Mnesia and the performance of variable size records

[Rudolph van Graan]

Hi all,

I have a question regarding the use of records with fields where the 
content size changes. As an example, take the following record:

-record(some_record,
     {key,field1,detailrecords=[]}).

This is stored in mnesia as an ordered set.

What would the performance implications be where you have a large 
number of records, where the content of detailrecords is a list, where 
the size is not constant? In a relational database, one would normally 
define a field with a specific size or of variable length and the 
database will allocate pages for the storage of the extra data. How 
does mnesia do this? If I  have to guess for best performance in 
mnesia, the records all need to be more or less the same size? We can 
potentially have millions of records like this, but the size of the 
variable field is a couple of entries, usually less than 10.

The reason I would want to do this, is to avoid having a table with 
detail rows and include the detail rows simply as a list in the master 
record. Doing this would avoid a query on a second table, but I am not 
sure how the performance of the master table would suffer  doing it 
this way.

The structure that I would like to represent looks somewhat like:

<Master Record 1>	
	<Detail Record 1>
	<Detail Record 2>
	. . .

<Master Record 2>
	<Detail Record 1>
	<Detail Record 2>
	. . .

The relational database schema would have been:

Table 1:
*Key
Data
Data

Table 2:
*Key
*Index
*Data
*Data

My mnesia implementation simply stores the detail records as items in a 
list, where the list is a field of the master record. Obviously, this 
makes the coding trivial, but what performance implications does this 
have?

Thank you again!

Rudolph