Interview question 5

1.explain sql Which columns to focus on ？
Need to pay attention to ,type Column ,const The type of unique value is this ,range Indicates the range of query ,index Indicates the index of walking ,all It means you have gone through a full scan . Focus on filter Column , Indicates the percentage value of filtered rows .
2.jvm What are the garbage collectors , What scenarios use which collectors ？
Serial、ParNew、Parallel Scavenge、Serial Old、Parallel Old、CMS、G1
1.4 It used to be Serial,Serial Old Use it with , Serial garbage collection ,1.4 after , With Concurrent Versions of garbage collection , The so-called concurrency refers to the use of concurrency when analyzing the accessibility of garbage , Execute with worker thread , That is to say parnew and cms, Behind the roll out Parallel Scavenge Parallel Old Combine garbage collection for the new generation and the elderly , This is a parallel garbage collector , It is suitable for throughput first scenarios ,1.9 After the default use g1 Garbage collector ,g1 Divide the memory one by one region Garbage collection , Within the limited collection time , Maximize throughput . Sum up 1. For response time priority cms Garbage collector ,2. Throughput priority Parallel Scavenge Parallel Old Garbage collector 3.g1 In a limited time , Maximize throughput 4. If the machine is single core , The garbage collector in the back is not applicable , Direct use Serial,Serial Old Just fine , The latter are all for multi-core cpu To have an advantage .
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