mysql流利的nhibernate问题nvarchar

Question

I have switched from sql server 2005 to mysql which was not really a problem.

I have a slight issue with (n)varchar which exist in sql server. Usually I have used:

mapping.Map(x => x.bla).Length(10000);

to set bla to nvarchar(max). does this work in mysql? I believe there is no nvarchar in mysql and you have to use something like this:

alter table sometable modify bla VARCHAR(21844) CHARACTER SET utf8

to update an existing column to 'nvarchar(max)'. Is this correct because I am getting:

"Row size too large. The maximum row size for the used table type"

If I am using:

alter table sometable modify bla VARCHAR(1000) CHARACTER SET utf8

things work but I am not sure whether this achieves 'nvarchar(max)' in mysql.

Answer 1

As explained in the manual :

Every table (regardless of storage engine) has a maximum row size of 65,535 bytes. Storage engines may place additional constraints on this limit, reducing the effective maximum row size.

The maximum row size constrains the number (and possibly size) of columns because the total length of all columns cannot exceed this size. For example, utf8 characters require up to three bytes per character, so for a CHAR(255) CHARACTER SET utf8 column, the server must allocate 255 × 3 = 765 bytes per value. Consequently, a table cannot contain more than 65,535 / 765 = 85 such columns.

Storage for variable-length columns includes length bytes, which are assessed against the row size. For example, a VARCHAR(255) CHARACTER SET utf8 column takes two bytes to store the length of the value, so each value can take up to 767 bytes.

You must therefore consider what other columns exist in your table and calculate the maximum size available for this VARCHAR .

However, if you require space for long text values, why not use the TEXT data types, which are not constrained by this limit (except for the 9 to 12 bytes they contribute toward it)?