On Sun, Apr 29, 2012 at 4:32 PM, Maxim Potekhin <email@example.com> wrote:
Looking at your example,as I think you understand, you forgo indexes
combining two conditions in one query, thinking along the lines of
often done in RDBMS. A scan is expected in this case, and there is
magic to avoid it.
This sounds like a mis-understanding of how RDBMSs work. If you combine two conditions in a single SQL query, the SQL execution optimizer looks at the cardinality of any indicies. If it can successfully predict that one of the conditions significantly reduces the set of rows that would be considered (such as a status match having 200 hits vs 1M rows in the table), then it selects this index for the first-iteration, and each index hit causes a record lookup which is then tested for the other conditions. (This is one of several query-execution types RDBMS systems use)
I'm no Cassandra expert, so I don't know what it does WRT index-selection, but from the page written on secondary indicies, it seems like if you just query on status, and do the other filtering yourself it'll probably do what you want...
However, if this query is important, you can easily index on two
using a composite type (look it up), or string concatenation for
This is not necessarily a good idea. Creating a composite index explodes the index size unnecessarily. If a condition can reduce a query to 200 records, there is no need to have a composite index including another condition.