String concatenation - which is the fastest way ?

przemolicc at poczta.fm przemolicc at poczta.fm
Wed Aug 10 15:31:46 CEST 2011


On Wed, Aug 10, 2011 at 01:32:06PM +0100, Chris Angelico wrote:
> On Wed, Aug 10, 2011 at 12:17 PM,  <przemolicc at poczta.fm> wrote:
> > Hello,
> >
> > I'd like to write a python (2.6/2.7) script which connects to database, fetches
> > hundreds of thousands of rows, concat them (basically: create XML)
> > and then put the result into another table. Do I have any choice
> > regarding string concatenation in Python from the performance point of view ?
> > Since the number of rows is big I'd like to use the fastest possible library
> > (if there is any choice). Can you recommend me something ?
> 
> First off, I have no idea why you would want to create an XML dump of
> hundreds of thousands of rows, only to store it in another table.
> However, if that is your intention, list joining is about as efficient
> as you're going to get in Python:
> 
> lst=["asdf","qwer","zxcv"] # feel free to add 399,997 more list entries
> xml="<foo>"+"</foo><foo>".join(lst)+"</foo>"
> 
> This sets xml to '<foo>asdf</foo><foo>qwer</foo><foo>zxcv</foo>' which
> may or may not be what you're after.

Chris,

since this process (XML building) is running now inside database (using native SQL commands)
and is one-thread task it is quite slow. What I wanted to do is to spawn several python subprocesses in parallel which
will concat subset of the whole table (and then merge all of them at the end).
Basically:
- fetch all rows from the database (up to 1 million): what is recommended data type ?
- spawn X python processes each one:
    - concat its own subset
- merge the result from all the subprocesses

This task is running on a server which has many but slow cores and I am trying to divide this task
into many subtasks.

Regards
Przemyslaw Bak (przemol)


















































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