9. Managing Transactions
A database transaction is a grouping of SQL statements that make a logical data change to the database.
Cursor.execute() executes a SQL statement, a transaction is
started or continued. By default, python-oracledb does not commit this transaction
to the database. The methods
Connection.rollback() methods can be used to explicitly commit
or rollback a transaction:
cursor.execute("INSERT INTO mytab (name) VALUES ('John')") connection.commit()
When a database connection is closed, such as with
or when variables referencing the connection go out of scope, any uncommitted
transaction will be rolled back.
An alternative way to commit is to set the attribute
autocommit of the connection to
True. This ensures all
DML statements (INSERT, UPDATE, and so on) are committed as they are
Connection.commit(), this does not require an
additional round-trip to the database so it is more
efficient when used appropriately.
Note that irrespective of the autocommit value, Oracle Database will always commit an open transaction when a DDL statement is executed.
When executing multiple DML statements that constitute a single transaction, it is recommended to use autocommit mode only for the last DML statement in the sequence of operations. Unnecessarily committing causes extra database load, and can destroy transactional consistency.
The example below shows a new customer being added to the table
SALES table is updated with a purchase of 3000 pens from
the customer. The final insert uses autocommit mode to commit both new
# Add a new customer id_var = cursor.var(int) connection.autocommit = False # make sure any previous value is off cursor.execute(""" INSERT INTO cust_table (name) VALUES ('John') RETURNING id INTO :bvid""", bvid=id_var) # Add sales data for the new customer and commit all new values id_val = id_var.getvalue() connection.autocommit = True cursor.execute("INSERT INTO sales_table VALUES (:bvid, 'pens', 3000)", bvid=id_val)
9.2. Explicit Transactions
Connection.begin() can be used to explicitly start a local
or global transaction.
Without parameters, this explicitly begins a local transaction; otherwise, this explicitly begins a distributed (global) transaction with the given parameters. See the Oracle documentation for more details.