change the default value of enable_bitmapscan to off
The default is enable_bitmapscan on. However, TPC-H.query17 get slower running on my NVMe SSD (WD SN850) after switching on the parameter: latency drop from 9secs to 16secs. During a B-tree Index Scan, bitmapscan optimization converts random I/O into sequential. However, many users use SSDs rather than HDDs. But they may not know the trick. Is there a possibility that can change the default value to off? Thanks!
Re: change the default value of enable_bitmapscan to off
Hi so 14. 1. 2023 v 15:51 odesÃlatel [email protected] < [email protected]> napsal: > The default is enable_bitmapscan on. However, TPC-H.query17 get slower > running on my NVMe SSD (WD SN850) after switching on the parameter: latency > drop from 9secs to 16secs. During a B-tree Index Scan, bitmapscan > optimization converts random I/O into sequential. However, many users use > SSDs rather than HDDs. But they may not know the trick. Is there a > possibility that can change the default value to off? > I don't think it can be disabled by default. When you have fast SSD disk, then common setting is decreasing random_page_cost to some value to 2 or maybe 1.5 Regards Pavel > > Thanks! >
Re: change the default value of enable_bitmapscan to off
"[email protected]" writes: > The default is enable_bitmapscan on. However, TPC-H.query17 get slower > running on my NVMe SSD (WD SN850) after switching on the parameter: latency > drop from 9secs to 16secs. During a B-tree Index Scan, bitmapscan > optimization converts random I/O into sequential. However, many users use > SSDs rather than HDDs. But they may not know the trick. Is there a > possibility that can change the default value to off? Use ALTER SYSTEM SET, or edit postgresql.conf: https://www.postgresql.org/docs/current/config-setting.html Note that changing planner parameters on the basis of a single query getting slower is a classic beginner error. You need to think about the totality of the installation's workload. regards, tom lane
