What are the levels of transaction isolation?

Transaction isolation levels in SQL determine how much a transaction can see uncommitted changes made by other transactions. They are designed to prevent concurrency issues: dirty reads, non-repeatable reads, and phantom reads.

There are four standard isolation levels:

1. READ UNCOMMITTED
   • The lowest level.
   • A transaction can see uncommitted changes of other transactions (allows dirty reads).
   • Does not prevent non-repeatable and phantom reads.
2. READ COMMITTED
   • Prevents dirty reads. A transaction only sees changes that were committed before or during the query.
   • Allows non-repeatable and phantom reads.
3. REPEATABLE READ
   • Prevents dirty and non-repeatable reads. If a transaction reads a row multiple times, it always sees the same value unless it itself has changed it.
   • Allows phantom reads (other transactions can insert new rows visible in range queries).
4. SERIALIZABLE
   • Eliminates all concurrency issues: dirty reads, non-repeatable reads, and phantom reads.
   • Transactions execute as if they were run sequentially, one after another.
   • The highest level but also the slowest, as it requires more locks.

Comparison table of issues prevented at each level:

Isolation Level	Dirty Reads	Non-Repeatable Reads	Phantom Reads
READ UNCOMMITTED	No	No	No
READ COMMITTED	Yes	No	No
REPEATABLE READ	Yes	Yes	No
SERIALIZABLE	Yes	Yes	Yes

Choosing an isolation level depends on the application's requirements for data consistency and performance. Higher levels provide greater integrity but may reduce concurrency. In Python, when working with databases (e.g., via sqlite3, PostgreSQL's psycopg2, MySQL's mysql.connector), the isolation level can usually be set through the connection or cursor methods.

Example of setting the isolation level in sqlite3:

import sqlite3
conn = sqlite3.connect('mydatabase.db', isolation_level='REPEATABLE READ')
# You can also set None (equivalent to READ UNCOMMITTED), 'DEFERRED', 'IMMEDIATE', 'EXCLUSIVE'

cursor = conn.cursor()

# Further database operations...

conn.commit()
conn.close()

Example of setting the isolation level in psycopg2:

import psycopg2
from psycopg2.extensions import ISOLATION_LEVEL_READ_COMMITTED, ISOLATION_LEVEL_SERIALIZABLE

conn = psycopg2.connect(database="mydatabase", user="myuser", password="mypassword", host="localhost")

conn.set_isolation_level(ISOLATION_LEVEL_SERIALIZABLE)
# You can also use ISOLATION_LEVEL_READ_UNCOMMITTED, ISOLATION_LEVEL_READ_COMMITTED, ISOLATION_LEVEL_REPEATABLE_READ, ISOLATION_LEVEL_SERIALIZABLE

cursor = conn.cursor()

conn.commit()
conn.close()

Standard isolation levels provide a balance between consistency and performance. In practice, READ COMMITTED and SERIALIZABLE are most commonly used.