Django—— ORM查询(sql优化)优化了解，Django(元信息)元类建索引

class Book(models.Model) name = models.CharField(max_length=64) class Meta: # 自定义表名 db_table = ‘table_name‘ # 联合索引: 索引的一个主要目的就是加快检索表中数据 index_together = (‘tag1‘, ‘tag2‘) # 联合唯一索引：两个字段全部重复才算重复 unique_together = (‘tag3‘, ‘tag4‘) # 排序字段 ordering = ‘ordering_tag‘ # /admin/中显示的表名称 verbose_name = ‘table_name‘

 

ORM查询(sql优化)优化 ，    了解

(1<span style="color: #000000;">)only() 只查某些字段，要查看其他的字段，后续也能点出其他字段
    uql1 </span>= Book.objects.all().only(<span style="color: #800000;">"</span><span style="color: #800000;">name</span><span style="color: #800000;">"</span>,<span style="color: #800000;">"</span><span style="color: #800000;">price</span><span style="color: #800000;">"</span>)   <span style="color: #008000;">#</span><span style="color: #008000;"> 只查了 "name","price" 两个字段</span>
    Book.objects.all()    <span style="color: #008000;">#</span><span style="color: #008000;"> 书的所有字段都查了一遍</span>
    <span style="color: #0000ff;">print</span>(uq11.first().gender)    <span style="color: #008000;">#</span><span style="color: #008000;"> 后续也能点出其他字段,不过自己又重新执行了查询该字段的sql，相当于在重新查了一遍</span>
<span style="color: #000000;">
(</span>2<span style="color: #000000;">)defer()  除了某些字段，其他的都查
    Book.objects.all().defer(</span><span style="color: #800000;">"</span><span style="color: #800000;">name</span><span style="color: #800000;">"</span>,<span style="color: #800000;">"</span><span style="color: #800000;">price</span><span style="color: #800000;">"</span>)  <span style="color: #008000;">#</span><span style="color: #008000;"> 除了 "name","price" 两个字段，其他的字段都查</span>
<span style="color: #000000;">
(</span>3)values  <span style="color: #008000;">#</span><span style="color: #008000;"> 后续不能再点出其他字段了</span>
    Book.objects.values(<span style="color: #800000;">"</span><span style="color: #800000;">name</span><span style="color: #800000;">"</span>,<span style="color: #800000;">"</span><span style="color: #800000;">price</span><span style="color: #800000;">"</span>)   <span style="color: #008000;">#</span><span style="color: #008000;"> 只查了 "name","price" 两个字段</span>

 

自定义聚合函数

    <span style="color: #0000ff;">from</span> django.db.models <span style="color: #0000ff;">import</span><span style="color: #000000;"> Aggregate, CharField
    </span><span style="color: #008000;">#</span><span style="color: #008000;"> 自定义聚合函数的名字</span>
    <span style="color: #0000ff;">class</span> Concat(Aggregate):  <span style="color: #008000;">#</span><span style="color: #008000;"> 写一个类继承Aggregate，</span>
        function = <span style="color: #800000;">‘</span><span style="color: #800000;">GROUP_CONCAT</span><span style="color: #800000;">‘</span><span style="color: #000000;">
        template </span>= <span style="color: #800000;">‘</span><span style="color: #800000;">%(function)s(%(distinct)s%(expressions)s)</span><span style="color: #800000;">‘</span>
        <span style="color: #0000ff;">def</span> <span style="color: #800080;">__init__</span>(self, expression, distinct=False, **<span style="color: #000000;">extra):
            super(Concat, self).</span><span style="color: #800080;">__init__</span><span style="color: #000000;">(
                expression,
                distinct</span>=<span style="color: #800000;">‘</span><span style="color: #800000;">DISTINCT </span><span style="color: #800000;">‘</span> <span style="color: #0000ff;">if</span> distinct <span style="color: #0000ff;">else</span> <span style="color: #800000;">‘‘</span><span style="color: #000000;">,
                output_field</span>=<span style="color: #000000;">CharField(),
                </span>**<span style="color: #000000;">extra)
    eg: Book.objects.aggregate( name</span>=Concat(<span style="color: #800000;">"</span><span style="color: #800000;">name</span><span style="color: #800000;">"</span>) )

 

Django—— ORM查询(sql优化)优化了解，Django(元信息)元类建索引

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