Quickest way to delete enormous MySQL table

Question

I have an enormous MySQL (InnoDB) database with millions of rows in the sessions table that were created by an unrelated, malfunctioning crawler running on the same server a

Answer 1

searlea's answer is nice, but as stated in the comments, you lose the foreign keys during the fight. this solution is similar: the truncate is executed within a second, but you keep the foreign keys.

The trick is that we disable/enable the FK checks.

SET FOREIGN_KEY_CHECKS=0;
CREATE TABLE NewFoo LIKE Foo;
insert into NewFoo SELECT * from Foo where What_You_Want_To_Keep  
    
truncate table Foo;
insert into Foo SELECT * from NewFoo;
SET FOREIGN_KEY_CHECKS=1;

---

Extended answer - Delete all but some rows

My problem was: Because of a crazy script, my table was for with 7.000.000 junk rows. I needed to delete 99% of data in this table, this is why i needed to copy What I Want To Keep in a tmp table before deleteting.

These Foo Rows i needed to keep were depending on other tables, that have foreign keys, and indexes.

something like that:

insert into NewFoo SELECT * from Foo where ID in (
 SELECT distinct FooID from TableA 
 union SELECT distinct FooID from TableB 
 union SELECT distinct FooID from TableC
)

but this query was always timing out after 1 hour. So i had to do it like this:

CREATE TEMPORARY TABLE tmpFooIDS  ENGINE=MEMORY  AS (SELECT distinct FooID from TableA);
insert into tmpFooIDS SELECT distinct FooID from TableB
insert into tmpFooIDS SELECT distinct FooID from TableC
insert into NewFoo SELECT * from Foo where ID in (select ID from tmpFooIDS);

I theory, because indexes are setup correctly, i think both ways of populating NewFoo should have been the same, but practicaly it didn't.

This is why in some cases, you could do like this:

SET FOREIGN_KEY_CHECKS=0;
CREATE TABLE NewFoo LIKE Foo;

-- Alternative way of keeping some data.
CREATE TEMPORARY TABLE tmpFooIDS  ENGINE=MEMORY  AS (SELECT * from Foo where What_You_Want_To_Keep);
insert into tmpFooIDS SELECT ID from Foo left join Bar where OtherStuff_You_Want_To_Keep_Using_Bar
insert into NewFoo SELECT * from Foo where ID in (select ID from tmpFooIDS);

truncate table Foo;
insert into Foo SELECT * from NewFoo;
SET FOREIGN_KEY_CHECKS=1;