如何更快地删除重复项？

Question

On a table with about 1.7M rows, I tried to delete duplicates posts:

delete a FROM comment a
  INNER JOIN comment a2
     WHERE a.id < a2.id
     AND   a.body = a2.body;

The result was:

  Query OK, 35071 rows affected (5 hours 36 min 48.79 sec)

This happened on my almost idle workstation with Intel(R) Core(TM) i7-4790 CPU @ 3.60GHz . I'm wondering if there are some tricks to make this delete operation faster?

Answer 1

For MySQL specifically you can try (assuming rows have the exact same columns information):

ALTER IGNORE TABLE comment ADD UNIQUE INDEX idx_name (id, body);

Source

Answer 2

The below query will be useful for you.

Delete  YourTableName 
From    (
Select  row_number() over(Partition by ColName1,ColName2,ColName3 order by ColName1,ColName2,ColName3 Asc)As RowNumber
        )YourTableName
Where   YourTableName.RowNumber>1

if it's working kindly mark as answer

Answer 3

Your query is attempting a zillion deletes for the same row. For instance, if you have this data:

body   id
  a     1
  a     2
  a     3
  a     4

Then your query attempts the following deletions:

 c.body   c.id  c2.id
  a         1      4
  a         1      3
  a         1      2
  a         2      4
  a         2      3
  a         3      4

You can see how this would result in lots of work for the database, as the number of id s on a given body increase.

You can fix this using group by instead:

delete c 
    from comment c join
         (select c2.body, max(c2.id) as max_id
          from comment c2
          group by c2.body
         ) c2
         on c2.body = c.body and c.id < c2.max_id;

In addition, you want an index on comment(body, id) .

You might also find that an anti-join works better than the join you are attempting:

delete c 
    from comment c left join
         comment c2
         on c2.body = c.body and c2.id > c.id
    where c2.id is null;