转载___国内比较有名的自动驾驶公司

2019-09-16

第一梯队

第一梯队的主要有两家：

第一家公司Momenta相对低调，但团队和实际进展却名声响于行业内。他们目前执掌研发的联合创始人，是目标检测和图像识别领域先进框架Faster R-CNN和ResNet的作者任少卿。

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高并发优化的一些经验

2019-09-09

首先顺一遍流量走的方向,

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写在使用es前

2019-09-09

es作为一个分布式全文检索引擎, 在用之前需要确认下一些常见的问题

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sql语法

2019-08-20

首先复杂的sql都是一个一个简单的sql拼的, 一个大的业务需要先拆分成小的业务来实现.

SELECT emp_no FROM employees.salaries union select emp_no from employees.salaries;

EXPLAIN SELECT emp_no FROM employees.salaries union  select emp_no from employees.salaries;

EXPLAIN SELECT emp_no FROM employees.salaries union all select emp_no from employees.salaries;

select count(*) from employees.salaries;

explain select *  from (select * from employees.salaries) as Pp;

select cc.first_name, cc.salary from (select salaries.emp_no, salaries.salary, employees.first_name from employees.salaries join employees.employees on salaries.emp_no=employees.emp_no where salary > (select avg(salaries.salary) from employees.salaries)) as cc;

select avg(salaries.salary) from employees.salaries;

desc select salaries.emp_no, salaries.salary, employees.first_name from employees.salaries join employees.employees on salaries.emp_no=employees.emp_no;

explain select count(distinct salary) from employees.salaries;

show indexes from employees.salaries;
alter table employees.salaries add index keke(salary);
explain select distinct salary from employees.salaries  order by salary ;

show variables like 'slow%';

set global slow_query_log='ON'; 
set global slow_launch_time=1;
use employees;
CREATE TABLE employees.t1 (s1 INT, s2 CHAR(5), s3 FLOAT);

INSERT INTO employees.t1 VALUES (1,'1',1.0);
INSERT INTO employees.t1 VALUES (2,'2',2.0);
explain SELECT sb1,sb2,sb3
  FROM (SELECT s1 AS sb1, s2 AS sb2, s3*2 AS sb3 FROM employees.t1) AS sb
  WHERE sb1 > 1;

SELECT AVG(SUM(column1)) FROM t1 GROUP BY column1;
-- 用了临时表 
explain SELECT AVG(sum_column1)
  FROM (SELECT SUM(s1) AS sum_column1
        FROM t1 GROUP BY s1) AS t1;
--  没有用临时表
explain SELECT AVG(sum_column1)
  FROM (SELECT SUM(s1) AS sum_column1
        FROM t1) AS t1;