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Seems to be a replicable issue in PostGis - ticket raised at
their end, so I'll wait for a resolution of the root cause.
Thanks for your help/thoughts.
Rgds
Bill
On 3/12/2016 2:41 AM, Daniel Blanch
Bataller wrote:
ANALYZE takes samples at random, so statistics might
be different even with same postgresql version:
For large tables, ANALYZE takes a random sample of the table contents,
rather than examining every row. This allows even very large
tables to be analyzed in a small amount of time. Note,
however, that the statistics are only approximate, and will
change slightly each time ANALYZE is run, even if the actual table contents did not
change. This might result in small changes in the planner's
estimated costs shown by EXPLAIN. In rare situations, this non-determinism will
cause the planner's choices of query plans to change after ANALYZE is run. To avoid this, raise the amount of
statistics collected by ANALYZE, as described below.
Though, having
that round (x 1000) difference, my bet is that you have
different statistics target whether on database, table or
columns, see:
The extent of analysis can be
controlled by adjusting the default_statistics_target configuration variable, or on a column-by-column
basis by setting the per-column statistics target with ALTER TABLE ...
ALTER COLUMN ... SET STATISTICS (see ALTER
TABLE). The target value sets the maximum
number of entries in the most-common-value list and the
maximum number of bins in the histogram. The default target
value is 100, but this can be adjusted up or down to trade
off accuracy of planner estimates against the time taken
for ANALYZE and the amount of space occupied in pg_statistic. In particular, setting the statistics target to
zero disables collection of statistics for that column. It
might be useful to do that for columns that are never used
as part of the WHERE, GROUP BY, or ORDER BY clauses of
queries, since the planner will have no use for statistics
on such columns.
Here is some
help on how to see statistics per column:
Check if this is the case.
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.
Rgds
Bill
On 2/12/2016 10:48 AM, Tom Lane wrote:
Bill Measday <bill@xxxxxxxxxxx> 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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