Machine Code Caching in PostgreSQL Query JIT-Compiler

Abstract

As the efficiency of main and external memory grows, alongside with decreasing hardware costs, the performance of database management systems (DBMS) on certain kinds of queries is more determined by CPU characteristics and the way it is utilized. Relational DBMS utilize diverse execution models to run SQL queries. Those models have different properties, but in either way suffer from substantial overhead during query plan interpretation. The overhead comes from indirect calls to handler functions, runtime checks and large number of branch instructions. One way to solve this problem is dynamic query compilation that is reasonable only in those cases when query interpretation time is larger than the time of compilation and optimized machine code execution. This requirement can be satisfied only when the amount of data to be processed is large enough. If query interpretation takes milliseconds to finish, then the cost of dynamic compilation can be hundreds of times more than the execution time of generated machine code. To pay off the cost of dynamic compilation, the generated machine code has to be reused in subsequent executions, thus saving the cost of code compilation and optimization. In this paper, we examine the method of machine code caching in our query JIT-compiler for DBMS PostgreSQL. The proposed method allows us to eliminate compilation overhead. The results show that dynamic compilation of queries with machine code caching feature gives a significant speedup on OLTP queries.