Why most SQL databases allow defining the same index (or constraint) twice?
For example in MySQL I can do:
CREATE TABLE testkey(id VARCHAR(10) NOT NULL, PRIMARY KEY(id));
ALTER TABLE testkey ADD KEY (id);
ALTER TABLE testkey ADD KEY (id);
SHOW CREATE TABLE testkey;
CREATE TABLE `testkey` (
`id` varchar(10) NOT NULL,
PRIMARY KEY (`id`),
KEY `id` (`id`),
KEY `id_2` (`id`)
)
I can see that some databases prevent duplicate indexes. Oracle Database prevents duplicate indexes https://www.techonthenet.com/oracle/errors/ora01408.php while other databases like MySQL and PostgreSQL do not have duplicate index prevention.