| From: | Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> |
|---|---|
| To: | "hehaochen(at)hotmail(dot)com" <hehaochen(at)hotmail(dot)com> |
| Cc: | "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: change the default value of enable_bitmapscan to off |
| Date: | 2023-01-14 15:00:29 |
| Message-ID: | CAFj8pRBKy7vWpN0-jxk24UnEiEbqA5-gFzyjbiO9gkHmuw-fig@mail.gmail.com |
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| Lists: | pgsql-performance |
Hi
so 14. 1. 2023 v 15:51 odesílatel hehaochen(at)hotmail(dot)com <
hehaochen(at)hotmail(dot)com> 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!
>
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