[Qgis-developer] Postgresql provider - Use ST_RemoveRepeatedPoints for server-side simplification

[kimaidou]

Hi,

New PR update with addition of missing SIP bindings and use of ST_Simplify
with third parameter "True" to decrease the size of data to download by
QGIS.
Please help to test with various data and report.

Some quick results, with the same data and query as mentioned in the PR, to
show how the number of vertexes decreases. It would lead to best
performance also because the data will be downloaded from the PostGIS faster
query number of vertexes "geom" 3 763 284 st_snaptogrid( "geom", 407.618 )
1 220 892 st_removerepeatedpoints( "geom", 407.618 ) 689 300 st_simplify(
st_removerepeatedpoints( "geom", 407.618 ), 356.665, true ) 303 292

Of course, this is done when viewing the whole layer (zoom to layer
extent). For big scales, this PR will not improve so drastically the
situation compared to the previous use if ST_SnapToGrid

Should we not accept the PR so that many more users can try it with master
branch ?

Cheers
Michaël

2015-11-27 14:27 GMT+01:00 kimaidou <kimaidou at gmail.com>:

> Hi all,
>
> I updated my PR.
> I made some quick tests, and it seems this PR can improve speed for
> PostGIS layers ( with PostGIS 2.2 )
> More details in the last comment [1]
>
> Please help to test it with a variety of data types and complexity.
>
> [1] https://github.com/qgis/QGIS/pull/2410#issuecomment-160138487
>
> Regards
> Michaël aka "kimaidou"
>
> 2015-10-30 11:50 GMT+01:00 kimaidou <kimaidou at gmail.com>:
>
>> Hi all
>>
>> Following the blog post written by Paul Ramsey [1], I decided to propose
>> using PostGIS 2.2 ST_RemoveRepeatedPoints function instead of ST_SnapToGrid
>> to do the server side simplification
>>
>> We could also investigate other new features, such as the 3rd parameter
>> for ST_Simplify, but we use St_SimplifyPreserveTopology in QGIS, and I do
>> not think this function has got this new parameter too.
>>
>> I made a pull request : see [2]
>> Please help me to test and review it
>>
>> [1] http://blog.cartodb.com/smaller-faster/
>> [2] https://github.com/qgis/QGIS/pull/2410
>>
>
>
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