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Is there a simple way to get a size of the record (in terms of the disk space it takes) with activerecord (my db is mysql)?

I have found several (1, 2) answers offering a way of doing it with sql, I wonder if there is something built in into Activerecord, like MyRecord.first.bytesize.

Size of the table would also do, I need an average row size.

The purpose of this is estimating disk space requirements for a database.

UPDATE

I have also found ObjectSpace.memsize_of

require 'objspace'
ObjectSpace.memsize_of(MyRecord.first)

Is the size of the activerecord object equal to the size of the record in the database?

This seems to give the combined size of all the fields. I am casting to String all the fields that do not respond to size, such as timestamps.

record.attributes.keys.map{ |attr_name| record.send(attr_name)}.inject{|sum, attr| sum + (attr.respond_to?(:size) ? attr.size : attr.to_s.size)  }

And here is the comparison of results of these methods for the same record:

  • combined size of all attributes (see above): 222
  • Marshal::dum(record).size: 2678
  • ObjectSpace.memsize_of(record): 128
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bosskovic
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1 Answers1

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An approximation of size can be found by using:

Marshal::dump(obj).size

Since you said, its for database sizing - The dump has name of classes and instance variables - it may be bit of overhead compared to absolute record size.

You may want to look at this answer for getting MySQL DB size - Database size calculation? and How to Get True Size of MySQL Database?

Based on number of entities currently stored in DB, and size thus determined using above methods - you can extrapolate for sizing.

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Wand Maker
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    what is the difference between `Marshal::dump(obj).size` and `ObjectSpace.memsize_of(obj)` ? The former returns 10+ time bigger size. – bosskovic Dec 14 '15 at 13:17
  • I am not very sure. Lets see if anybody else can throw some light. – Wand Maker Dec 14 '15 at 13:24
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    `ObjectSpace.memsize_of` will be an accurate representation of the size of the Ruby object in memory, and more accurate than `Marshal`. Neither will be the same as the size of the database row. – Jordan Running Dec 14 '15 at 15:37
  • Marshal.dump will include all the instance variables, eg those related to change tracking, associations, various caches etc. Memsize_of counts ruby memory footprint (eg an empty array is 40 bytes on a 64 bit platform) and doesn't include a lot of things – Frederick Cheung Dec 14 '15 at 15:47
  • I thought so, that would explain the difference in size (when compared to other two approaches I listed). – bosskovic Dec 14 '15 at 15:51