Database Reference
In-Depth Information
Apart from the bloom filter for row keys, there exists one bloom filter for each row in the
SSTable. This secondary bloom filter is created to detect whether the requested column
names exist in the SSTable. Now, Cassandra will take SSTables one by one from younger
to older, and use the index file to locate the offset for each column value for that row key
and the bloom filter associated with the row (built on the column name). On the bloom fil-
ter being positive for the requested column, it looks into the SSTable file to read the
column value. Note that we may have a column value in other yet-to-be-read SSTables,
but that does not matter, because we are reading the most recent SSTables first, and any
value that was written earlier to it does not matter. So, the value gets returned as soon as
the first column in the most recent SSTable is allocated.
