Nested-Loops
Table Physical I/O
•In the
ideal case (which is surprisingly close to the common case!), the
joined-to table is accessed roughly in row-creation order.
•Read-ahead
in the disk subsystem effectively converts reads like this
into multi-block reads.
•These
two effects lead to physical I/O much like a fractional table scan
of only the top of the heap table (which is likely largely cached, anyway).