tailieunhanh - MySQL Enterprise Solutions phần 8

nơi được sử dụng: Điều này có nghĩa rằng trong thời gian tham gia mệnh đề WHERE được sử dụng để loại trừ một số hàng từ tập hợp kết quả có thể. Nếu bạn không nhìn thấy điều này, nó có nghĩa rằng tôi ưu hoa "lừa" bằng cách thực hiện tra cứu const hoặc eq_ref hoặc có thể một số thủ thuật thông minh khác, | L274 Analyzing and Improving Server Performance In our case even with one table the number of rows is already 600 000 which is not good. The Extra column provides room for the optimizer to pass along some nonnumeric information sometimes by naming a key or by picking one of the predefined types. It writes a little essay telling you perhaps with a bit of a Swedish accent what is happening as the query is being processed. Here are some common expressions it can use where used This means that during a join the WHERE clause was used to eliminate some rows from the possible result subset. If you don t see this it means that either the optimizer cheated by doing a const or eq_ref lookup or possibly some other clever trick or it scanned the entire table and could not eliminate any rows by evaluating the expression in the WHERE clause. range checked for each record index map This means that MySQL did not find a good key to look up rows in this table however MySQL specifies that depending on the values of the preceding table records in the join order it might sometimes be possible to use a key in this table. This is a rather uncommon condition but if you see this you should ask yourself why it happened and how to make things better so that MySQL can find a key to use for this table. Using index This means that the data for the table was retrieved completely by using the index without accessing the data records. This approach is usually a plus although in some rare cases it can be a detriment if the optimizer erroneously decides that the index scan would be faster than reading the data. The advantage of scanning the index as opposed to accessing the data records is that the index scan will require less I O. However the disadvantage is that it requires more CPU resources. Not exists This means that the not exists optimization technique was applied in processing a LEFT JOIN command. This means that the data in the table did not have to be examined at all because the optimizer