Question

I’m using Cassandra 1.1.2 I’m trying to convert a RDBMS application to Cassandra. In my RDBMS application I have following table called table1:

| Col1 | Col2 | Col3 | Col4 |

1. Col1: String (primary key)
2. Col2: String (primary key)
3. Col3: Bigint (index)
4. Col4: Bigint

This table counts over 200 million records. Mostly used query is something like:

Select * from table where col3 < 100 and col3 > 50;

In Cassandra I used following statement to create the table:

create table table1 (primary_key varchar, col1 varchar, 
col2 varchar, col3 bigint, col4 bigint, primary key (primary_key));

create index on table1(col3);

I changed the primary key to an extra column (I calculate the key inside my application).
After importing a few records I tried to execute following cql:

select * from table1 where col3 < 100 and col3 > 50;

This result is:

Bad Request: No indexed columns present in by-columns clause with Equal operator

The Query select col1,col2,col3,col4 from table1 where col3 = 67 works

Google said there is no way to execute that kind of queries. Is that right? Any advice how to create such a query?

Answer 1

Cassandra indexes don’t actually support sequential access; see http://www.datastax.com/docs/1.1/ddl/indexes for a good quick explanation of where they are useful. But don’t despair; the more classical way of using Cassandra (and many other NoSQL systems) is to denormalize, denormalize, denormalize.

It may be a good idea in your case to use the classic bucket-range pattern, which lets you use the recommended RandomPartitioner and keep your rows well distributed around your cluster, while still allowing sequential access to your values. The idea in this case is that you would make a second dynamic columnfamily mapping (bucketed and ordered) col3 values back to the related primary_key values. As an example, if your col3 values range from 0 to 10^9 and are fairly evenly distributed, you might want to put them in 1000 buckets of range 10^6 each (the best level of granularity will depend on the sort of queries you need, the sort of data you have, query round-trip time, etc). Example schema for cql3:

CREATE TABLE indexotron (
    rangestart int,
    col3val int,
    table1key varchar,
    PRIMARY KEY (rangestart, col3val, table1key)
);

When inserting into table1, you should insert a corresponding row in indexotron, with rangestart = int(col3val / 1000000). Then when you need to enumerate all rows in table1 with col3 > X, you need to query up to 1000 buckets of indexotron, but all the col3vals within will be sorted. Example query to find all table1.primary_key values for which table1.col3 < 4021:

SELECT * FROM indexotron WHERE rangestart = 0 ORDER BY col3val;
SELECT * FROM indexotron WHERE rangestart = 1000 ORDER BY col3val;
SELECT * FROM indexotron WHERE rangestart = 2000 ORDER BY col3val;
SELECT * FROM indexotron WHERE rangestart = 3000 ORDER BY col3val;
SELECT * FROM indexotron WHERE rangestart = 4000 AND col3val < 4021 ORDER BY col3val;