Sorry to hijack the thread, I also recently have similar observation that the statement about random_page_cost on SSD is ambiguous. The current document says that
> 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. However, this statement does not clarify what values might be good. For some workload, the default value 4.0 would cause bad performance and lowering random_page_cost to a value 3.0 or 2.0 does not solve the performance problem. Only when the random_page_cost is lowered to below 1.2 will the bad performance be mitigated. Thus, I would suggest elaborating on this description further as: > Storage that has a low random read cost relative to sequential, e.g. solid-state drives, might also be better modeled with a value that is close to 1 for random_page_cost. Detail: I run the PostgreSQL 11 on an SSD hardware. The database has two small tables with 6MB and 16MB separately. The pgbench runs a select join query in 1 min. The result shows that when the random_page_cost is 1, the average latency is 14ms. When the random_page_cost is 1.5, 2, 3 or 4, the average latency is 26ms. This result suggests that setting random_page_cost to a value larger than 1.5 would cause almost 2x latency. If I increase the 6MB table to 60MB and rerun the sysbench, the result shows that when the random_page_cost is 1, the average latency is 13ms. When the random_page_cost is 1.5,2,3 or 4, the average latency is 17ms. I attached my testing script, the postgresql configuration file, and planner output. On Mon, Apr 27, 2020 at 3:19 AM Олег Самойлов <spl...@ya.ru> wrote: > Yep. Unclear. What parameter is recommended for SSD? Lower? 3? 2? 1? > > Much better will be write: if you use SSD set 1. > > Олег > > > 19 марта 2020 г., в 23:56, Bruce Momjian <br...@momjian.us> написал(а): > > > > On Thu, Feb 27, 2020 at 02:48:44PM +0000, PG Doc comments form wrote: > >> The following documentation comment has been logged on the website: > >> > >> Page: https://www.postgresql.org/docs/12/runtime-config-query.html > >> Description: > >> > >> Explanation for random_page_cost is rather outdated, because it did > only for > >> case of mechanical hdd. But all modern database servers, which I know, > made > >> upon SSD. Do or not do default value for random_page_cost equal to 1 is > the > >> question, but, IMHO, at list in the documentation about > random_page_cost > >> need to add in a speculation about SSD. > >> > >> It's important because a business programming now is mostly web > programming. > >> Most database is poorly designed by web programmer, tables looked like a > >> primary key and a huge json (containing all) with large gin index upon > it. > >> Now I am seeing a table with a GIN index 50% of the table size. The > database > >> is on SSD, of cause. With default random_page_cost=4 GIN index don't > used > >> by planner, but with random_page_cost=1 the result may be not > excellent, but > >> acceptable for web programmers. > > > > Does this sentence in the random_page_cost docs unclear or not have > enough > > visibility: > > > > > https://www.postgresql.org/docs/12/runtime-config-query.html#RUNTIME-CONFIG-QUERY-CONSTANTS > > > > 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. > > > > -- > > Bruce Momjian <br...@momjian.us> https://momjian.us > > EnterpriseDB https://enterprisedb.com > > > > + As you are, so once was I. As I am, so you will be. + > > + Ancient Roman grave inscription + > > > > > >
postgresql.conf
Description: Binary data
query_analytical.sh
Description: application/shellscript
planner_output
Description: Binary data
