In almost 20 years of using Analysis Services I have never, ever used HOLAP storage. However the other week I was with a customer that was using ROLAP storage on top of Exasol and while query performance was generally good, I wondered whether building an aggregation or two in SSAS might help query performance in some cases. Surely HOLAP could be useful here? Sadly not.
What I hadn’t realised was that when you use HOLAP storage and process a partition, SSAS generates exactly the same fact table-level SQL query that is used to process a MOLAP partition. It then uses this data to build any aggregations you have defined and after that throws the data it has read away, leaving only the aggregations stored in MOLAP mode. Therefore you get exactly the same, slow processing performance as pure MOLAP storage and worse query performance! It even executes the SQL query when there are no aggregations defined. I had assumed SSAS would generate one SQL query for each aggregation and get just the summarised data needed by the aggregation but I was wrong. This means that for the kind of scenarios where ROLAP on a fast relational database is becoming more popular (for example when working with large data volumes and/or real-time data) HOLAP is not a viable option.
