From: Tom Lane <tgl@sss.pgh.pa.us>
To: Bill Measday <bill@measday.com>
cc: pgsql-performance@postgresql.org
Subject: Re: Substantial different index use between 9.5 and 9.6
In-reply-to: <c5d43892-7382-5105-67d0-c7800eac77f8@measday.com>
References: <c5d43892-7382-5105-67d0-c7800eac77f8@measday.com>
Comments: In-reply-to Bill Measday <bill@measday.com>
	message dated "Fri, 02 Dec 2016 10:38:50 +1100"
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Date: Thu, 01 Dec 2016 18:48:12 -0500
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Bill Measday <bill@measday.com> writes:
> Substantial different index use between 9.5 and 9.6

Maybe you missed an ANALYZE after migrating?  The plan difference
seems to be due to a vast difference in rowcount estimate for the
m_elevations condition:

>       ->  Bitmap Heap Scan on m_elevations e 
> (cost=282802.21..37401439.43 rows=3512160 width=8)

>       ->  Seq Scan on m_elevations e 
> (cost=10000000000.00..13296950520.12 rows=3512159563 width=8)

If you don't know where that factor-of-1000 came from, maybe take
it up with the postgis folk.  It'd mostly be coming out of their
selectivity estimation routines.

			regards, tom lane


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