size estimation of null fields and indexes with nulls

[richard horan]

I am trying to determine sizing information for various designs for our application. I am trying to determine the best way to layout our DB with the least amount of memory (because it is very large, ~8GB) and still get optimal performance. Any information would be greatly appreciated.

I am trying to determine how much space nulls take, in particular the following:

1. for Varchars, the cluster documentation on dataMemory settings says the following:
"For variable-size table attributes in MySQL 5.1, the data is stored on separate datapages, allocated from DataMemory. Variable-length records use a fixed-size part with an extra overhead of 4 bytes to reference the variable-size part. The variable-size part has 2 bytes overhead plus 2 bytes per attribute. "

is this overhead always included even if the field is null? does one have to include this overhead for the index that is created too?

2. when sizing the amount of memory is needed for a unique index and that unique index allows nulls how do I size that index? how much space do null entries take? is there an entry in that index for every single null that is in the index?