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Martin Hope
Мелкий
Asked: 2020-09-23 02:47:51 +0000 UTC
  • 13

有一个这样的表：

create table email_stats (
    id bigserial primary key,
    mailing_list_id int not null,
    sended_date timestamp not null,
    emails_send bigint not null default 0,
    emails_clicks bigint not null default 0
);

现在我们需要向它添加一个新字段。所以任务很简单，

alter table email_stats 
    add column emails_paid_clicks bigint not null default 0;

这是唯一的问题：平板电脑的大小已经有几十 GB，而且这台平板电脑会alter table长时间阻塞所有写入表的内容。

如何在不停机的情况下添加字段？

PS：奇怪，但是在这里没有找到这么常见的问题

sql
Martin Hope
Kunoichi
Asked: 2020-09-08 17:50:33 +0000 UTC
  • 13

我知道在 C# 中创建对象的一种方法：

public static class ObjectCreator
{
    public static T GetObject<T>() where T : class
    {
        return (T)Activator.CreateInstance(typeof(T));
    }
}

有没有更高产的？

用已知方式更新类：

 public class ObjectCreator<T> where T: new()
{
    protected Func<T> V4Lambda;

    protected Func<T> V5Lambda;

    public ObjectCreator()
    {
        Type sType = typeof(T);

        //V4
        V4Lambda = Expression.Lambda<Func<T>>(Expression.New(sType)).Compile();

        //V5
        V5Lambda = DynamicModuleLambdaCompiler.GenerateFactory<T>();
    }

    public T V1()
    {
        return (T)Activator.CreateInstance(typeof(T));
    }

    public T V2()
    {
        return new T();
    }

    public T V3()
    {
        return CustomActivator.CreateInstance<T>();
    }

    public T V4()
    {
        return V4Lambda();
    }

    public T V5()
    {
        return V5Lambda();
    }
}

public static class CustomActivator
{
    public static T CreateInstance<T>() where T : new()
    {
        return ActivatorImpl<T>.Factory();
    }

    private class ActivatorImpl<T> where T : new()
    {
        private static readonly Expression<Func<T>> _expr = () => new T();

        public static readonly Func<T> Factory = _expr.Compile();
    }
}

public static class DynamicModuleLambdaCompiler
{
    public static Func<T> GenerateFactory<T>() where T : new()
    {
        Expression<Func<T>> expr = () => new T();
        NewExpression newExpr = (NewExpression)expr.Body;

        var method = new DynamicMethod(
            name: "lambda",
            returnType: newExpr.Type,
            parameterTypes: new Type[0],
            m: typeof(DynamicModuleLambdaCompiler).Module,
            skipVisibility: true);

        ILGenerator ilGen = method.GetILGenerator();
        // Constructor for value types could be null
        if (newExpr.Constructor != null)
        {
            ilGen.Emit(OpCodes.Newobj, newExpr.Constructor);
        }
        else
        {
            LocalBuilder temp = ilGen.DeclareLocal(newExpr.Type);
            ilGen.Emit(OpCodes.Ldloca, temp);
            ilGen.Emit(OpCodes.Initobj, newExpr.Type);
            ilGen.Emit(OpCodes.Ldloc, temp);
        }

        ilGen.Emit(OpCodes.Ret);

        return (Func<T>)method.CreateDelegate(typeof(Func<T>));
    }
}

我的测试结果：

在此处输入图像描述

c#
Martin Hope
kizoso
Asked: 2020-08-10 18:56:00 +0000 UTC
  • 13

有必要为这些块自适应地布置连接线：

布局

是否可以将css一行的开头写在一个relative块中，而将结尾写在另一个块中？是否有这样的解决css方案svg？Js我不想仅仅为了这些线路而搞砸。

对于固定宽度的块，没有问题，一切正常，如果文本没有改变，那么一切正常：

*,
*:before,
*:after {
  box-sizing: border-box;
}

.wrapper {
  position: relative;
  display: flex;
  flex-wrap: wrap;
  width: 500px;
  margin: 1rem auto;
}

.item {
  position: relative;
  width: 100%;
  min-height: 1px;
  flex: 0 0 33.333333%;
  max-width: 33.333333%;
  padding: 0 1rem;
  margin: 0 0 2rem
}

.item__num {
  position: relative;
  display: inline-block;
  height: 2rem;
  width: 2rem;
  text-align: center;
  vertical-align: middle;
  border-radius: 50%;
  font: 400 1.5rem/2rem sans-serif;
  color: #fff;
  background: cadetblue;
}

.item__num span {
  position: relative;
  z-index: 2;
}

.item__num_lg {
  margin-top: .5rem;
  height: 3rem;
  width: 3rem;
  font: 400 2.5rem/3rem sans-serif;
}

.item__text {
  position: relative;
  z-index: 3;
  display: block;
  padding: 1rem;
  margin: .5rem -1rem 0;
  font: 400 1rem/1.4 sans-serif;
  color: gray;
  background: rgba(255, 255, 255, 0.8);
}


/*мое не адаптивное решение:*/

.item__num:before {
  content: '';
  position: absolute;
  z-index: 1;
  height: .5rem;
  background: cadetblue;
}

.wrapper .item:nth-child(1) .item__num:before {
  top: 1.25rem;
  left: 50%;
  width: 166px;
  transform: rotate(7deg);
}

.wrapper .item:nth-child(2) .item__num:before {
  top: .75rem;
  left: 50%;
  width: 166px;
  transform: rotate(-7deg);
}

.wrapper .item:nth-child(4) .item__num:before {
  top: -93px;
  left: -46px;
  width: 300px;
  transform: rotate(-55deg);
}

.wrapper .item:nth-child(5) .item__num:before {
  top: 1.25rem;
  left: 50%;
  width: 166px;
  transform: rotate(7deg);
}
<link href="https://necolas.github.io/normalize.css/7.0.0/normalize.css" rel="stylesheet" />

<div class=wrapper>
  <div class=item>
    <div class=item__num><span>1</span></div>
    <div class=item__text>Текст короткий</div>
  </div>

  <div class=item>
    <div class="item__num item__num_lg"><span>2</span></div>
    <div class=item__text>Текст немного побольше, возможно в две строки, возможно в одну</div>
  </div>

  <div class=item>
    <div class=item__num><span>3</span></div>
    <div class=item__text>Текст небольшой, средний текст</div>
  </div>

  <div class=item>
    <div class="item__num item__num_lg"><span>4</span></div>
    <div class=item__text>Текст немного побольше, возможно в две строки, возможно в одну</div>
  </div>

  <div class=item>
    <div class=item__num><span>5</span></div>
    <div class=item__text>Текст небольшой, средний текст</div>
  </div>

  <div class=item>
    <div class="item__num item__num_lg"><span>6</span></div>
    <div class=item__text>Текст короткий</div>
  </div>
</div>

如何对自适应块做同样的事情？

*,
*:before,
*:after {
  box-sizing: border-box;
}

.wrapper {
  position: relative;
  display: flex;
  flex-wrap: wrap;
  width: 90%;
  margin: 1rem auto;
}

.item {
  position: relative;
  width: 100%;
  min-height: 1px;
  flex: 0 0 33.333333%;
  max-width: 33.333333%;
  padding: 0 1rem;
  margin: 0 0 2rem
}

.item__num {
  position: relative;
  display: inline-block;
  height: 2rem;
  width: 2rem;
  text-align: center;
  vertical-align: middle;
  border-radius: 50%;
  font: 400 1.5rem/2rem sans-serif;
  color: #fff;
  background: cadetblue;
}

.item__num span {
  position: relative;
  z-index: 2;
}

.item__num_lg {
  margin-top: .5rem;
  height: 3rem;
  width: 3rem;
  font: 400 2.5rem/3rem sans-serif;
}

.item__text {
  position: relative;
  z-index: 3;
  display: block;
  padding: 1rem;
  margin: .5rem -1rem 0;
  font: 400 1rem/1.4 sans-serif;
  color: gray;
  background: rgba(255, 255, 255, 0.8);
}


/*мое не адаптивное решение:*/

.item__num:before {
  content: '';
  position: absolute;
  z-index: 1;
  height: .5rem;
  background: cadetblue;
}

.wrapper .item:nth-child(1) .item__num:before {
  top: 1.25rem;
  left: 50%;
  width: 166px;
  transform: rotate(7deg);
}

.wrapper .item:nth-child(2) .item__num:before {
  top: .75rem;
  left: 50%;
  width: 166px;
  transform: rotate(-7deg);
}

.wrapper .item:nth-child(4) .item__num:before {
  top: -93px;
  left: -46px;
  width: 300px;
  transform: rotate(-55deg);
}

.wrapper .item:nth-child(5) .item__num:before {
  top: 1.25rem;
  left: 50%;
  width: 166px;
  transform: rotate(7deg);
}
<link href="https://necolas.github.io/normalize.css/7.0.0/normalize.css" rel="stylesheet" />

<div class=wrapper>
  <div class=item>
    <div class=item__num><span>1</span></div>
    <div class=item__text>Текст короткий</div>
  </div>

  <div class=item>
    <div class="item__num item__num_lg"><span>2</span></div>
    <div class=item__text>Текст немного побольше, возможно в две строки, возможно в одну</div>
  </div>

  <div class=item>
    <div class=item__num><span>3</span></div>
    <div class=item__text>Текст небольшой, средний текст</div>
  </div>

  <div class=item>
    <div class="item__num item__num_lg"><span>4</span></div>
    <div class=item__text>Текст немного побольше, возможно в две строки, возможно в одну</div>
  </div>

  <div class=item>
    <div class=item__num><span>5</span></div>
    <div class=item__text>Текст небольшой, средний текст</div>
  </div>

  <div class=item>
    <div class="item__num item__num_lg"><span>6</span></div>
    <div class=item__text>Текст короткий</div>
  </div>
</div>

html
Martin Hope
Glazgo Cronwerk
Asked: 2020-07-27 00:21:36 +0000 UTC
  • 13

面试的时候，他们问：向量遍历和列表遍历哪个更快，输出值到控制台？我们如何绕过，没有具体说明。我回答说没有区别。我说得对吗？说明：有一个向量容器，我们可以在其上运行，例如，使用迭代器：

std::vector<int> vector{1, 2, 3};
for (auto it = vector.begin(); it != vector.end(); ++it)
{
    qDebug() << *it;
}

有一个容器列表，我们还在其上运行一个迭代器：

std::list<int> values{1, 2, 3};
for (auto it = values.begin(); it != values.end(); ++it)
{
    qDebug() << *it;
}

哪个循环会运行得更快，为什么？使用其他绕过这些容器的方法，哪个循环会运行得更快？这些周期不在采访中，为了清楚起见，我添加了它们。

PS 在这个问题被问到之后：什么是缓存？也许第二个问题与第一个问题有某种关系？如果不缓存整个vector或list，容器遍历时间会不会增加？

c++
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