why do I have to commit explicitly when doing an UPDATE

Question

Here's my code:

import cx_Oracle

conn = cx_Oracle.connect(usr, pwd, url)
cursor = conn.cursor()
cursor.execute("UPDATE SO SET STATUS='PE' WHERE ID='100'")
conn.commit()

If I remove the conn.commit(), the table isn't updated. But for select statements, I don't need that conn.commit(). I'm curious why?

Answer 1

commit is used to tell the database to save all the changes in the current transaction.

Select does not change any data so there is nothing to save and thus nothing to commit

See wikipedia for transactions

Answer 2

The DB-API spec requires that by default connecting to the database begins a new transaction. You must commit to confirm any changes you make, or rollback to discard them.

Note that if the database supports an auto-commit feature, this must be initially off.

Pure SELECT statements, since they never make any changes to the database, don't have to have their changes committed.