Query Compilation
Umbra performs Just-In-Time (JIT) compilation of queries into Umbra IR, a custom intermediate representation (IR) similar to LLVM IR but optimized for use in a database system. After generating Umbra IR, the code is lowered using one of two backends:
- LLVM
- Flying Start
The LLVM backend emits LLVM IR, compiled at optimization level -O3. This backend is the slowest but generates the fastest executing code, making it suitable for long-running queries.
The Flying Start backend emits x86 machine code using asmJIT, generating x86 in a single pass. In addition, the Flying Start backend implements Stack Space Reuse, Machine Register Allocation, Lazy Address Calculation, and Comparison-Branch Fusion optimizations. As a result, the code generated by Flying Start has performance on par with code generated by LLVM -O0 (i.e., with optimizations disabled). Additionally, Flying Start outperforms interpretation of Umbra IR, making Flying Start suitable for all but the longest-running queries.
Umbra supports adaptive execution, pioneered by HyPer, allowing the DBMS to switch execution strategies while processing a single query. Umbra first generates x86 machine code using the Flying Start backend and then switches to the code generated by the LLVM backend for long-running queries.
