Mysql – Using JOIN to find duplicates in another table taking too long

In the spirit of @yercube's answer, I have an answer that has an added twist.

CREATE TABLE stage
(
    id int not null auto_increment,
    name varchar(20),
    primary key (id)
);
CREATE TABLE stage2 LIKE stage;
INSERT INTO stage (name) SELECT name FROM item;
INSERT INTO stage2 (id) SELECT min_id FROM
(SELECT MIN(id) min_id,name FROM stage GROUP BY name) A;
UPDATE stage2 A INNER JOIN stage B USING (id) SET A.name=B.name;
TRUNCATE TABLE item;
INSERT INTO item (name) SELECT name FROM stage2;
DROP TABLE stage;
DROP TABLE stage2;

This will load stage2 with the first occurrence of each name from item, zap the item table, and load the unique occurrences back.

If you look back in @yercube's answer and compare it to my answer, his is much more simplistic because

• @yercube uses one temp table, while I use two
• I had to create a column for iteration control, @yercube did not need to
• @yercube has fewer steps
• both answers achieve the same thing

I do not expect my answer to be accepted. The sole purpose of my answer was demonstrate that other answers lose the concise clarity needed to solve your problem. Again, hats off to @yercube.

The problem appears to have been that the cardinality of my indices was NULL. I am unsure why the indices would revert to this state when they were apparently fine yesterday.

I discovered this problem by running SHOW INDEX FROM tickets.

I ran ANALYZE TABLE tickets. Now the EXPLAIN looks subtly different-- the rows have dropped to 1:

mysql> explain select count(*) from tickets WHERE id = 214;
|  1 | SIMPLE      | tickets | const | PRIMARY       | PRIMARY | 2       | const |    1 | Using index |