As we all know, full table scans can be very expensive, poor old SQL is forced to read every single row in a table (of course that doesn’t always mean that it’s a bad choice for SQL).
Lets assume we’ve got a table scan happening that results in 1,000,000 page reads, that’s quite a bit of work for SQL to do. Now imagine another query comes in and needs to scan the same table, that’s also going to need to do 1,000,000 reads to get the data that it needs. If this table happens to be frequently accessed, this is soon going add up.
There’s a clever solution which tends to work better and better as you have more and more queries scanning the table.