[Tutor] Re: memory problem (II part)
dyoo at hkn.eecs.berkeley.edu
Mon Jan 19 17:19:00 EST 2004
> I am perplexed, what product are you using and how heavily are you
> pounding on it?
> Why are you using a cursor, which is a known memory hog?
We should clarify: the DB API specifies that we access the database
through a Python Cursor:
So if we follow the API, there's no way we can get around "cursors".
I think, though, that we're just confusing multiple related uses of the
> I was forbidden using cursors (as were the others in our group), because
> it impacts performance so extremely under Sybase. Oracle seems to have
> relied upon cursors much longer than competing products due to its tie
> in with the user interface (and packaged tools). However, finally they
Your use of "cursor" here may be different from the "cursor" that the DB
API is talking about.
As a concrete example, Pysqlite actually doesn't support real database
cursors --- they are simulated by storing all the results in a temporary
list, and the "cursor" object there is just iterating across that
Ah! Ok, I think I'm understanding what's happening. Pysqlite's
connections themselves hold onto cursors in a list, so that when a
connection closes, all its associated cursors can be collected.
But as a result, large queries may end up in memory while we're holding
onto the connection, since those cursors may not necessarily be freed till
I see that the pysqlite code is using weak references to keep track of all
the cursors associated with a connection:
so the cursors should get reclaimed eventually when the garbage collector
Guillermo notes that:
> The only thing that makes the RAM go down is to close the database. I'll
> try to live with that (closing and opening the database for each
Instead of closing the connections, what happens if you call each cursor's
close() method after you do a fetch? Doing a close() on the connection
may be working because it calls close() on all open cursors. Let's verify
that its the cursors that are holding onto database results in memory.
Hope this helps!
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