Hi Anthony,
Thanks for your response. I suspect you are right that sets are not regular
enough. Are all column types of fixed size? Is this a necessary condition for
pytables? I though I’d seen an example of variable-length strings with
pytables, but I maybe I was mistaken.
I was thinking of solving the problem by creating a new table the set-valued
column and inserting values (along with row IDs) into it. I could then do a
join with the main table to retrieve the rows I want. Although pytables can’t
do table joins naturally, I could use the join method described in “Hints for
SQL Users”. Though I’m not using indices, this may be fast enough.
Thanks again,
-Tom
On Jan 30, 2011, at 12:35 AM, Anthony Scopatz wrote:
> Hey Tom,
>
> I am not sure what you are asking is possible. I don't think sets are regular
> enough* container type. Perhaps if you had a sets of consistent (or maximal)
> length and primitive data type, this kind of thing would be possible. (Eg
> every set in your column only contained integers and has less than or equal
> to 10 elements.) However, this is functionally equivalent to an array**.
>
> If you need the data in set form why not use something like the following:
>
> for row in table:
> s = set(row['col_name'])
>
> Additionally, you could store a set as an attribute on a Node
> (http://www.pytables.org/docs/manual/ch04.html#AttributeSetClassDescr) or
> perhaps you could try storing set definitions or pickled sets in a series of
> filenodes (http://www.pytables.org/docs/manual/ch06.html). It should be
> noted that either of these methods will likely be slower than the array/set
> idea.
>
> Be Well
> Anthony
>
> *ie you can't memory map into a set the same way that you can an array.
> Elements of a set can be anywhere in memory. Elements of an array are
> guaranteed to be available sequentially.
>
> **You could probably hack an array column that reads out into a set
> automatically, rather than a numpy array. However, it would still be stored
> under the covers as an array, and thus subject to all the array-constraints.
>
> On Sun, Jan 30, 2011 at 7:20 AM, Tom Fawcett <tom.fawc...@gmail.com> wrote:
> Hi. I recently discovered pytables, and I’m impressed with its speed and
> flexibility. But there’s one feature I’d like and I can’t figure out how
> whether pytables can do it.
>
> I’d like to have essentially a set-valued column in a table. I’d like a
> column data type that is very similar to a Python set. I’m not sure how to
> achieve this naturally with pytables – it looks like none of the Cols
> subclasses can be variable length. Any ideas?
>
> Thanks,
> -Tom
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