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Re: Slow Update on large table myisam
Posted by: BOZAPE PEDRO BOZA
Date: January 16, 2014 08:23PM

* Let's focus on the indexes. Why, you ask? Because some of them need to be updated, and that is part of the performance problem. More in a minute.

* You are asking to update (I think) _all_ 5 million rows. This necessarily takes time, since all the data will need changing, plus much of the indexes -- many GB of I/O. It may take hours, simply because disks are not fast enough.

One with 5 million records using 1.2 GB of disk space.

>>>nacimientos is with 5 million record

Which table is that??

* Do not index ENUMs, sex, and other fields that have only a few distinct values. Such INDEXes will never be used. Meanwhile, they slow down INSERT and UPDATE and DELETE.

* All of your indexes have a single field. The MySQL optimizer (almost) never uses two indexes in a single SELECT. You probably need some 'compound' indexes. Let's see some of your SELECTs.

* You are using MyISAM. So, the UPDATE that died changed some of the rows, but not all. Ugh. InnoDB is a better choice. If you change, then the second table would need a PRIMARY KEY.

>>>The reason I use myisam is cause the UPDATE procedure it is made no more than one a month and the database is getting a lot of SELECT query by customers. So as I understand innodb is slow with SELECT.

* It is not good practice to duplicate data between two tables. Please justify it before we discuss that further. Normally one would not have sex (etc) duplicated, but instead do a JOIN in the SELECT.

>>>The reason I duplicate if cause I think it is fast to do a select in just 1 single table with less records the in 2 tables, one with 5 million records.

>>>Thanks so so muchs for the reply

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