Performance of list.index - how to speed up a silly algorithm?

Laszlo Nagy gandalf at shopzeus.com
Fri Apr 30 00:21:24 CEST 2010


I have some ten thousand rows in a table. E.g.

columns = ["color","size","weight","value"]
rows = [
     [ "Yellow", "Big", 2, 4 ],
     [ "Blue", "Big", 3, -4 ],
     [ "Blue", "Small", 10, 55 ],
     ...
]

Some columns are dimensions, others are measures. I want to convert this 
to an indexed version, that looks like this:

dimension_names = ["color","size"] # List of dimension names
dimension_cols = [0,1] # Column indexes for dimension values
dimension_values = { # Dimension value occurences for each dimension
     0: ["Yellow","Blue",....],
     1: ["Big","Small",...],
}
measure_names = ["weight","value"] # List of measure names
measure_cols = [2,3] # List of measure columns
facts = [ # Facts, containing tuples of (dimension_value_incides, 
measure_values )
     ( (0,0) , (2,4) ),
     ( (1,0) , (3,-4) ),
     ( (1,1) , (10,55) ),
     ...
]

This is how I try to convert to the indexed version:

#1. Iterate over all rows, and collect possible dimension values.

cnt = 0
for row in iterator_factory():
     cnt += 1
     for dimension_idx,col_idx in enumerate(dimension_cols):
         dimension_values[colidx].append(row[cold_idx])
         if cnt%10000:
             dimension_values[colidx] = list(set(dimension_values[colidx]))

#2. Index facts by dimension values

facts = []
for row in iterator_factory():
     dv = []
     for dimension_idx,col_idx in enumerate(dimension_cols):
         dv.append( dimension_values[col_idx].index(row[col_idx]) ) # 
THIS IS THE PROBLEMATIC LINE!
     mv = [ row[col_idx] for col_idx in measure_cols ]
     facts.append( dv,mv )

(In reality, rows and facts are not stored in memory, because there can 
be 100 000+ facts. I did not want to bore you with the full source code.)

And finally, here is my problem. If a dimension has many possible 
values, then the list.index() code above slows down eveything. In my 
test case, the problematic dimension had about 36 000 different values, 
and the number of rows was about 60 000. Calling index() on a list of 36 
000 values, times 60 000, is too slow.

Test performance was 262 rows/sec. If I use dv.append(0) instead of " 
dv.append( dimension_values[col_idx].index(row[col_idx]) ) " then it 
goes up to 11520 rows/sec. If I exclude the problematic dimension, and 
only leave the others (with 1000 or less values) then goes up to 3000 
rows/sec.

Maybe this should be implemented in C. But I believe that the algorithm 
itself must be wrong (regardless of the language). I really think that 
I'm doing something wrong. Looks like my algorithm's processing time is 
not linear to the number of rows. Not even log(n)*n. There should be a 
more effective way to do this. But how?

I had the idea of sorting the rows by a given dimension. Then it would 
be obvious to speed up the indexing part - for that dimension. PROBABLY 
sorting all rows would be faster than calling list.index() for each row. 
But... it does not seem very efficient either. What if I have 1 million 
rows and 10 dimensions? Do I sort 1 million rows on the disk 10 times? 
Some of you might have ran into the same problem before, and can tell me 
which is the most efficient way to do this.

Thanks,

    Laszlo




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