random_page_cost v10

Parameter Type: Floating point

Default Value: 4

Range: 0 to 1.79769e+308

Minimum Scope of Effect: Per session

When Value Changes Take Effect: Immediate

Required Authorization to Activate: Session user

Sets the planner's estimate of the cost of a non-sequentially-fetched disk page. The default is 4.0. This value can be overridden for a particular tablespace by setting the tablespace parameter of the same name. (Refer to the ALTER TABLESPACE command in the PostgreSQL Core Documentation.)

Reducing this value relative to seq_page_cost will cause the system to prefer index scans; raising it will make index scans look relatively more expensive. You can raise or lower both values together to change the importance of disk I/O costs relative to CPU costs, which are described by the cpu_tuple_cost and cpu_index_tuple_cost parameters.

The default value assumes very little caching, so it's frequently a good idea to reduce it. Even if your database is significantly larger than physical memory, you might want to try setting this parameter to 2 (rather than its default of 4) to see whether you get better query plans that way. If your database fits entirely within memory, you can lower this value much more, perhaps to 0.1.

Although the system will let you do so, never set random_page_cost less than seq_page_cost. However, setting them equal (or very close to equal) makes sense if the database fits mostly or entirely within memory, since in that case there is no penalty for touching pages out of sequence. Also, in a heavily-cached database you should lower both values relative to the CPU parameters, since the cost of fetching a page already in RAM is much smaller than it would normally be.