I ntroducon to MemSQL Figure 3. Compiled plans in MemSQL ulizing and bypassing the code generator Aer code generaon, the compiled plans are saved for later use in a plan cache. Each MemSQL node has its own plans and plan cache. A plan cache consists of two layers: the in-memory plan cache and the on-disk plan cache. Plans stored in the in-memory plan cache remain unl they expire, or unl the memSQL node restarts. When a plan expires, it stays put in the on-disk plan cache, and is loaded back into memory the next me the query is executed. By interpreng SQL statements and implemenng compiled query plans, MemSQL removes interpretaon overhead and minimizes code execuon paths. Another key feature of MemSQL’s compiled plans is that they do not pre-specify values for the parameters. Query parameters are dynamically extracted from a query template, producing a normalized query that is then transformed into a specialized nave representaon (MemSQL Plan Language, or MPL). The generated execuon plan is wrien in C++ and compiled to machine code. When queries that match the query template are executed, MemSQL substutes the parameter values, allowing the request to reuse already-compiled plans and run quickly. Addionally, compiled plans are also reused across server restarts, so they need to be only compiled once in an applicaon’s lifeme. 8