On Tue, Jun 26, 2012 at 4:19 PM, Aquil H. Abdullah <aquil.abdul...@gmail.com
> wrote:
> Hello All,
>
> In my newbist state, I called createIndex on two columns in one of my
> tables:
>
> import tables
> table_desc = {'timestamp':tables.Time32Col(),
> 'symbol':tables.StringCol(8), 'observation':tables.Float32Col()}
> h5f = tables.openFile('test.h5',mode='w')
> group = h5f.createGroup('/','data')
> table = h5f.createTable(group, 'test',table_desc,'Test Table')
> table.cols.timestamp.createIndex()
> table.cols.symbol.createIndex()
> …
>
> Now from what I've been able to find on the internet an index is only
> associated with one column:
>
> class tables.Index
> Represents the index of a column in a table.
>
> This class is used to keep the indexing information for columns in a
> Table dataset (see The Table class). It is actually the descendant of the
> Group class (see The Group class), with some added functionality. An
> Index is always associated with one and only one column in a table.
>
> - PyTables 2.3.1 User's Guide - Library Reference/The Index Class
> http://pytables.github.com/usersguide/libref.html#indexclassdescr
> - Efficient way to verify that records are unique in Python/PyTables
> http://stackoverflow.com/questions/1315129/efficient-way-to-verify-that-records-are-unique-in-python-pytables
> - Hints For SQL Users (Creating an index)
> http://www.pytables.org/moin/HintsForSQLUsers#Creatinganindex
>
> So how does PyTables interpret a table with multiple column indices? The
> best solution that I've found is creating a hash from the two fields that I
> am interested in indexing and then indexing that table on that hash.
>
> The other solution would be to shard my data by symbol and then index each
> symbol table by timestamp.
>
> Can anyone explain what effect two index columns has on Pytables?
> Also, can anyone tell me if they've come up with a better solution for
> dealing with tables that require multiple indices than any that I've
> mentioned?
>
I don't have a lot of time right now, but maybe create a nested column or a
column with a compound data type that is just a tuple of the two data types
you are interested in. Then index against the super column. Storing a
hash in another column is probably not the greatest way to do this...
Hopefully someone else can jump in and answer this one.
>
> Regards,
>
> --
> Aquil H. Abdullah
>
>
>
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