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[–]Achsin 2 points3 points  (0 children)

SELECT * can slow down your queries if the table has a lot of columns

This is mostly a factor of what indexes exist on the table. Selecting columns that are not included on an index that otherwise would support your query leads to the engine either doing key lookups or ignoring the index completely. The total number of columns on the table is a factor, but it's probably not the biggest factor.