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What does the St _ Union of the geometry column of?
In your query you are using the simple one that is creating the union of two single geometries. What you are doing is cross joining your two tables, and make union of every combination of the geometries of your two tables, resulting in n*m multipolygons, where n is the row count of your first table and m the count of the second one.
How to Union a set of geometry objects?
UnionAggregate (geometry Data Type) Performs a union operation on a set of geometry objects. Syntax. Arguments. Is a geometry type table column that holds the set of geometry objects on which to perform a union operation. Return Types. Exceptions. Throws a FormatException when there are input values that are not valid.
Which is the aggregate variant of the St Union function?
Array variant: returns a geometry that is the union of an array of geometries. Aggregate variant: returns a geometry that is the union of a rowset of geometries. The ST_Union () function is an “aggregate” function in the terminology of PostgreSQL.
How to perform a Union in SQL Server?
Performs a union operation on a set of geometry objects. To view Transact-SQL syntax for SQL Server 2014 and earlier, see Previous versions documentation. Is a geometry type table column that holds the set of geometry objects on which to perform a union operation.
When to use St _ Union or St _ collect?
ST_Collect may sometimes be used in place of ST_Union, if the result is not required to be non-overlapping. ST_Collect is usually faster than ST_Union because it performs no processing on the collected geometries.
How does the St Union array in PostGIS 14 work?
The ST_Union array and set variants use the fast Cascaded Union algorithm described in http://blog.cleverelephant.ca/2009/01/must-faster-unions-in-postgis-14.html A gridSize can be specified to work in fixed-precision space. The inputs are snapped to a grid of the given size, and the result vertices are computed on that same grid.
What does the St Union function do in PostgreSQL?
The ST_Union() function is an “aggregate” function in the terminology of PostgreSQL. That means that it operates on rows of data, in the same way the SUM() and AVG() functions do and like most aggregates, it also ignores NULL geometries.