Comments on: Clone a table in MySQL without killing your server

By: Bill

Bill — Wed, 10 Mar 2010 20:02:17 +0000

I recently looked this article back up because I am a new company and needed this script again. Great script with one exception. The limit command starts to deteriorate in performance when you have larger tables (at least with MyISAM which we were using). I had a database with 40M rows and after a million or two performance came to a crawl. I don’t have access to the script anymore, but at the time I rewrote it to use the Primary key column instead. You do run into dead spots if they table has had deletes but for the most part it is quick. Be sure to order on the primary key if you use that method.

By: Ryan

Ryan — Mon, 13 Oct 2008 12:37:02 +0000

@Skip

Yeah but the “CREATE TABLE new LIKE old;” method will create the new table with the same indexes as the old table. The idea in this case seems to be to create the table without any indexes. “CREATE TABLE new AS SELECT * from old;” will create new with the same schema as old, but without any indexes.

By: Seun Osewa

Seun Osewa — Mon, 13 Oct 2008 01:55:17 +0000

Methinks the best way to deal with unused indexes is to just drop them. It doesn’t take any time at all because indexes are stored separately.

By: Shantanu Oak

Shantanu Oak — Sun, 12 Oct 2008 12:47:40 +0000

For medium sized tables, use outfile. Test case…
mysql> SELECT * INTO OUTFILE ’shantanu.txt’ FIELDS TERMINATED BY ‘|’ OPTIONALLY ENCLOSED BY ‘”‘ LINES TERMINATED BY ‘\n’ FROM bad;
Query OK, 2681793 rows affected (59.72 sec)

mysql> alter table good disable keys;
Query OK, 0 rows affected (0.00 sec)

mysql> LOAD DATA INFILE ’shantanu.txt’ INTO TABLE good
FIELDS TERMINATED BY ‘|’ OPTIONALLY ENCLOSED BY ‘”‘;
Query OK, 2681793 rows affected (3 min 49.91 sec)
Records: 2681793 Deleted: 0 Skipped: 0 Warnings: 0

mysql> alter table good enable keys;
Query OK, 0 rows affected (11 min 1.68 sec)

You can use Lock on source table so that writes will temporarily halt but can continue once the data is out.
LOCK TABLES bad WRITE;
UNLOCK TABLES;

By: Skip

Skip — Fri, 10 Oct 2008 16:41:20 +0000

An even simpler way to create the new table would be:

CREATE TABLE new LIKE old;