<para>

Random access to mechanical disk storage is normally much more expensive

than four-times sequential access. However, a lower default is used

(4.0) because the majority of random accesses to disk, such as indexed

reads, are assumed to be in cache. The default value can be thought of

as modeling random access as 40 times slower than sequential, while

expecting 90% of random reads to be cached.

</para>

<para>

If you believe a 90% cache rate is an incorrect assumption

for your workload, you can increase random_page_cost to better

reflect the true cost of random storage reads. Correspondingly,

if your data is likely to be completely in cache, such as when

the database is smaller than the total server memory, decreasing

random_page_cost can be appropriate. Storage that has a low random

read cost relative to sequential, e.g. solid-state drives, might

also be better modeled with a lower value for random_page_cost.

</para>