postgresql:md5 消息摘要的数据类型? [英] postgresql: data type for md5 message digest?

问题描述

我想使用某个字符串的 MD5 消息摘要作为表的主键.我应该为这样的字段使用什么数据类型?我应该为该字段编写哪些 select 和 insert 语句?

I want to use the MD5 message digest of some string as the primary key of a table. What datatype should I use for such a field? What select and insert statements should I write for the field?

推荐答案

作为 bytea 的 md5 哈希将仅使用 16 个字节而不是 32 个字节用于六进制表示:

The md5 hash as bytea will use only 16 bytes in instead of 32 for the hexa representation:

create table t (d bytea);
insert into t (d) values
    (digest('my_string', 'md5')),
    (decode(md5('my_string'), 'hex'));

以上两种形式都可以使用，但要使用更简单的 digest 功能，必须以超级用户身份安装 pgcrypto 扩展:

Both forms above will work but to use the simpler digest function it is necessary to install the pgcrypto extension as superuser:

create extension pgcrypto;

使用上面的digest函数或者decode和md5的组合来搜索某个字符串:

Use the digest function or the combination of decode and md5 as above to search for a certain string:

select
    octet_length(d) ba_length,
    pg_column_size(d) ba_column,
    encode(d, 'hex') hex_representation,
    octet_length(encode(d, 'hex')) h_length,
    pg_column_size(encode(d, 'hex')) h_column
from t
where d = digest('my_string', 'md5')
;
 ba_length | ba_column |        hex_representation        | h_length | h_column 
-----------+-----------+----------------------------------+----------+----------
        16 |        17 | 3d212b21fad7bed63c1fb560c6a5c5d0 |       32 |       36
        16 |        17 | 3d212b21fad7bed63c1fb560c6a5c5d0 |       32 |       36

pg_column_size 值是存储大小.与六进制表示相比，bytea 不到一半.

The pg_column_size value is the storage size. It is less than half for the bytea compared to the hexa representation.

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