0727~ sorting out interview questions

Principles of index creation ?

where Judge the conditions often used ;

Do not use indexes for frequently updated columns ;

For columns that are rarely involved in queries , Don't index ;

Do not index classes with a small cardinality ;

When defining columns with foreign keys, you must build indexes ;

ArrayList and LinkList The difference between ?

ArrayList The bottom layer is based on dynamic array , and LinkList The bottom layer is based on a two-way linked list ;

ArrayList Quick query and modification , and LinkList Is to delete and add fast ;

What is the parent delegate model ?

In the class loader , Delegating up is looking for cache , Then delegate down to find the loading path ;

Delegate up to the top loader , Delegation down is until the initiated loader ;

stay mybatis in $ and # The difference between ?

mybatis Medium $ It's string concatenation , stay sql I will put ${} Switch to statement Assign values to variables directly , So there will be sql Injection problem ;

mybatis Medium # It's a placeholder , Pre compile , stay sql I will put #{} Switch to ?, And then use preperdstatement Assign a value to a variable , So it can effectively prevent sql Injection problem ,

MySql Have the slow queries in it been counted ? How to optimize ?

First, look at what causes slow queries , Is an index miss , Or is the judgment condition of the query statement loaded with more data , Or the amount of data is too large ;

First check whether the index hits , Then modify the query statement , Hit the index as much as possible ;

Check whether the query statements and judgment conditions have loaded redundant data , It invalidated the index , Then modify the statement ;

See if the database is too large , Resulting in slow query , It is necessary to divide the database and table ;

Three paradigms of database ?

The first paradigm : Columns and data cannot be further divided , Keep the data atomic , No more data ;

The second paradigm : Data records are unique , A watch can only say one thing ;

The third paradigm : The data in the database cannot have a transfer relationship , It must have a direct relationship with the primary key , It cannot be an indirect relationship ;

Runnable and callable The difference between ?

Runnable Interface has no return value ,callable Interface has a return value ;

Runnable The interface cannot catch exceptions ,callable Can catch exceptions ;