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Slow UPDATE's on a large table
Posted by: Srdan Dukic
Date: June 24, 2010 10:11PM

Hi,

I have a MyISAM table of approximately 13 million rows. This table contains rows which must be processed. Once a row is processed, its status is changed from "unprocessed" to "processed".

I've written a program to grab 1000 rows at a time, process them and then update the status of the rows to "processed".

My problem is that the program starts out fast, but the "update" part of the program slows down, at what seems, an exponential rate. This means that when I've left the program running for 5-6 hours, waiting for the "update" step to finish takes 99% of the programs running time, while at the start it's only about 20%. Consequently, the overall program running time increases as well.

How can I improve the update performance of this very large MyISAM table? I've tried using a 'LOCK TABLE aTable WRITE' command before the update, with a 'UNLOCK TABLES' after the update, but this resulted only in a marginal improvement in performance, with the time to update still increasing.

Are there any server variables which I can tweak?

Thank you

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