mysql- Prevent id increment on unique - mysql

Question

I have a table with unique constraint, which is working fine.
But the problem is when a duplicate record is tried to be inserted it fails due to unique constraint but, increments the id and next valid record get the id with double increment.
How to prevent id from incrementing if insertion fails?
UPDATE
Here is the table structure
CREATE TABLE `crawl_links` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`url` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
`priority` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
`last_crawled_date` date DEFAULT NULL,
`crawl_status` varchar(255) COLLATE utf8_unicode_ci DEFAULT 'New',
`server_id` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
`created_at` datetime DEFAULT NULL,
`updated_at` datetime DEFAULT NULL,
`site_name_id` int(11) DEFAULT NULL,
`last_fetched_by` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `unique_url` (`url`)
) ENGINE=InnoDB AUTO_INCREMENT=149 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
NOTE: I have taken this from dump file created
I have a links database. I collect links from a crawler and store it in the db. The links collected can be duplicate. So to prevent, I added UNIQ constraint on url field. The constraint is successfully preventing the insertion of duplicate records. But however it increments the id even if the insert is failed. My id column is INT(11) and current snapshot of my db shows max(id) = 128961841 but number of records count(*) crawl_links = 700231.

Answer 1

First of all - I agree to SergeS, in general it should not matter at all if there are gaps between the ids.
But still I am curious, since I never had such behavior, so I tried to reproduce:
mysql> CREATE TABLE foo (
id TINYINT UNSIGNED NOT NULL auto_increment,
bar CHAR(2) NOT NULL,
PRIMARY KEY( id ),
UNIQUE( bar ) ) ENGINE=InnoDB;
Query OK, 0 rows affected (0.01 sec)
mysql> INSERT INTO foo SET bar = 'a';
Query OK, 1 row affected (0.00 sec)
mysql> INSERT INTO foo SET bar = 'a';
ERROR 1062 (23000): Duplicate entry 'a' for key 2
mysql> INSERT INTO foo SET bar = 'b';
Query OK, 1 row affected (0.00 sec)
mysql> SELECT * FROM foo;
+----+-----+
| id | bar |
+----+-----+
| 1 | a |
| 2 | b |
+----+-----+
2 rows in set (0.00 sec)
I tried with InnoDB and MyISAM but was not able to build gaps that way. Could you please describe your table setup and your insert?

Answer 2

There is a command to manually set the next ID to be used:
ALTER TABLE tbl AUTO_INCREMENT = 100;
You'll probably have to check if the last INSERT falied due to violation of the UNIQUE contraint, then apply the query above when necessary.