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Re: text vs varchar
Posted by: Øystein Grøvlen
Date: May 03, 2016 02:19AM

From the reference manual (http://dev.mysql.com/doc/refman/5.7/en/innodb-restrictions.html):

"If a row is less than half a page long, all of it is stored locally within the page. If it exceeds half a page, variable-length columns are chosen for external off-page storage until the row fits within half a page."

Default page size is 16k. Hence, if a column value is more than 8k, it will definitely be stored off-page. Also, smaller values may be stored off-page if total row size is larger than 8k. Note that the number of bytes used per character depends on the character set used. If stored off-page, it will not be read when during table scans unless the column is actually needed.

Whether it is useful to store columns in a separate table will depend on your access pattern. If you do a lot of table scans or range scans on PRIMARY key, you would want the table to be as small as possible. If you do mostly key look-ups, table size does not matter than much. If you store the columns in a separate table, you will have extra overhead when fetching these columns since a join will be needed.

Hope this helps,

Øystein Grøvlen,
Senior Principal Software Engineer,
MySQL Group, Oracle,
Trondheim, Norway

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