cx_Oracle: How do I iterate over a result set?
PythonSqlDatabaseOracleCx OraclePython Problem Overview
There are several ways to iterate over a result set. What are the tradeoff of each?
Python Solutions
Solution 1 - Python
The canonical way is to use the built-in cursor iterator.
curs.execute('select * from people')
for row in curs:
print row
You can use fetchall()
to get all rows at once.
for row in curs.fetchall():
print row
It can be convenient to use this to create a Python list containing the values returned:
curs.execute('select first_name from people')
names = [row[0] for row in curs.fetchall()]
This can be useful for smaller result sets, but can have bad side effects if the result set is large.
-
You have to wait for the entire result set to be returned to your client process.
-
You may eat up a lot of memory in your client to hold the built-up list.
-
It may take a while for Python to construct and deconstruct the list which you are going to immediately discard anyways.
If you know there's a single row being returned in the result set you can call fetchone()
to get the single row.
curs.execute('select max(x) from t')
maxValue = curs.fetchone()[0]
Finally, you can loop over the result set fetching one row at a time. In general, there's no particular advantage in doing this over using the iterator.
row = curs.fetchone()
while row:
print row
row = curs.fetchone()
Solution 2 - Python
My preferred way is the cursor iterator, but setting first the arraysize property of the cursor.
curs.execute('select * from people')
curs.arraysize = 256
for row in curs:
print row
In this example, cx_Oracle will fetch rows from Oracle 256 rows at a time, reducing the number of network round trips that need to be performed
Solution 3 - Python
There's also the way psyco-pg
seems to do it... From what I gather, it seems to create dictionary-like row-proxies to map key lookup into the memory block returned by the query. In that case, fetching the whole answer and working with a similar proxy-factory over the rows seems like useful idea. Come to think of it though, it feels more like Lua than Python.
Also, this should be applicable to all PEP-249 DBAPI2.0 interfaces, not just Oracle, or did you mean just fastest using Oracle?