Erstellt `SERIAL PRIMARY KEY` zwei Indizes für die Tabelle?

Da SERIALes sich um eine Abkürzung für handeltBIGINT UNSIGNED NOT NULL AUTO_INCREMENT UNIQUE , sieht es so aus, als würde sich das SERIAL PRIMARY KEYverdoppeln UNIQUEund PRIMARY KEY:

> CREATE TEMPORARY TABLE foo (id SERIAL PRIMARY KEY);

> SHOW CREATE TABLE foo \G
*************************** 1. row ***************************
       Table: foo
Create Table: CREATE TABLE `foo` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`),
  UNIQUE KEY `id` (`id`)
)

Es gibt definitiv den WTF- Faktor, beide PRIMARY KEYund UNIQUE KEY idin zu sehen SHOW CREATE TABLE, aber sind das wirklich zwei physische Indizes, die ich sehe?

Sollte man die Verwendung PRIMARY KEYmit SERIALund stattdessen das Schreiben bigint unsigned NOT NULL AUTO_INCREMENT PRIMARY KEYoder die Verwendung SERIALohne Primärschlüssel auf dem Tisch vermeiden ?

Ich sehe dieses Verhalten auch in 5.7.18 MySQL Community Server.

Basierend auf der Ausgabe von SHOW INDEX FROM foodort wird ein doppelter Index erstellt und verwaltet:

mysql> CREATE TEMPORARY TABLE foo (id SERIAL PRIMARY KEY);
Query OK, 0 rows affected (0.00 sec)

mysql> show index from foo;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| foo   |          0 | PRIMARY  |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
| foo   |          0 | id       |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)

Bearbeiten: Dies passiert auch für normale Tabellen:

mysql> CREATE TABLE foo (id SERIAL PRIMARY KEY);
Query OK, 0 rows affected (0.01 sec)

mysql> show index from foo;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| foo   |          0 | PRIMARY  |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
| foo   |          0 | id       |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)

mysql> select * from INFORMATION_SCHEMA.TABLE_CONSTRAINTS where TABLE_NAME = 'foo';
+--------------------+-------------------+-----------------+--------------+------------+-----------------+
| CONSTRAINT_CATALOG | CONSTRAINT_SCHEMA | CONSTRAINT_NAME | TABLE_SCHEMA | TABLE_NAME | CONSTRAINT_TYPE |
+--------------------+-------------------+-----------------+--------------+------------+-----------------+
| def                | my_db             | PRIMARY         | my_db        | foo        | PRIMARY KEY     |
| def                | my_db             | id              | my_db        | foo        | UNIQUE          |
+--------------------+-------------------+-----------------+--------------+------------+-----------------+
2 rows in set (0.00 sec)