插入具有自动增量ID的新行未添加为数据库中的最后一个条目

Question

I have a SQL - MyIsam table called links with the following structure

| field      | type         | null | predefined       | extra          |
|------------|--------------|------|------------------|----------------|
| id         | int(11)      | no   | none             | auto_increment |
| link       | varcar(2083) | no   | none             | none           |
| created_at | timestamp    | no   | curret_timestamp | none           |
| origin     | varcar(100)  | no   | none             | none           |

my insert statement is

public function createLink($user_id, $link, $origin) {        
        $stmt = $this->conn->prepare("INSERT INTO links(link, origin) VALUES(?,?)");
        $stmt->bind_param("ss", $link, $origin);
        $result = $stmt->execute();
        $stmt->close();

        if ($result) {
            // link row created
            // now assign the link to user
            $new_link_id = $this->conn->insert_id;
            $res = $this->createUserLink($user_id, $new_link_id);
            if ($res) {
                // link created successfully
                return $new_link_id;
            } else {
                // user_link failed to create
                return NULL;
            }
        } else {
            // link failed to create
            return NULL;
        }
    }

unfortunately sometimes i get an unexpected behavior which i'm not totally able to reproduce.

as you can see the rows with id 301 and 304 has been inserted between 286 and 293 as well as row with id 300 is between 298 and 296.

Since id is auto incremental, i would have expected to find all of them ordered from the smaller to the higher value.

This is bad for my application because i need that the chronological order of creation is respected and followed whenever i query the db to get all the links

 public function getAllUserLinks($user_id) {
        $stmt = $this->conn->prepare("SELECT l.* FROM links l, users_links ul WHERE l.id = ul.link_id AND ul.user_id = ?");
        $stmt->bind_param("i", $user_id);
        $stmt->execute();
        $links = $stmt->get_result();
        $stmt->close();
        return $links;
    }

i know i could add an ORDER BY... to my query but this would further slow down things.

Besides, new links should be added as last row of the table, not in the middle , right?!?

Answer 1

So the physical order you see will be down to your clustered index, if you haven't defined one then MySQL will do it for you, from the InnoDB docs here :

If the table has no PRIMARY KEY or suitable UNIQUE index, InnoDB internally generates a hidden clustered index on a synthetic column containing row ID values. The rows are ordered by the ID that InnoDB assigns to the rows in such a table. The row ID is a 6-byte field that increases monotonically as new rows are inserted. Thus, the rows ordered by the row ID are physically in insertion order.

I'm assuming you're using InnoDB but MyISAM would also use the clustered index to determine physical order. Some more info here about clustered indexes.

Edit

The comment below is correct to say clustered indexes are how data is stored and strictly speaking not how data is retrieved. In practice there's a good chance you'll find data returned in physical order if you don't specify an ORDER BY.

I was attempting to explain why the questioner might be seeing the ordering he was but I'm not saying this should be relied upon, no guarantees are given about the order of data without an ORDER BY. From the SQL 92 spec (in various places):

If an is not specified, then the ordering of the rows of Q is implementation-dependent