July 2, 2009

Notes on columnar/TPC-H compression

I was chatting with Omer Trajman of Vertica, and he said that a 70% compression figure for ParAccel’s recent TPC-H filing sounded about right.*  When I noted that seemed kind of low, Omer pointed out that TPC-H data is pseudo-random, while real-life data has much more correlation among the values in different columns. E.g., in retail, a customer is likely to consistently shop at the same stores and to put similar items into his shopping basket).

*Omer was involved in Vertica’s TPC-H-data-based load speed benchmark, and is Vertica’s representative to the TPC.

But why does this matter? After all, Vertica compresses one column at a time (unlike, say, Clearpace).  Well, the reason is that Vertica — like other column stores — wants to store different columns in the same row order, for obvious benefits in both reading and writing.  So, for example, if all the rows that include Gotham City are grouped sequentially, then all the rows mentioning Bruce Wayne are likely to be near each other as well, while none of the rows that mention Clark Kent will be mixed in.

And when a set of consecutive entries has low cardinality, it’s easier to get high levels of compression.


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