問題描述
當我執(zhí)行以下命令時:
ALTER TABLE `mytable` ADD UNIQUE (
`column1` ,
`column2`
);
我收到此錯誤消息:
#1071 - Specified key was too long; max key length is 767 bytes
關(guān)于 column1 和 column2 的信息:
Information about column1 and column2:
column1 varchar(20) utf8_general_ci
column2 varchar(500) utf8_general_ci
我認為 varchar(20) 只需要 21 個字節(jié)，而 varchar(500) 只需要 501 個字節(jié).所以總字節(jié)數(shù)是 522，小于 767.那么為什么我會收到錯誤消息?
I think varchar(20) only requires 21 bytes while varchar(500) only requires 501 bytes. So the total bytes are 522, less than 767. So why did I get the error message?
#1071 - Specified key was too long; max key length is 767 bytes
推薦答案
767 字節(jié)是 規(guī)定的前綴限制在 MySQL 5.6 版(和之前的版本)中的 InnoDB 表.MyISAM 表的長度為 1,000 字節(jié).在 MySQL 5.7 及更高版本中，此限制已增加到 3072 字節(jié).
767 bytes is the stated prefix limitation for InnoDB tables in MySQL version 5.6 (and prior versions). It's 1,000 bytes long for MyISAM tables. In MySQL version 5.7 and upwards this limit has been increased to 3072 bytes.
您還必須注意，如果您在 utf8mb4 編碼的 big char 或 varchar 字段上設置索引，則必須將 767 字節(jié)(或 3072 字節(jié))的最大索引前綴長度除以 4，結(jié)果為 191.這是因為 utf8mb4 字符的最大長度是四個字節(jié).對于 utf8 字符，它將是三個字節(jié)，導致最大索引前綴長度為 254.
You also have to be aware that if you set an index on a big char or varchar field which is utf8mb4 encoded, you have to divide the max index prefix length of 767 bytes (or 3072 bytes) by 4 resulting in 191. This is because the maximum length of a utf8mb4 character is four bytes. For a utf8 character it would be three bytes resulting in max index prefix length of 254.
您可以選擇的一種方法是為您的 VARCHAR 字段設置下限.
One option you have is to just place lower limit on your VARCHAR fields.
另一種選擇(根據(jù)對此問題的回應)是獲得列的子集而不是整個金額，即:
Another option (according to the response to this issue) is to get the subset of the column rather than the entire amount, i.e.:
ALTER TABLE `mytable` ADD UNIQUE ( column1(15), column2(200) );
調(diào)整，因為您需要獲取要應用的密鑰，但我想知道是否值得檢查有關(guān)此實體的數(shù)據(jù)模型，以查看是否有改進可以讓您在不影響 MySQL 的情況下實現(xiàn)預期的業(yè)務規(guī)則限制.
Tweak as you need to get the key to apply, but I wonder if it would be worth it to review your data model regarding this entity to see if there's improvements that would allow you to implement the intended business rules without hitting the MySQL limitation.
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