I am wondering how's column slicing in CQL WHERE clause affects read performance. Does Cassandra have some optimization, which is able to only fetch specific columns with the value or have to retrieve all the columns of a row and check one after another? e.g.: I have a primary key as (key1, key2), key2 is the clustering key. I only want to find columns that match a certain key2, say value2?
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Cassandra saves the data as cells - each value for a key+column is a cell. If you save several values for the key in once they will be placed together in same file. Also, since cassandra writes to sstables, you can have several values saved for same key-column/cell in different files, and cassandra will read all of them and return you the last written one, until comperssion or repair is occured, and irrelevant values are deleted.
Good article about deletes/reads/tombstones: http://thelastpickle.com/blog/2016/07/27/about-deletes-and-tombstones.html
