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?
The DB-API spec requires that connecting to the database begins a new transaction, by default. 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.
Others have explained why a commit is not necessary on a SELECT statement. I just wanted to point out you could utilize the autocommit property of the Connection object to avoid having to manually execute commit yourself:
import cx_Oracle
with cx_Oracle.connect(usr, pwd, url) as conn:
conn.autocommit = True
cursor = conn.cursor()
cursor.execute("UPDATE SO SET STATUS='PE' WHERE ID='100'")
cursor.close()
This is especially useful when you have multiple INSERT, UPDATE, and DELETE statements within the same connection.
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
Related
Sometimes I have a need to execute a query from psycopg2 that is not in a transaction block.
For example:
cursor.execute('create index concurrently on my_table (some_column)')
Doesn't work:
InternalError: CREATE INDEX CONCURRENTLY cannot run inside a transaction block
I don't see any easy way to do this with psycopg2. What am I missing?
I can probably call os.system('psql -c "create index concurrently"') or something similar to get it to run from my python code, however it would be much nicer to be able to do it inside python and not rely on psql to actually be in the container.
Yes, I have to use the concurrently option for this particular use case.
Another time I've explored this and not found an obvious answer is when I have a set of sql commands that I'd like to call with a single execute(), where the first one briefly locks a resource. When I do this, that resource will remain locked for the entire duration of the execute() rather than for just when the first statement in the sql string was running because they all run together in one big happy transaction.
In that case I could break the query up into a series of execute() statements - each became its own transaction, which was ok.
It seems like there should be a way, but I seem to be missing it. Hopefully this is an easy answer for someone.
EDIT: Add code sample:
#!/usr/bin/env python3.10
import psycopg2 as pg2
# -- set the standard psql environment variables to specify which database this should connect to.
# We have to set these to 'None' explicitly to get psycopg2 to use the env variables
connDetails = {'database': None, 'host': None, 'port': None, 'user': None, 'password': None}
with (pg2.connect(**connDetails) as conn, conn.cursor() as curs):
conn.set_session(autocommit=True)
curs.execute("""
create index concurrently if not exists my_new_index on my_table (my_column);
""")
Throws:
psycopg2.errors.ActiveSqlTransaction: CREATE INDEX CONCURRENTLY cannot run inside a transaction block
Per psycopg2 documentation:
It is possible to set the connection in autocommit mode: this way all the commands executed will be immediately committed and no rollback is possible. A few commands (e.g. CREATE DATABASE, VACUUM, CALL on stored procedures using transaction control…) require to be run outside any transaction: in order to be able to run these commands from Psycopg, the connection must be in autocommit mode: you can use the autocommit property.
Hence on the connection:
conn.set_session(autocommit=True)
Further resources from psycopg2 documentation:
transactions-control
connection.autocommit
Context
So I am trying to figure out how to properly override the auto-transaction when using SQLite in Python. When I try and run
cursor.execute("BEGIN;")
.....an assortment of insert statements...
cursor.execute("END;")
I get the following error:
OperationalError: cannot commit - no transaction is active
Which I understand is because SQLite in Python automatically opens a transaction on each modifying statement, which in this case is an INSERT.
Question:
I am trying to speed my insertion by doing one transaction per several thousand records.
How can I overcome the automatic opening of transactions?
As #CL. said you have to set isolation level to None. Code example:
s = sqlite3.connect("./data.db")
s.isolation_level = None
try:
c = s.cursor()
c.execute("begin")
...
c.execute("commit")
except:
c.execute("rollback")
The documentaton says:
You can control which kind of BEGIN statements sqlite3 implicitly executes (or none at all) via the isolation_level parameter to the connect() call, or via the isolation_level property of connections.
If you want autocommit mode, then set isolation_level to None.
i need some help with python an mysql.
I have the following code, which is executing in infinite loop:
db = MySQLdb.connect("127.0.0.1","user","password","dbname" )
while True:
cursor = db.cursor()
cursor.execute("SELECT * FROM requests WHERE status <> 'Finished'")
all_pending_requests = cursor.fetchall()
cursor.close()
And that works fine the first time i run it. But when i go to a tool like mysql workbench or i type it myself in in terminal, i update some rows and set their status to something that is not "Finished". So by doing that the next time the loop executes i should get those rows as a result but i get nothing. Do you guys now why this is happening maybe?
Thanks for help.
I am not certain but would assume that you are using InnoDB storage engine in MySQL and MySQLdb version >=1.2.0. You need to commit before the changes are being reflected. As of version 1.2.0, MySQLdb disables auto-commit by default. Confirmation of the same is here. Try adding db.commit() as the last line in the loop.
I'm trying to use an SQLite insert operation in a python script, it works when I execute it manually on the command line but when I try to access it on the web it won't insert it in the database. Here is my function:
def insertdb(unique_id,number_of_days):
conn = sqlite3.connect('database.db')
print "Opened database successfully";
conn.execute("INSERT INTO IDENT (ID_NUM,DAYS_LEFT) VALUES (?,?)",(unique_id,number_of_days));
conn.commit()
print "Records created successfully";
conn.close()
When it is executed on the web, it only shows the output "Opened database successfully" but does not seem to insert the value into the database. What am I missing? Is this a server configuration issue? I have checked the database permissions on writing and they are correctly set.
The problem is almost certainly that you're trying to create or open a database named database.db in whatever happens to be the current working directory, and one of the following is true:
The database exists and you don't have permission to write to it. So, everything works until you try to do something that requires write access (like commiting an INSERT).
The database exists, and you have permission to write to it, but you don't have permission to create new files in the directory. So, everything works until sqlite needs to create a temporary file (which it almost always will for execute-ing an INSERT).
Meanwhile, you don't mention what web server/container/etc. you're using, but apparently you have it configured to just swallow all errors silently, which is a really, really bad idea for any debugging. Configure it to report the errors in some way. Otherwise, you will never figure out what's going on with anything that goes wrong.
If you don't have control over the server configuration, you can at least wrap all your code in a try/except and manually log exceptions to some file you have write access to (ideally via the logging module, or just open and write if worst comes to worst).
Or, you can just do that with dumb print statements, as you're already doing:
def insertdb(unique_id,number_of_days):
conn = sqlite3.connect('database.db')
print "Opened database successfully";
try:
conn.execute("INSERT INTO IDENT (ID_NUM,DAYS_LEFT) VALUES (?,?)",(unique_id,number_of_days));
conn.commit()
print "Records created successfully";
except Exception as e:
print e # or, better, traceback.print_exc()
conn.close()
For the life of me I can't figure out why the below module won't add new rows to my DB. I can add them using the command line interface. I can also add them by using other means (ie. writing commands to a script file and using os.system('...'), but if I use cursor.execute(), no rows are added (even though the table is created). Here is a minimal script for your viewing pleasure. Note that I am getting no errors or warnings when I run this script
#!/usr/bin/env python
import MySQLdb
if __name__ == '__main__':
db = MySQLdb.connect ( host="localhost", user="user", passwd="passwd", db="db" )
cursor = db.cursor()
cursor.execute (
"""
CREATE TABLE IF NOT EXISTS god_i_really_hate_this_stupid_library
(
id INT NOT NULL auto_increment,
username VARCHAR(32) NOT NULL UNIQUE,
PRIMARY KEY(id)
) engine=innodb;
"""
)
cursor.execute (
"""
INSERT INTO god_i_really_hate_this_stupid_library
( username )
VALUES
( 'Booberry' );
"""
)
cursor.close()
you need to call commit on your connection, otherwise all the changes made will be rolled back automatically.
From the FAQ of MySQLdb:
Starting with 1.2.0, MySQLdb disables autocommit by default, as required by the DB-API standard (PEP-249). If you are using InnoDB tables or some other type of transactional table type, you'll need to do connection.commit() before closing the connection, or else none of your changes will be written to the database.
Conversely, you can also use connection.rollback() to throw away any changes you've made since the last commit.
Important note: Some SQL statements -- specifically DDL statements like CREATE TABLE -- are non-transactional, so they can't be rolled back, and they cause pending transactions to commit.
You can call db.autocommit(True) to turn autocommit on for the connection or just call db.commit() manually whenever you deem it necessary.