MySQL – Updating Rows from Other Rows in a Large Table

MySQLsubqueryupdate

I have a huge table with items

item-id | name | type | extra

Many of them are "old versions" of type 0.

I have to update the extra field in all new items (same item-id, but type > 0) with the extra data we have for the old version.

Since this is a very large table with millions of rows, I have to be very careful with joining the table with itself.

Basically I want to do this

update mytable t1 set extra = (select extra from mytable where t1.itemid = mytable.itemid and type=0) where type > 0;

But MySQL doesn't accept t1 in the sub-select.

Best Answer

Recently read an article that might help you, see: https://dzone.com/articles/specifying-target-tables-in-mysql

Still think a self join would be faster than a correlated sub-query. Especially on MySQL 5.5 and less, where these type of sub queries are badly optimized.

Self join like:

UPDATE mytable t1
  JOIN mytable t2
    ON t1.item-id = t2.item-id
SET t1.extra = t2.extra
WHERE t1.type > 0;

Related Solutions

Mysql – How to index this table (a_level, b_level, item_id)

Since your table only three columns, this setup can be straightforward.

Here is an ideal table layout for items

CREATE TABLE items
(
    item_id int not null auto_increment,
    a_level int not null,
    b_level int not null,
    KEY  ai  (a_level,item_id),
    KEY  bi  (b_level,item_id),
    KEY  abi (a_level,b_level,item_id),
    KEY  bai (b_level,a_level,item_id),
    PRIMARY KEY     (item_id)
);

Here are the realm of possible queries

SELECT * FROM items WHERE id = 100; (uses the PRIMARY KEY)
SELECT * FROM items WHERE a_level = 200; (uses KEY ai)
SELECT * FROM items WHERE b_level = 300; (uses KEY bi)
SELECT * FROM items WHERE a_level = 400 order by b_level (uses KEY abi ordering all b_level values under a_level 400 and orders item_id under each b_level)

YYou may try out other combinations of SELECT...GROUP BY...ORDER BY.... You should also run EXPLAIN on every SELECT query and see which indexes are summoned for query execution. Indexes that do not appear as a chosen index can be eliminated (dropped) over the course of time.

Here were questions you asked?

If i INDEX both on a_level, b_level (and of course item_id) and SELECT WHERE a_level= b_level= item_id= will it be bad? I guess only INDEX on b_level and item_id and SELECT WHERE b_level= AND item_id= will be enough/the best solution?

Those queries are not bad given the indexes I just presented.

The only reason why the item_id appears in all the indexes is to make those indexes covering indexes. That way, the retrieval of data is done using the index only, bypassing the table.

Mysql – Very slow update with subquery (different tables)

The subquery will be executed for each row from the outer query, and no index will be used. In addition, 0.6 sec is "relatively" long. For example, if you have 1000 rows in domain table, then expect more them 600 sec (10 minutes).

In the second query, MySQL is able to use the index on 'domain'. This is a feature of using "IN" instead of subquery.

Best Answer

Related Solutions

Mysql – How to index this table (a_level, b_level, item_id)

Mysql – Very slow update with subquery (different tables)

Related Question