[dba-SQLServer] Undo Query

Just idle curiosity, does an "undo query" happen at the same 
rate as a "do query"?

I started a query running to append records from a ninety 
million record table to a table that contains a subset of 
the fields.  Basically I have a denormalized source table 
with name, name2, name3 etc fields.  Each of these have 
fname, mname, gender, age etc.  Since these are denormalized 
"family" records, there are fewer Name2 records than Name1, 
fewer still Name3 etc.

I neglected to put in a "where name2 is not null" clause and 
30 minutes into the second append I realized that.  I 
canceled the query and it is still "undoing" the query. 
Which led me to wonder the relative efficiency of "doing" vs 
"undoing".

No Indexes in place on the target table of course.

Any ideas on the relative efficiencies?  Is undoing an 
append much slower than the append?

-- 
John W. Colby
www.ColbyConsulting.com