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On 16/09/2016 10:01 PM, <postgis-users-requ...@lists.osgeo.org> wrote:

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> Today's Topics:
>
>    1. what does it really mean for one geometry to be   equal to
>       another (Rhys A.D. Stewart)
>    2. Re: what does it really mean for one geometry to be equal to
>       another (Sandro Santilli)
>    3. Re: what does it really mean for one geometry to be equal to
>       another (Paul Norman)
>    4. Re: what does it really mean for one geometry to be equal to
>       another (Paul Norman)
>    5. Re: what does it really mean for one geometry to be equal to
>       another (Paul Norman)
>    6. Re: what does it really mean for one geometry to be equal to
>       another (Sandro Santilli)
>    7. pl/pgsql function to write table in parallel mode (Nicolas Ribot)
>    8. Re: fuzzy tolerance (Willy-Bas Loos)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Thu, 15 Sep 2016 20:53:25 -0500
> From: "Rhys A.D. Stewart" <rhys.stew...@gmail.com>
> To: PostGIS Users Discussion <postgis-users@lists.osgeo.org>
> Subject: [postgis-users] what does it really mean for one geometry to
>         be      equal to another
> Message-ID:
>         <CACg0vTkjgR1ZhuDsHK1dZOKi=K=RBXbxa+qmq6E46Q0ZurK2=w@mail.
> gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> Greetings all,
>
> I maintain a medium size table of customer locations, which, for business
> purposes now needs to not have any coincident points. Table definition
> follows:
> =====================================================================
> service.location
> (
>   premises text NOT NULL,
>   matchtype text,
>   matchdate date,
>   connectedtransformer text,
>   g geometry(Point,3448),
>   CONSTRAINT servicelocation_pkey PRIMARY KEY (premises),
>   CONSTRAINT servicelocation_premisesnumber_check CHECK
> (char_length(premises) = 6 OR char_length(premises) = 7),
>   CONSTRAINT servicelocation_premisesnumber_is_a_number_check CHECK
> (premises !~* '[A-z]+'::text)
> )
> =====================================================================
>
> There are approximately 866k rows, and a gist index on g. I update the
> table so that no geometries are coincident ( see
> https://gist.github.com/rhysallister/bcb4bb07a99d69938fff88f150883bee for
> the sql to remove the coincident geoms) I ran the sql in the gist until it
> said 0 rows affected.
>
> To prevent one from inserting or updating a coincident geometry I try to
> create a unique index on g. Since gist doesn't support unique indices I do:
>
> =====================================================================
> CREATE unique INDEX unique_g ON service.location (st_astext(g) );
> ---------------------------------------------------------------------
> ERROR:  could not create unique index "unique_g"
> DETAIL:  Key (st_astext(g))=(POINT(727895.4 663599.3)) is duplicated.
> =====================================================================
> This makes me slightly flummoxed. I'm pretty sure the query in the gist
> returned 0 affected rows. But, maybe I missed a step. I try to find the
> offending rows with:
>
> =====================================================================
> select premises, st_astext(g), g from service.location
> where st_equals(g, 'SRID=3448;POINT(727895.4 663599.3)'::geometry)
> ---------------------------------------------------------------------
> premises st_astext g
> 267077 POINT(727895.4 663599.3)
> 0101000020780D0000CDCCCCCCAE3626419A9999995E402441
> =====================================================================
> Strange. I now move to being slightly perturbed. I'm very sure the previous
> error message made mention of duplicity. I then run
>
> =====================================================================
> select premises, st_astext(g), g from service.location
> where st_astext(g) = 'POINT(727895.4 663599.3)'
> ---------------------------------------------------------------------
> premises st_astext g
> 267077 POINT(727895.4 663599.3)
> 0101000020780D0000CDCCCCCCAE3626419A9999995E402441
> 267053 POINT(727895.4 663599.3)
> 0101000020780D0000CDCCCCCCAE362641999999995E402441
> =====================================================================
>
> Now I'm just confused, the 2 premises have the same st_astext, but
> different wkb representations and as such are not being caught in the
> st_equals call.
>
>
> Is there some gotcha that I don't know about, maybe something in the docs
> that I missed or is this not supposed to happen?
>
> Rhys
> Peace & Love|Live Long & Prosper
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> ------------------------------
>
> Message: 2
> Date: Fri, 16 Sep 2016 07:11:55 +0200
> From: Sandro Santilli <s...@kbt.io>
> To: PostGIS Users Discussion <postgis-users@lists.osgeo.org>
> Subject: Re: [postgis-users] what does it really mean for one geometry
>         to be equal to another
> Message-ID: <20160916051155.GB4828@localhost>
> Content-Type: text/plain; charset=us-ascii
>
> On Thu, Sep 15, 2016 at 08:53:25PM -0500, Rhys A.D. Stewart wrote:
>
> > Now I'm just confused, the 2 premises have the same st_astext, but
> > different wkb representations and as such are not being caught in the
> > st_equals call.
> >
> > Is there some gotcha that I don't know about, maybe something in the docs
> > that I missed or is this not supposed to happen?
>
> What you describe is expected:
> the text representation is approximated/truncated.
>
> You could create your index on ST_AsBinary(geometry) if you wanted
> to check binary-level equality, but as your subject asks: what's
> your concept of equality ? Do you really want those practically
> identical points in your db ? Or you could create an index on
> ST_SnapToGrid(geom, <tolerance>), for points to be no closer
> than <tolerance>. Or (for lines) you could use ST_HausdorffDistance.
>
> Many ways to interpret equality, which is why the equality operator
> is currently just checking for approximated minimum bounding box equality
> (probably still equal for your two almost-identical points).
>
> --strk;
>
>   ()   Free GIS & Flash consultant/developer
>   /\   https://strk.kbt.io/services.html
>
>
> ------------------------------
>
> Message: 3
> Date: Thu, 15 Sep 2016 23:19:49 -0700
> From: Paul Norman <penor...@mac.com>
> To: postgis-users@lists.osgeo.org
> Subject: Re: [postgis-users] what does it really mean for one geometry
>         to be equal to another
> Message-ID: <702ce217-2a87-be77-13f1-63bd1a34c...@mac.com>
> Content-Type: text/plain; charset="utf-8"; Format="flowed"
>
> On 9/15/2016 6:53 PM, Rhys A.D. Stewart wrote:
> > =====================================================================
> > select premises, st_astext(g), g from service.location
> > where st_astext(g) = 'POINT(727895.4 663599.3)'
> > ---------------------------------------------------------------------
> > premisesst_astextg
> > 267077POINT(727895.4
> > 663599.3)0101000020780D0000CDCCCCCCAE3626419A9999995E402441
> > 267053POINT(727895.4
> > 663599.3)0101000020780D0000CDCCCCCCAE362641999999995E402441
> > =====================================================================
> >
> > Now I'm just confused, the 2 premises have the same st_astext, but
> > different wkb representations and as such are not being caught in the
> > st_equals call.
> >
> >
> > Is there some gotcha that I don't know about, maybe something in the
> > docs that I missed or is this not supposed to happen?
>
> A safe bet is that you're hitting floating point issues. EWKB is the
> canonical format for geometries, but the conversion to a text
> representation could lose some precision. If you did want to require
> unique geometries, you could do it with a btree index on the geometry,
> not st_astext of the geometry.
>
> As a general rule, comparing two floating point numbers for equality is
> tricky. What you probably want is an exclusion constraint which prevents
> two points from being within a small distance of each other.
>
> I don't know of a great way to do this, but a bad way that might work is
> EXCLUDE USING GIST (ST_Buffer(geom, 0.1) WITH &&). See
> https://www.postgresql.org/docs/current/static/rangetypes.html#RANGETYPES-
> CONSTRAINT
> and the links from there. I haven't tested this.
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> ------------------------------
>
> Message: 4
> Date: Thu, 15 Sep 2016 23:37:38 -0700
> From: Paul Norman <penor...@mac.com>
> To: postgis-users@lists.osgeo.org
> Subject: Re: [postgis-users] what does it really mean for one geometry
>         to be equal to another
> Message-ID: <9221777f-f4c2-ce2b-e19a-680ce943c...@mac.com>
> Content-Type: text/plain; charset=utf-8; format=flowed
>
> On 9/15/2016 11:19 PM, Paul Norman wrote:
> > I don't know of a great way to do this, but a bad way that might work
> > is EXCLUDE USING GIST (ST_Buffer(geom, 0.1) WITH &&). See
> > https://www.postgresql.org/docs/current/static/
> rangetypes.html#RANGETYPES-CONSTRAINT
> > and the links from there. I haven't tested this.
>
> After further though, this is doing a bounding box comparison so it's
> not quite 0.1 projected units and you can use ST_Expand instead of
> ST_Buffer.
>
>
> ------------------------------
>
> Message: 5
> Date: Thu, 15 Sep 2016 23:58:28 -0700
> From: Paul Norman <penor...@mac.com>
> To: postgis-users@lists.osgeo.org
> Subject: Re: [postgis-users] what does it really mean for one geometry
>         to be equal to another
> Message-ID: <4ac8f86f-fae3-b5f5-6880-88fe0e5c0...@mac.com>
> Content-Type: text/plain; charset=utf-8; format=flowed
>
> On 9/15/2016 10:11 PM, Sandro Santilli wrote:
> > you could create an index on
> > ST_SnapToGrid(geom, <tolerance>), for points to be no closer
> > than <tolerance>.
>
> This won't check that points are no closer than <tolerance>, nor will it
> solve floating point issues. It's the same as rounding. If two points
> are on either side of the line where they round to different grid
> points, even if they are arbitrarily close, up to the limit of precision.
>
> It does mean that you will see fewer values where you get floating point
> equality issues, but the difference between the rounded floating points
> will increase.
>
>
>
> ------------------------------
>
> Message: 6
> Date: Fri, 16 Sep 2016 09:11:50 +0200
> From: Sandro Santilli <s...@kbt.io>
> To: PostGIS Users Discussion <postgis-users@lists.osgeo.org>
> Subject: Re: [postgis-users] what does it really mean for one geometry
>         to be equal to another
> Message-ID: <20160916071150.GA9706@localhost>
> Content-Type: text/plain; charset=us-ascii
>
> On Thu, Sep 15, 2016 at 11:19:49PM -0700, Paul Norman wrote:
>
> > If you did want to require
> > unique geometries, you could do it with a btree index on the
> > geometry, not st_astext of the geometry.
>
> The btree opclass for geometry only checks bounding box equality,
> not geometry equality (see lwgeom_eq, aka OPERATOR=).
>
> --strk;
>
>   ()   Free GIS & Flash consultant/developer
>   /\   https://strk.kbt.io/services.html
>
>
> ------------------------------
>
> Message: 7
> Date: Fri, 16 Sep 2016 12:32:43 +0200
> From: Nicolas Ribot <nicolas.ri...@gmail.com>
> To: PostGIS Users Discussion <postgis-users@lists.osgeo.org>
> Subject: [postgis-users] pl/pgsql function to write table in parallel
>         mode
> Message-ID:
>         <CAGAwT=2AM14QetUCihrKAr+1+aQ+bdebkYiAYhp9WUtj4i0zpQ@mail.
> gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> Hi,
>
> Playing with new PG9.6rc1 / Pgis 2.3beta1, I found parallel query mode to
> be really efficient to process big tables.
> Unfortunately, it is not possible to directly create tables with parallel
> plan (create table as select...) (see:
> https://wiki.postgresql.org/wiki/Parallel_Query).
> It is possible, though, to use copy mode with psql feed to create a table
> with parallel plan enabled.
>
> To allow creating tables directly in pure SQL script, I developed a small
> hack function that takes a SQL query and creates a table from it, using
> COPY command with psql PROGRAM executing the query.
>
> Usage:
>
> select * from create_table_parallel(
>     'table_name',
>     'select p.id as idparc, c.gid as idcarreau
>         st_intersection(p.geom, c.geom) as geom
>       from parcelle_sample2 p
>       join carreau_sample2 c on st_intersects(p.geom, c.geom)',
>     '/usr/local/pgsql-9.6/bin/psql -A -t -p 5439 -d nicolas -c',
>     8, -- workers
>     true);
>
> Limitations:
>
> • delimiter used for copy operation defaults to '|'
> • a 'LIMIT 0' clause is inserted at the end of the passed query to create
> table structure: query to run cannot contain a LIMIT clause.
> • The function is not safe, as it injects user parameters to build psql
> command, and it's not extensively tested.
>
> Perfomance expected:
>
> Depends on the number of workers configured and planned:
> On a small dataset (~15 000 pg intersected with 360 000 pg), with 8 workers
> configured and 3 choosen by the planner, table creation took *24s vs 1m25s*
> with a traditionnal create table as select...
>
> Nicolas
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> ------------------------------
>
> Message: 8
> Date: Fri, 16 Sep 2016 17:20:05 +0200
> From: Willy-Bas Loos <willy...@gmail.com>
> To: PostGIS Users Discussion <postgis-users@lists.osgeo.org>, Daniel
>         Baston <dbas...@gmail.com>,  Lars Aksel Opsahl <
> lars.ops...@nibio.no>
> Subject: Re: [postgis-users] fuzzy tolerance
> Message-ID:
>         <CAHnozTi76La74wU0TXE68YLn9TQPHJUbDMYcyR0hCMruROmkRg@mail.
> gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> On Thu, Sep 15, 2016 at 1:58 PM, Daniel Baston <dbas...@gmail.com> wrote:
>
> > > Any words of warning about using a trigger and storing the data on a 10
> > cm
> > > grid like i suggest?
> >
>
> Wow, thanks for the great responses. Lars Opsahl, nice to see you in the
> mailing list :D
> So what i gather from this is that it is not ideal to use st_snaptogrid. It
> solves some problems, but it creates some new ones too.
> Maybe a second geometry column would be a better idea, so that the original
> is still there (and will consume your server's memory &hdd space :/ )
> Anyway, there is no automatic way to solve the problem right now.
>
> So how big are the problems that arise from this?
> For me i have to say that we often have problems with errors in overlays,
> and we have to keep using st_makevalid after every step of a process.
> Decreasing the supersmall artifacts in the geometries would probably help
> with that.
> @Lars Opsahl, you describe a lot of problems or very reasonable wishes in
> your presentation (link to abstract in previous mail). Do you think those
> could be solved with a concept similar to fuzzy tolerance?
> @Daniel Baston could you describe some of the problems that the
> hyperprecise coordinates cause for you?
>
> Cheers,
>
> --
> Willy-Bas Loos
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