slow start update big table

Hi,

We have a table of 12 million records and we  want update 1 column of 1,9 million rows so we know the present records are old values. No indexes on that table, primary key= 4 columns, update column not in pk.

When we do a simple count(*) of these rows, we see (taskmanager) dbeng9.exe increase from 36 MB to 400 MB and it takes a while. It's like the table become total in memory. Is this normal behavior? How can we speed this up? Because it's a 1 time a week this must be happen and we can't keep the table in memory just for that.

ASA 9.0.2.3702

PB 11.5