chanyan 最近的时间轴更新


V2EX 第 370135 号会员,加入于 2018-12-13 13:51:28 +08:00
今日活跃度排名 24098
chanyan 最近回复了
22 天前
回复了 qweruiop 创建的主题 PostgreSQL 请教一个经典的 postgres 的 sql 怎么写
count(t2.*) cnt
t_student t1
left join
t_transactions t2
on = t2.student_id
group by,
68 天前
回复了 xpl8435 创建的主题 生活 冬至快乐,吃汤圆还是饺子!
看不见妳的笑 我怎么睡得着 妳的声音这么近我却抱不到
@chanyan #4 mysql8
For example, a VARCHAR(255) column can hold a string with a maximum length of 255 characters. Assuming that the column uses the latin1 character set (one byte per character), the actual storage required is the length of the string (L), plus one byte to record the length of the string. For the string 'abcd', L is 4 and the storage requirement is five bytes. If the same column is instead declared to use the ucs2 double-byte character set, the storage requirement is 10 bytes: The length of 'abcd' is eight bytes and the column requires two bytes to store lengths because the maximum length is greater than 255 (up to 510 bytes).

The effective maximum number of bytes that can be stored in a VARCHAR or VARBINARY column is subject to the maximum row size of 65,535 bytes, which is shared among all columns. For a VARCHAR column that stores multibyte characters, the effective maximum number of characters is less. For example, utf8mb4 characters can require up to four bytes per character, so a VARCHAR column that uses the utf8mb4 character set can be declared to be a maximum of 16,383 characters.
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