索引是否可以帮助包含大量列但记录数量较少的表？

Question

I am under the assumption that indexes help you find particular records based on hashing on a combination of 1 or more columns. If this is the case, then a large table with a lot of columns but few records will not benefit much from indexes. Is this correct?

Answer 1

That's correct. On smaller tables (and depending on the query) it is going to be more efficient to just scan the whole table than to assemble the results based on a seek on a specific index and then a lookup to get the rest of the results.

There is no magic row count or other formula here, though. Too many variables to make a blanket statement. Just something you should be aware of.