Date: Thu, 1 Dec 2022 19:21:24 -0600
From: Justin Pryzby <pryzby@telsasoft.com>
To: Paul McGarry <paul@paulmcgarry.com>
Cc: pgsql-performance@lists.postgresql.org
Subject: Re: Odd Choice of seq scan
Message-ID: <20221202012124.GA14156@telsasoft.com>
References: 
 <CAPrE0Sahe_J1R3iN1kfyFR1j7prXE-SaqH5rK5JG-u2ToAzUJA@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: 
 <CAPrE0Sahe_J1R3iN1kfyFR1j7prXE-SaqH5rK5JG-u2ToAzUJA@mail.gmail.com>
User-Agent: Mutt/1.9.4 (2018-02-28)
Archived-At: 
 <https://www.postgresql.org/message-id/20221202012124.GA14156%40telsasoft.com>
Precedence: bulk

On Fri, Dec 02, 2022 at 11:52:19AM +1100, Paul McGarry wrote:
> Hi there,
> 
> I'm wondering if anyone has any insight into what might make the database
> choose a sequential scan for a query (table defs and plan below) like :

> Plan - seq scan of table:
> =====
> > explain select orders.orderid FROM orders WHERE (orders.orderid IN ('546111') OR orders.orderid IN  (select orderid FROM orderstotrans WHERE (transid IN ('546111'))));

> Plan - Seq scan and filter of index:
> =====
> > explain select orders.orderid FROM orders WHERE (orders.orderid IN ('546111') OR orders.orderid IN  (select orderid FROM orderstotrans WHERE (transid IN ('546111'))));

Could you show explain analyze ?

Show the size of the table and its indexes 
And GUC settings
And the "statistics" here:
https://wiki.postgresql.org/wiki/Slow_Query_Questions#Statistics:_n_distinct.2C_MCV.2C_histogram
Maybe on both a well-behaving instance and a badly-beving instance.

-- 
Justin