问题
I have a table encoded in latin1 and collate latin1_bin.
In my table there is a column comments of type 'TEXT', as you know this column inherits table's encoding and collation, but from now on I should change it to be utf8 and utf8_general_ci because I'm starting to store special characters in comments.
Would it cause any downside effect if I'd use a command like the following?
alter table notebooks modify comments text CHARACTER SET utf8 COLLATE utf8_general_ci;
Thank you for your answer.
回答1:
Danger I think that ALTER will destroy existing text.
Also, ... Your 'name' looks Chinese, so I would guess that you want to store Chinese characters? In that case, you should use utf8mb4, not just utf8. This is because some of the Chinese characters take 4 bytes (and are not in the Unicode BMP).
I believe you need 2 steps:
ALTER TABLE notebooks MODIFY comments BLOB;
ALTER TABLE notebooks MODIFY comments TEXT
CHARACTER SET utf8mb4 COLLATE utf8mb4_general_520_ci;
Otherwise the latin1 characters will be "converted" to ut8. But if you really have Chinese in the column, you do not have latin1. The 2-step alter, above, does (1) turn off any knowledge of character set, and (2) establish that the bytes are really utf8mb4-encoded.
To be safer, first do
RENAME TABLE notebooks TO old;
CREATE TABLE notebooks LIKE old;
INSERT INTO notebooks SELECT * FROM old;
Then do the two ALTERs and test the result. If there is trouble, you can RENAME to get back the old copy.
回答2:
Specifying any collating sequence that does not involve direct integral comparison of a NATIVE character set will slow down your query. Whether it will slow it down noticeably is another issue. Looking up the ranking of this, and the ranking of that, in a table and comparing the two results is much, MUCH faster than retrieving on-disk information from a database, wouldn't you imagine?
来源：https://stackoverflow.com/questions/38268802/are-there-any-downside-effects-when-changing-mysql-table-encoding