What methods can be used to ensure transaction isolation in PostgreSQL database?

In PostgreSQL, transaction isolation is ensured through isolation levels, which define how data modifications are visible across different transactions. The main methods:

1. Transaction Isolation Levels:

   • Read Uncommitted — in PostgreSQL, it behaves like Read Committed.
   • Read Committed — by default, a transaction sees only data committed before the start of the current query.
   • Repeatable Read — the transaction sees data as it was at the start of the transaction, preventing non-repeatable reads.
   • Serializable — the strictest level, providing full isolation as if transactions were executed sequentially.

2. Use of Locks:

   • PostgreSQL uses MVCC (Multi-Version Concurrency Control), which minimizes locking, but explicit locks (e.g., FOR UPDATE) can be used when necessary.

3. Setting Isolation Level in SQL:

BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
-- or
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;

4. In Python with psycopg2:

import psycopg2
conn = psycopg2.connect(...)
conn.set_isolation_level(psycopg2.extensions.ISOLATION_LEVEL_SERIALIZABLE)
cur = conn.cursor()
cur.execute('BEGIN')
# operations
conn.commit()

Thus, isolation is achieved by choosing the appropriate transaction isolation level and using locking mechanisms built into PostgreSQL.