![]() However this particular kind of query is quite used. That said, we should not take performances into consideration when designing queries unless necessary, as we can expect optimizers to improve over time. SELECT timestamp, value, cardīut without an index, two passes on the data will be necessary whereas the previous query can find the solution with only one scan. To circumvent this, one can use MAX in the subquery instead of the correlation. The drawback is that our intent is not immediately clear when having a glimpse on this query. It is my favorite solution, and the one I tend to use. The typical way of doing this is to check that no row has a higher timestamp than any row we retrieve. Finally, only one row is returned when several records share the maximum timestamp. However, LIMIT is not standard and as stated by Wikipedia, The SQL standard's core functionality does not explicitly define a default sort order for Nulls. Input must be a point.Easy way: ORDER BY in conjunction with LIMIT SELECT timestamp, value, card ST_Y: Returns the Y coordinate of the point, or NULL if not available. ST_X: Returns the X coordinate of the point, or NULL if not available. ST_StartPoint: Returns the first point of a LINESTRING geometry as a POINT. ST_SRID: Returns the spatial reference identifier for the ST_Geometry as defined in spatial_ref_sys table. ST_Perimeter: Returns the length measurement of the boundary of an ST_Surface or ST_MultiSurface value. ST_NumGeometries: If geometry is a GEOMETRYCOLLECTION (or MULTI*) returns the number of geometries, otherwise return NULL. ST_NRings: If the geometry is a polygon or multi-polygon returns the number of rings. ![]() ST_NPoints: Returns the number of points (vertexes) in a geometry. ST_NDims: Returns coordinate dimension of the geometry as a small int. geometry are in units of spatial reference and geography are in meters (default spheroid) ST_Length: Returns the 2d length of the geometry if it is a linestring or multilinestring. Return NULL if the geometry is not a polygon or the given N is out of range. ST_InteriorRingN: Returns the Nth interior linestring ring of the polygon geometry. ST_GeometryType: Returns the geometry type of the ST_Geometry value. ST_GeomFromWKB: Creates a geometry instance from a Well-Known Binary geometry representation (WKB) and optional SRID. ST_GeomFromText: Returns a specified ST_Geometry value from Well-Known Text representation (WKT). ST_GeomFromKML: Takes as input KML representation of geometry and outputs a PostGIS geometry object ST_GeomFromGML: Takes as input GML representation of geometry and outputs a PostGIS geometry object. ST_GeometryN: Returns the 1-based Nth geometry if the geometry is a GEOMETRYCOLLECTION, MULTIPOINT, MULTILINESTRING, MULTICURVE or MULTIPOLYGON. Return NULL if the geometry is not a polygon. ST_ExteriorRing: Returns a line string representing the exterior ring of the POLYGON geometry. ST_AsSVG: Returns a Geometry in SVG path data given a geometry or geography object. ST_AsKML: Returns the geometry as a KML element. ST_AsGML: Returns the geometry as a GML version 2 or 3 element. ST_AsGeoJSON: Returns the geometry as a GeoJSON element. ST_AsEWKT: Returns the Well-Known Text (WKT) representation of the geometry with SRID meta data. ST_AsEWKB: Returns the Well-Known Binary (WKB) representation of the geometry with SRID meta data. ST_EndPoint: Returns the last point of a LINESTRING geometry as a POINT. ![]() ST_AsBinary: Returns the Well-Known Binary (WKB) representation of the geometry/geography without SRID meta data. ST_AsText: Returns the Well-Known Text (WKT) representation of the geometry/geography without SRID metadata. For “geography” area is in square meters. For “geometry” type area is in SRID units. ST_Area: Returns the area of the surface if it is a polygon or multi-polygon. The following SQL query shows an example of WKB representation (the call to encode() is required to convert the binary output into an ASCII form for printing): Note that in addition to a text parameter with a geometry representation, we also have a numeric parameter providing the SRID of the geometry. The most common use of a constructor is to turn a text representation of a geometry into an internal representation: ST_GeomFromText(text, srid) returns geometry ![]() Fortunately, PostGIS supports emitting and consuming geometries in a large number of formats: In order for external programs to insert and retrieve useful geometries, they need to be converted into a format that other applications can understand. ![]() Within the database, geometries are stored on disk in a format only used by the PostGIS program. ![]()
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