Thanks Tom.

First, this wasn't a migration but new db loaded from scratch (if that matters).

As per the end of the original post "I have vacuum analysed both tables". I assume this is what you meant?

My gut feel was that it isn't a postgis issue since the third example I gave uses the index, but I will take it up with them too.



On 2/12/2016 10:48 AM, Tom Lane wrote:
Bill Measday <> 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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