psycopg2:DictCursor 与 RealDictCursor

问题描述

AFAIU 和文档中的 RealDictCursor 是一个专门的 DictCursor，它只能从键(又名列名)访问列，而 DictCursor 可以从键或索引号访问数据.
我想知道如果 DictCursor 提供了更大的灵活性，为什么要实现 RealDictCursor?它在性能方面(或内存方面)是否如此不同(我认为有利于 RealDictCursor ......)?
换句话说，什么是 RealDictCursor 用例与 DictCursor?

AFAIU and from docs, RealDictCursor is a specialized DictCursor that enables to access columns only from keys (aka columns name), whereas DictCursor enables to access data both from keys or index number.
I was wondering why RealDictCursor has been implemented if DictCursor offers more flexibility? Is it performance-wise (or memory-wise) so different (in favor of RealDictCursor I imagine...)?
In other words, what are RealDictCursor use cases vs DictCursor?

推荐答案

真正的字典游标的主要优点是可以很容易地获得 json 格式的查询输出.

The main advantage of real dictionary cursor is the easiness to get a query output as json.

比较:

with psycopg2.connect('dbname=test') as connection:
    with connection.cursor(cursor_factory=RealDictCursor) as cursor:
        cursor.execute("select * from my_table")
        print(json.dumps(cursor.fetchall()))

对比

with psycopg2.connect('dbname=test') as connection:
    with connection.cursor() as cursor:
        cursor.execute("select * from my_table")
        columns = [desc[0] for desc in cursor.description]
        real_dict = [dict(zip(columns, row)) for row in cursor.fetchall()]
        print(json.dumps(real_dict))

就性能而言，这些选项之间没有重要区别.

There is no important difference between these options when it comes to performance.

对于常规或类似字典的游标，您无法使用 json.dumps(cursor.fetchall()) 获得预期的 json，并且需要进行上述转换.另一方面，真正的字典游标会产生更大的结果，所以如果你真的不需要它，你不应该使用它.

You cannot get an expected json using json.dumps(cursor.fetchall()) for regular or dictionary-like cursors and need the conversion showed above. On the other hand, real dictionary cursor produces a much larger result so you should not use it if you really do not need it.

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