The database server can process only data in memory. It must read rows into memory to evaluate those rows against the filters of a query. Once the database server finds rows that satisfy those filters, it prepares an output row in memory by assembling the selected columns.
Most of these activities are performed quickly. Depending on the computer and its workload, the database server can perform hundreds or even thousands of comparisons each second. As a result, the time spent on in-memory work is usually a small part of the execution time.
Although some in-memory activities, such as sorting, take a significant amount of time, it takes much longer to read a row from disk than to examine a row that is already in memory.
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