Обсуждение: 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!
Hi
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!
"hehaochen@hotmail.com" <hehaochen@hotmail.com> writes: > The default is enable_bitmapscan on. However, TPC-H.query17 get slower running on my NVMe SSD (WD SN850) after switchingon the parameter: latency drop from 9secs to 16secs. During a B-tree Index Scan, bitmapscan optimization convertsrandom I/O into sequential. However, many users use SSDs rather than HDDs. But they may not know the trick. Is therea 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