September 26, 2008 at 10:05 am
I've inherited a job that reads millions of rows and updates an IMAGE data type field to NULL based on a few conditions, including data older than 90 days. It's quite possible a lot of rows are already NULL. Do you think performance would be improved by adding:
AND Image_field is not NULL ?
Or does checking the value of the field add more overhead ?? I suppose it partly depends on the total percentage that will get updated, but I though I'd ask if there's a general "rule".
Thanks
EDIT.... looks as though less than 10% of the rows will actually need to get updated. 90% - 98% are already NULL
September 26, 2008 at 11:16 am
It can help if there's an index, but not sure that it would help on an index column.
September 26, 2008 at 11:59 am
Including that in the Where clause should at least reduce the amount of logging overhead from the update, since less rows will be logged. I generally include that kind of thing in my code.
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