django count(*) 慢查询优化

　　分页显示是web开发常见需求，随着表数据增加，200万以上时，翻页越到后面越慢，这个时候慢查询成为一个痛点，关于count(*)慢的原因，简单说会进行全表扫描，再排序，导致查询变慢。这里介绍postgresql一种解决方案。对于大表，我们有时候并不需要返回精确的数值，可以采用模糊的总数代替。

　　原始语句

SELECT COUNT(*) AS "__count" FROM "my_table"

　　优化语句

SELECT reltuples::numeric FROM pg_class WHERE relname = table_name

介绍Django admin 分页优化

　　

from django.contrib.auth.admin import UserAdmin
from django.core.paginator import Paginator, cached_property
from django.db import connections

class UserAdmin(UserAdmin):
    paginator = TablePaginator

class TablePaginator(Paginator):

    @cached_property
    def count(self):
        return (
            self._get_pgsql_estimate_count()
            if not self.object_list.query.where
            else super(LargeTablePaginator, self).count
        )

    def _get_pgsql_estimate_count(self):
        with connections["default"].cursor() as cursor:
            cursor.execute(
                "SELECT reltuples::numeric FROM pg_class WHERE relname = %s",
                [self.object_list.query.model._meta.db_table],
            )

            total_count = int(cursor.fetchone()[0])
        return total_count