Question

We have cassandra column family.
each row have multiple columns. columns have name, but value is empty.
if we have 5-10 row keys, how we can find column names that appear in all of these keys.
e.g.

row1: php, programming, accounting
row2: php, bookkeeping, accounting
row3: php, accounting

must return:

result: php, accounting

note we can not easily load whole row into the memory, because it may contain 1M+ columns
solution not need to be fast.

Answer 1

In order to do intersection of several rows, we will need to intersect two of them first, then to intersect the result with third and so on.

Looks like in cassandra we can query the data by column names and this is relatively fast operation.

So we first get Column Slice of 10k rows. Making list of column names (in PHP Cassa – put them in array). Then select those from second row.

Code may be looking like this:

$x = $cf->get($first_key, <some column slice>);

$column_names = array();
foreach(array_keys($x) as $k)
   $column_names[] = $k;

$result = $cf->get($second_key, $column_slice = null, $column_names);

// write result somewhere, and proceed with next slice