Re: Odd Choice of seq scan

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