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Concatenating MyISAM files

 | November 19, 2012 |  Posted In: MySQL


Recently, I found myself involved in the migration of a large read-only InnoDB database to MyISAM (eventually packed). The only issue was that for one of the table, we were talking of 5 TB of data, 23B rows. Not small… I calculated that with something like insert into MyISAM_table… select * from Innodb_table… would take about 10 days. The bottleneck was clearly the lack of concurrency on the read part from InnoDB and then the key management for MyISAM. The server has many dozen drives so it was easy to add more concurrency so I kicked off, from a script, insertions into 16 identical MyISAM files for distinct parts of the table. That was much faster and would complete within a day.

Then, while the Innodb extraction was running at a nice pace, I thought about the next phase. My first idea was simply to do insert into MyISAM_table select * from MyISAM_table1 and so on for the 16 files. Since MyISAM are flat files, that should be faster, especially with the keys disabled. At that point, I remembered, from a previous disaster recovery work where a database directory has been wiped out that the MyISAM files have no headers which make them difficult (read almost impossible) to locate on a drive with tools like ext3grep. No headers… that means the first byte of byte of a file is the first byte of the first row… So we should be able to concatenate these files. Let’s see.

Then, at the shell command line:

And then, back in mysql:

So, yes, you can concatenate MyISAM files, even when multiple keys are defined. Not for everyday use but still pretty cool.


Following Peter’s comment, I added varchar and deleted rows to the mix:

So varchar columns are supported without any issue but, deleted rows prevent the use of the quick option for myisamchk.

Yves Trudeau

Yves is a Principal Consultant at Percona, specializing in distributed technologies such as MySQL Cluster, Pacemaker and XtraDB cluster. He was previously a senior consultant for MySQL and Sun Microsystems. He holds a Ph.D. in Experimental Physics.


  • Yves,

    It works in simple case I wonder how it works if tables have deleted rows (would not delete chain be all weird) and also how about dynamic row format (Varchar etc) would it also work or is it fixed row format feature only ?

  • Wouldn’t it be safer to use something like Mydumper/Myloader which will do parallel dump/load.

    Combine that with dropping the secondary indexes and it speeds up things.

    Here is an article about Mydumper


    And here is a presentation on deleting the secondary indexes, though it is for InnoDB, but should work for MyISAM too


  • Yves, You sir are an evil genius. Not only for figuring out how to do this, but for all the support calls from when it is inappropriately applied. I mean this in only the greatest terms of respect 😉

  • I have been using ‘myisamchk -rq’ to quickly drop, create, alter table indexes with great success, so long as the table does not have any deleted records. This can be done by copying the .frm and .MYI from a table with exact same column definitions, having different indexes, back onto the original table. This technique cuts the cpu/clock time by nearly 2/3rd compared to traditional SQL ‘ALTER TABLE x ADD/DROP INDEX’, which rewrites the whole .MYD.

    CREATE TABLE z LIKE x; — duplicate the structure of the original table.
    ALTER TABLE z DROP INDEX PRIMARY, DROP INDEX my_idx, ADD INDEX PRIMARY ( f1, f2, f3, … ) — Define new keys
    LOCK TABLE x WRITE, z WRITE; — safety first, lock the tables involved
    FLUSH TABLE x,z; — safety, allow usage of cp and myisamchk while databse is HOT.
    unix> cp z.frm x.frm — take new table
    unix> cp z.MYI x.MYI — take empty index
    unix> myisamchk -rqaS x — quickly rebuild the index

    Note: ‘Lock Table x Write’ and ‘Flush Table x’ commands are required to safely run myisamchk ( or myisampack ) when the database is hot.

    — J Jorg —

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