有关MySql索引和Asp.net C＃的问题

Question

In Asp.net C# I use the following code to select from a MySql-database:

string userid = "12";
MySqlCommand mysqlcmd = new MySqlCommand("SELECT productid FROM table WHERE userid=?userid", mysqlconn);
mysqlcmd.Parameters.AddWithValue("?userid", userid);
mysqlcmd.Connection.Open();
mysqlcmd.ExecuteReader();
...

In the table the userid is of the type int. If I have an index in the table on userid what will happen with this query, will it be able to use the index in a good way even if the userid I add to the parameter is a string and the index in the table is int?

Would it give me better performance to before adding userid to the parameter convert userid to an int like this:

int userid_int = Int32.Parse(userid);
MySqlParameter param = new MySqlParameter("?userid", MySqlDbType.Int32);
param.Value = userid_int;
cmd.Parameters.Add(param);

Or is it ok to only use:

mysqlcmd.Parameters.AddWithValue("?userid", userid);

Answer 1

Indexes are used no matter what kind of datatype the indexed column is. The questions is, how efficient this will happen.

For instance, if you use string-columns with indexes for search you should ensure, that the values in that columns are differing from each other. If you have records like

index_column
------------
DHDHDSNDH233
DHDHDSNDH234
DHDHDSNDH235
DHDHDSNDH236

the index is a little bit useless because the engine has to load a lot of data for finding your keys.

This is the reason why a lot of people are using GUIDs when search-indexing a string-column.

If the column is'nt the PK, you have to live with that possible bottleneck.