From: Tom Lane <tgl@sss.pgh.pa.us>
To: hehaochen@hotmail.com <hehaochen@hotmail.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: change the default value of enable_bitmapscan to off
Date: Sat, 14 Jan 2023 10:57:07 -0500
Message-ID: <1005543.1673711827@sss.pgh.pa.us> (raw)
In-Reply-To: <OSZP286MB1725E184F8E925D4A10B2687A4C39@OSZP286MB1725.JPNP286.PROD.OUTLOOK.COM>
References: <OSZP286MB1725E184F8E925D4A10B2687A4C39@OSZP286MB1725.JPNP286.PROD.OUTLOOK.COM>
"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 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
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, hehaochen@hotmail.com
Subject: Re: change the default value of enable_bitmapscan to off
In-Reply-To: <1005543.1673711827@sss.pgh.pa.us>
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