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I have a table that holds information about cities in a game, you can build one building each turn and this is recorded with the value ‘usedBuilding’.
Each turn I will run a script that alters usedBuilding to 0, the question is, which of the following two ways is faster and does it actually matter which way is used?
UPDATE cities SET usedBuilding = 0; UPDATE cities SET usedBuilding = 0 WHERE usedBuilding = 1;
In general, the 2nd case (with the WHERE) clause would be faster – as it won’t cause trigger evaluation, transaction logging, index updating, etc. on the unused rows.
Potentially – depending on the distribution of 0/1 values, it could actually be faster to update all rows rather than doing the comparison – but that’s a pretty degenerate case.
Since ~95% of your query costs are I/O, using the WHERE clause will either make no difference (since the column is not indexed, and you’re doing a table scan) or a huge difference (if the column is indexed, or the table partitioned, etc.). Either way, it doesn’t hurt.
I’d suspect that for the amount of data you’re talking, you won’t notice a difference in either execution plans or speed – which makes it academic at best, premature optimization at worst. So, I’d advise to go with whatever logically makes sense for your app.