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(mysql.info)Update speed

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Speed of `UPDATE' Queries
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Update queries are optimized 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.  *Note `OPTIMIZE TABLE':
OPTIMIZE TABLE.