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The <emphasis>raison d'etre</emphasis> of spatial databases is to perform queries inside the database which would ordinarily require desktop GIS functionality. Using PostGIS effectively requires knowing what spatial functions are available, how to use them in queries, and ensuring that appropriate indexes are in place to provide good performance.
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An SP-GiST index can accelerate queries involving the following operators:
 
&lt;&lt;, &amp;&lt;, &amp;&gt;, &gt;&gt;, &lt;&lt;|, &amp;&lt;|, |&amp;&gt;, |&gt;&gt;, &amp;&amp;, @&gt;, &lt;@, and ~=, for 2-dimensional indexes,
 
&amp;/&amp;, ~==, @&gt;&gt;, and &lt;&lt;@, for 3-dimensional indexes.
 
There is no support for kNN searches at the moment.
 
Tuning Index Usage
 
Ordinarily, indexes invisibly speed up data access: once an index is built, the PostgreSQL query planner automatically decides when to use it to improve query performance. But there are some situations where the planner does not choose to use existing indexes, so queries end up using slow sequential scans instead of a spatial index.
 
If you find your spatial indexes are not being used, there are a few things you can do:
 
Examine the query plan and check your query actually computes the thing you need. An erroneous JOIN, either forgotten or to the wrong table, can unexpectedly retrieve table records multiple times. To get the query plan, execute with <code>EXPLAIN</code> in front of the query.
 
Make sure statistics are gathered about the number and distributions of values in a table, to provide the query planner with better information to make decisions around index usage. <command>VACUUM ANALYZE</command> will compute both.
 
You should regularly vacuum your databases anyways. Many PostgreSQL DBAs run <command>VACUUM</command> as an off-peak cron job on a regular basis.
 
If vacuuming does not help, you can temporarily force the planner to use the index information by using the command <command>SET ENABLE_SEQSCAN TO OFF;</command>. This way you can check whether the planner is at all able to generate an index-accelerated query plan for your query. You should only use this command for debugging; generally speaking, the planner knows better than you do about when to use indexes. Once you have run your query, do not forget to run <command>SET ENABLE_SEQSCAN TO ON;</command> so that the planner will operate normally for other queries.
 
If <command>SET ENABLE_SEQSCAN TO OFF;</command> helps your query to run faster, your Postgres is likely not tuned for your hardware. If you find the planner wrong about the cost of sequential versus index scans try reducing the value of <varname>RANDOM_PAGE_COST</varname> in <code>postgresql.conf</code>, or use <command>SET RANDOM_PAGE_COST TO 1.1;</command>. The default value for <varname>RANDOM_PAGE_COST</varname> is 4.0. Try setting it to 1.1 (for SSD) or 2.0 (for fast magnetic disks). Decreasing the value makes the planner more likely to use index scans.
 
If <command>SET ENABLE_SEQSCAN TO OFF;</command> does not help your query, the query may be using a SQL construct that the Postgres planner is not yet able to optimize. It may be possible to rewrite the query in a way that the planner is able to handle. For example, a subquery with an inline SELECT may not produce an efficient plan, but could possibly be rewritten using a LATERAL JOIN.
 
For more information see the Postgres manual section on <link xlink:href="https://www.postgresql.org/docs/current/runtime-config-query.html">Query Planning</link>.
 
Spatial Queries
Interrogazioni spaziali
The <emphasis>raison d'etre</emphasis> of spatial databases is to perform queries inside the database which would ordinarily require desktop GIS functionality. Using PostGIS effectively requires knowing what spatial functions are available, how to use them in queries, and ensuring that appropriate indexes are in place to provide good performance.
La ragion d'essere delle basi di dati spaziali è quella di effettuare all'interno del database le interrogazioni che normalmente richiederebbero l'impiego di un sistema GIS desktop. Usare PostGIS in maniera efficace richiede la conoscienza di quali funzioni sono disponibili, come usarle nelle interrogazioni e la creazione di indici che garantiscano una buona performance.
Determining Spatial Relationships
Determinare le relazioni spaziali
Spatial relationships indicate how two geometries interact with one another. They are a fundamental capability for querying geometry.
Le relazioni spaziali indicano come due geometrie interagiscono tra loro. Rappresentano la principale capacità di interrogazione delle geometries.
Dimensionally Extended 9-Intersection Model
Modello dimensionale esteso 9-Intersection
According to the <link xlink:href="http://www.opengeospatial.org/standards/sfs">OpenGIS Simple Features Implementation Specification for SQL</link>, "the basic approach to comparing two geometries is to make pair-wise tests of the intersections between the Interiors, Boundaries and Exteriors of the two geometries and to classify the relationship between the two geometries based on the entries in the resulting 'intersection' matrix."
Secondo la specifica <link xlink:href="http://www.opengeospatial.org/standards/sfs">OpenGIS Simple Features Implementation Specification for SQL</link>, "l'approccio di base per comparare due geometrie è quello di determinare l'interesezione tra gli interni, i confini e gli esterni di ognuna, classificando così la relazione tra le due geometrie attraverso la risultante matrice di 'intersezione'."
In the theory of point-set topology, the points in a geometry embedded in 2-dimensional space are categorized into three sets:
Nella teoria della topologia degli insiemi di punti, i punti di una geometria nello spazio bidimensionale sono categorizzate in 3 insiemi:
<glossterm>Boundary</glossterm>
 
The boundary of a geometry is the set of geometries of the next lower dimension. For <varname>POINT</varname>s, which have a dimension of 0, the boundary is the empty set. The boundary of a <varname>LINESTRING</varname> is the two endpoints. For <varname>POLYGON</varname>s, the boundary is the linework of the exterior and interior rings.
Il confine di una geometria è l'insieme di geometrie della dimensione immediatamente inferiore. Per i <varname>POINT</varname>, che hanno dimensione 0, il confine è l'insieme vuoto. Il confine di una <varname>LINESTRING</varname> è l'insieme dei due punti estremi. Per i <varname>POLYGON</varname>, il confine è l'insieme delle linee che ne formano il ring esterno e i ring interni.
<glossterm>Interior</glossterm>
 
The interior of a geometry are those points of a geometry that are not in the boundary. For <varname>POINT</varname>s, the interior is the point itself. The interior of a <varname>LINESTRING</varname> is the set of points between the endpoints. For <varname>POLYGON</varname>s, the interior is the areal surface inside the polygon.
L'interno di una geometria è costituito da quei punti della geometria che non sono parte del confine. Per i <varname>POINT</varname>, l'interno è il punto stesso. L'interno di una <varname>LINESTRING</varname> è l'insieme dei punti tra i due estremi. Per i <varname>POLYGON</varname>, l'interno è la superficie all'interno del poligono.
<glossterm>Exterior</glossterm>
 
The exterior of a geometry is the rest of the space in which the geometry is embedded; in other words, all points not in the interior or on the boundary of the geometry. It is a 2-dimensional non-closed surface.
L'esterno di una geometria è lo spazio attorno al quale la geometria è immersa; in altre parole, tutti i punti che non fanno parte dell'interno o del confine della geometria. È una superficie non chiusa a 2 dimensioni.
The <link xlink:href="http://en.wikipedia.org/wiki/DE-9IM">Dimensionally Extended 9-Intersection Model</link> (DE-9IM) describes the spatial relationship between two geometries by specifying the dimensions of the 9 intersections between the above sets for each geometry. The intersection dimensions can be formally represented in a 3x3 <emphasis role="bold">intersection matrix</emphasis>.
L'articolo <link xlink:href="http://en.wikipedia.org/wiki/DE-9IM">Dimensionally Extended 9-Intersection Model</link> (DE-9IM) descrive la relazione spatiale tra due geometrie specificando le dimensioni delle 9 intersezioni tra gli insiemi di punti sopracitati di ogni geometria. La dimensione dell'intersezione può essere formalmente rappresentata in una <emphasis role="bold">matrice d'intersezione</emphasis> 3x3.
For a geometry <emphasis>g</emphasis> the <emphasis>Interior</emphasis>, <emphasis>Boundary</emphasis>, and <emphasis>Exterior</emphasis> are denoted using the notation <emphasis>I(g)</emphasis>, <emphasis>B(g)</emphasis>, and <emphasis>E(g)</emphasis>. Also, <emphasis>dim(s)</emphasis> denotes the dimension of a set <emphasis>s</emphasis> with the domain of <literal>{0,1,2,F}</literal>:
Per una geometria <emphasis>g</emphasis> <emphasis>Interno</emphasis>, <emphasis>Confine</emphasis> e <emphasis>Esterno</emphasis> si denotano usando la notazione <emphasis>I(g)</emphasis>, <emphasis>B(g)</emphasis> ed <emphasis>E(g)</emphasis>. Inoltre, <emphasis>dim(s)</emphasis> denota la dimensione di un insieme <emphasis>s</emphasis> nel dominio <literal>{0,1,2,F}</literal>:
<literal>0</literal> =&gt; point
 
<literal>1</literal> =&gt; line
 

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  1. postgis
  2. PostGIS Manual
  3. Italian
The <emphasis>raison d'etre</emphasis> of spatial databases is to perform queries inside the database which would ordinarily require desktop GIS functionality. Using PostGIS effectively requires knowing what spatial functions are available, how to use them in queries, and ensuring that appropriate indexes are in place to provide good performance.
La ragion d'essere delle basi di dati spaziali è quella di effettuare all'interno del database le interrogazioni che normalmente richiederebbero l'impiego di un sistema GIS desktop. Usare PostGIS in maniera efficace richiede la conoscienza di quali funzioni sono disponibili, come usarle nelle interrogazioni e la creazione di indici che garantiscano una buona performance.
10 months ago
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String updated in the repository

  1. postgis
  2. PostGIS Manual
  3. Italian
The <emphasis>raison d'etre</emphasis> of spatial databases is to perform queries inside the database which would ordinarily require desktop GIS functionality. Using PostGIS effectively requires knowing what spatial functions are available, how to use them in queries, and ensuring that appropriate indexes are in place to provide good performance.
 
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