Develop Reference

Sequence nextval/currval in two sessions

Setup:
Oracle DB running on a windows machine
Mac connected with the database, both in the same network
Problem:
When I created a sequence in SQL Developer, I can see and use the sequence in this session. If I logoff and login again the sequence is still there. But if I try to use the sequence via Python and cx_Oracle, it doesn't work. It also doesn't work the other way around.
[In SQL Developer: user: uc]
create SEQUENCE seq1;
select seq1.nextval from dual; ---> 1
commit; --> although the create statement is a DDL method, just in case
[login via Python, user: uc]
select seq1.currval from dual;--> ORA-08002 Sequence seq1.currval isn't defined in this session
The python code:
import cx_Oracle
cx_Oracle.init_oracle_client(lib_dir="/Users/benreisinger/Documents/testclients/instantclient_19_8", config_dir=None, error_url=None, driver_name=None)
# Connect as user "hr" with password "hr" to the "orclpdb" service running on a remote computer.
connection = cx_Oracle.connect("uc", "uc", "10.0.0.22/orcl")
cursor = connection.cursor()
cursor.execute("""
select seq1.currval from dual
""")
print(cursor)
for seq1 in cursor:
print(seq1)
The error says, that [seq1] wasn't defined in this session, but why does the following work:
select seq1.nextval from dual
--> returns 2
Even after issuing this, I can't use seq1.currval
Btw., select sequence_name from user_sequences returns seq1in Python
[as SYS user]
select * from v$session
where username = 'uc';
--> returns zero rows
Why is seq1 not in reach for the python program ?
Note: With tables, everything just works fine
EDIT:
also with 'UC' being upper case, no rows returned
first issuing
still doesn't work

Not sure how to explain this. The previous 2 answers are correct, but somehow you seem to miss the point.
First, take everything that is irrelevant out of the equation. Mac client on Windows db: doesn't matter. SQLDeveloper vs python: doesn't matter. The only thing that matters is that you connect twice to the database as the same schema. You connect twice, that means that you have 2 separate sessions and those sessions don't know about each other. Both sessions have access to the same database objects, so you if you execute ddl (eg create sequence), that object will be visible in the other session.
Now to the core of your question. The oracle documentation states
"To use or refer to the current sequence value of your session, reference seq_name.CURRVAL. CURRVAL can only be used if seq_name.NEXTVAL has been referenced in the current user session (in the current or a previous transaction)."
You have 2 different sessions, so according to the documentation, you should not be able to call seq_name.CURRVAL in the other session. That is exactly the behaviour you are seeing.
You ask "Why is seq1 not in reach for the python program ?". The answer is: you're not correct, it is in reach for the python program. You can call seq1.NEXTVAL from any session. But you cannot invoke seq1.NEXTVAL from one session (SQLDeveloper) and then invoke seq1.CURRVAL from another session (python) because that is just how sequences works as stated in documentation.
Just to confirm you're not in the same session, execute the following statement for both clients (SQLDeveloper and python):
select sys_context('USERENV','SID') from dual;
You'll notice that the session id is different.

CURRVAL returns the last allocated sequence number in the current session. So it only works when we have previously executed a NEXTVAL. So these two statements will return the same value when run in the same session:
select seq1.nextval from dual
/
select seq1.currval from dual
/
It's not entirely clear what you're trying to achieve, but it looks like your python code is executing a single statement for the connection, so it's not tapping into an existing session.
This statement returns zero rows ...
select * from v$session
where username = 'uc';
... because database objects in Oracle are stored in UPPER case (at least by default, but it's wise to stick with that default. So use where username = 'UC' instead.

Python established a new session. In it, sequence hasn't been invoked yet, so its currval doesn't exist. First you have to select nextval (which, as you said, returned 2) - only then currval will make sense.
Saying that
Even after issuing this, I can't use seq1.currval
is hard to believe.
This: select * From v$session where username = 'uc' returned nothing because - by default - all objects are stored in uppercase, so you should have ran
.... where username = 'UC'
Finally:
commit; --> although the create statement is a DDL method, just in case
Which case? There's no case. DDL commits. Moreover, commits twice (before and after the actual DDL statement). And there's nothing to commit either. Therefore, what you did is unnecessary and pretty much useless.

sqlalchemy connection flask

I have the following code in flask
sql = text('select * from person')
results = self.db.engine.execute(sql)
for row in results:
print(".............", row) # prints nothing
people = Person.query.all() # shows all person data
Now given this situation, it's obvious, the self.db is not using the same connection somehow that Person.query is using. However, given this situation, can I get the connection somehow from Person.query object?
PS. This is for testing and I'm using SQLite3 database. I tried this in postgres, but outcome is the same.

Just figured out. Try Person.query.session.execute(sql). Voila!

Mongodb: Shard a collection within my python code

I have a distributed MongoDB 3.2 database. The system is mounted on two nodes with RedHat operating system.
Using python and the PyMongo driver (or some other), I want enable the sharding of a collection, specifying the compound shard key.
In the mongo shell this works:
> use mongotest
> sh.enableSharding("mongotest")
> db.signals.createIndex({ valueX: 1, valueY: 1 }, { unique: true })
> sh.shardCollection("mongotest.signals", { valueX: 1, valueY: 1 })
('mongotest' is the DB, and 'signals' is the collection)
The last two lines I want to make them within my code. Does anyone know if this is possible in python? If so, how is it done?
thank you very much,
sorry for my bad English

A direct translation of your shell commands to python is as shown below.
from pymongo import MongoClient
client = MongoClient()
db = client.admin # run commands against admin database.
db.command('enableSharding', 'mongotest')
db.command({'shardCollection': 'mongotest.signals', 'key': {'valueX': 1, 'valueY': 1}})
However, you may want to confirm that both enableSharding and shardCollection are exposed in your db by running
db.command('listCommands')

Connecting to a SQL db using python

I currently have a python script that can connect to a mySQL db and execute queries. I wish to modify it so that I can connect to a different SQL db to run a separate query. I am having trouble doing this, running osx 10.11. This is my first question and I'm a newbie programmer so please take it easy on me...
Here is the program i used to for mySQL
sf_username = "user"
sf_password = "pass"
sf_api_token = "token"
sf_update = beatbox.PythonClient()
password = str("%s%s" % (sf_password, sf_api_token))
sf_update.login(sf_username, password)
t = Terminal()
hub = [stuff]
def FINAL_RUN():
cnx = alternate_modify(hub)
cur = cnx.cursor()
queryinput = """
SQL QUERY I WANT
"""
cur.execute(queryinput)
rez = cur.fetchone()
while rez is not None:
write_to_sf(rez)
rez = cur.fetchone()
FINAL_RUN()

You can use a Python library called SQLAlchemy. It abstracts away the "low-level" operations you would do with a database (e.g. specifying queries manually).
A tutorial for using SQLAlchemy can be found here.

I was able to get connected using SQL Alchemy--thank you. If anyone else tries, I think you'll need a ODBC driver per this page:
http://docs.sqlalchemy.org/en/latest/dialects/mssql.html
Alternatively, pymssql is a nice tool. If you run into trouble installing like I did, there is a neat workaround here:
mac - pip install pymssql error
Thanks again

MySQL driver issues with INFORMATION_SCHEMA?

I'm trying out the Concurrence framework for Stackless Python. It includes a MySQL driver and when running some code that previously ran fine with MySQLdb it fails.
What I am doing:
Connecting to the MySQL database using dbapi with username/password/port/database.
Executing SELECT * FROM INFORMATION_SCHEMA.COLUMNS
This fails with message:
Table 'mydatabase.columns' doesn't exist
"mydatabase" is the database I specified in step 1.
When doing the same query in the MySQL console after issuing "USE mydatabase", it works perfectly.
Checking the network communication yields something like this:
>>>myusername
>>>scrambled password
>>>mydatabase
>>>CMD 3 SET AUTOCOMMIT = 0
<<<0
>>>CMD 3 SELECT * FROM INFORMATION_SCHEMA.COLUMNS
<<<255
<<<Table 'mydatabase.columns' doesn't exist
Is this a driver issue (since it works in MySQLdb)? Or am I not supposed to be able to query INFORMATION_SCHEMA this way?
If I send a specific "USE INFORMATION_SCHEMA" before trying to query it, I get the expected result. But, I do not want to have to sprinkle my code all over with "USE" queries.

It definitely looks like a driver issue. Maybe the python driver don't support the DB prefix.
Just to be sure, try the other way around: first use INFORMATION_SCHEMA and then SELECT * FROM mydatabase.sometable

I finally found the reason.
The driver just echoed the server capability flags back in the protocol handshake, with the exception of compression:
## concurrence/database/mysql/client.py ##
client_caps = server_caps
#always turn off compression
client_caps &= ~CAPS.COMPRESS
As the server has the capability...
CLIENT_NO_SCHEMA 16 /* Don't allow database.table.column */
...that was echoed back to the server, telling it not to allow that syntax.
Adding client_caps &= ~CAPS.NO_SCHEMA did the trick.