There's a bitmap of which columns in the table allow NULL. This
contains as many bits as there are NULL-able columns in the table,
rounded up to the nearest byte. When a column is NULL, the bit is set to 1.
That said, I'm not sure whether the CHAR's storage space is still
present in the row when the column is NULL. I think it is. Maybe
someone else knows that offhand.
Baron
Josh wrote:
Thanks for all of your help/information. One additional question... do NULL
values take up any space?
For example, if I have a column defined as:
repAccess char(1) default null
When a user should have access to run a particular report, repAccess will be
set to 'T'. If not, it is left null.
In this example, the rows with 'T' will occupy an additional 1 btye for storing
the single character, however, for rows with null... does that take up space?
I'm only asking to give me an idea of what sort of space NULL values take up.
One of my tables has hundreds of thousands of rows and could potentially have
many null values... I'm trying to get an idea of whether or not those null
values are taking up much space.
Thanks.
----- Original Message ----
From: Baron Schwartz <[EMAIL PROTECTED]>
To: Dan Nelson <[EMAIL PROTECTED]>
Cc: Josh <[EMAIL PROTECTED]>; mysql@lists.mysql.com
Sent: Sunday, October 28, 2007 9:25:11 AM
Subject: Re: Table Size
Dan Nelson wrote:
In the last episode (Oct 27), Baron Schwartz said:
InnoDB has the following extra things, plus some things I might forget:
a) the primary key B-Tree
b) row versioning information for every row
c) 16k page size; each page might not be completely full
Those are all counted towards the table size.. Actually, the primary
key B-Tree might not be; I'd need to look that up. But I think it
is. Hmmmm. I just tested -- yes, the PK counts towards table size.
In fact, in InnoDB, all indexes count towards table size, since there
is a single .ibd file for the whole thing. So you've got the space
taken up by your `repid` index to consider as well..
It's true they're in the same file, but the secondary indexes show up in
the 'Index_length' column in SHOW TABLE STATUS. I was double-checking
that the primary key contributes to the 'Data_length' column, not the
'Index_length' column.
--
Baron Schwartz
Xaprb LLC
http://www.xaprb.com/
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