Common Sub Expression Optimization at Facebook

Common Sub Expression Optimization at Facebook

In complex analytics queries, we often see repeated expressions, for example parsing the same JSON column but extracting different fields, elaborate CASE statement with common predicates and different ones. Previously, Presto will compute the same expression many times as they appear in query. With common sub expression optimization, we would only evaluate the same expression once within the same project operator or filter operator. In our workload, we’ve seen 3x improvements on certain queries with expensive common sub expressions like JSON_PARSE. Microbenchmark also shows a consistent ~10% performance improvement for simple common sub-expressions like x + y. In this talk, we will talk about how this is implemented.

Query Execution Optimization for Broadcast Join using Replicated-Reads Strategy – George Wang, Ahana

Query Execution Optimization for Broadcast Join using Replicated-Reads Strategy – George Wang, Ahana

Today presto supports broadcast join by having a worker to fetch data from a small data source to build a hash table and then sending the entire data over the network to all other workers for hash lookup probed by large data source. This can be optimized by a new query execution strategy as source data from small tables is pulled directly by all workers which is known as replicated reads from dimension tables. This feature comes with a nice caching property given that all worker nodes N are now participating in scanning the data from remote sources. The table scan operation for dimension tables is cacheable per all worker nodes. In addition, there will be better resource utilization because the presto scheduler can now reduce the number plan fragment to execute as the same workers run tasks in parallel within a single stage to reduce data shuffles.