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Asked: 2026-05-19T00:52:04+00:00

I have a table with quite a lot entries. I need an additional column

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I have a table with quite a lot entries.
I need an additional column with an integer value or null.
The thing is that only very few rows will have that field populated.

So i wonder whether its better to create a seperate table where i link the entries in an 1:1 relation.

I know one integer entry takes 4 bytes in mysql/myisam. If I have the column set to allow null values, and only 100 of 100 000 rows have the field populated, will the rest still consume 4 bytes for every null value?

Or is mysql intelligent enough to set the value where it is populated and just regard everything as null, where nothing is set?

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1 Answer

  1. Editorial Team

    This depends on the ROW_FORMAT value you give when you create your table.

    Before version 5.0.3, the default format is set to “REDUNDANT” : any fixed-length field will use the same space, even if it’s value is NULL.

    Starting with version 5.0.3, the value is set to “COMPACT” : NULL values will never use any space in your database.

    You can do an ALTER TABLE to be sure to use the correct format :

    ALTER TABLE ... ROW_FORMAT=COMPACT

    More details here :
    http://dev.mysql.com/doc/refman/5.1/en/data-size.html

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