Straight from the MySQL Documentation, which is where you should look first

    5.2.10 Speed of UPDATE Queries

    Update queries are optimised as a SELECT query with the additional
    overhead of a write. The speed of the write is dependent on the size of
    the data that is being updated and the number of indexes that are
    updated. Indexes that are not changed will not be updated.

    Also, another way to get fast updates is to delay updates and then do
    many updates in a row later. Doing many updates in a row is much
    quicker than doing one at a time if you lock the table.

    Note that, with dynamic record format, updating a record to a longer
    total length may split the record. So if you do this often, it is very
    important to OPTIMIZE TABLE sometimes. See section 4.5.1 OPTIMIZE TABLE

If you aren't doing an update of 50K per row, updating 3 columns instead of
1 will be "much quicker" (quoted from the manual).  If you are doing 10,000
updates, read more of the manual on the syntax of UPDATE because you can
delay your updates which allows MYSQL to update the table at its leisure
which offers better performance.

Peter, who reminds you to always read the manual (or less kindly RTFM!).

On Sun, 10 Nov 2002, Leif K-Brooks wrote:

> I'm wondering how significant the performance differences between:
> mysql_query("update table set col1='val1' where whatever='whatever'");
> and
> mysql_query("update table set col1='val1',col2='val2',col3='val3'...
> where whatever='whatever'");
> --
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Peter Beckman            Systems Engineer, Fairfax Cable Access Corporation
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