT 0 1

Query the table from the first transaction:

α=> BEGIN ISOLATION LEVEL REPEATABLE READ;

BEGIN

α=> SELECT * FROM t;

 n  
----
 42
(1 row)

Delete the row in the second transaction and commit the change:

α=> DELETE FROM t;

DELETE 1

Query the table:

α=> SELECT * FROM t;

 n  
----
 42
(1 row)

On this level the first transaction doesn't see the committed changes.

α=> COMMIT;

COMMIT