ST_Simplify
ST_Simplify returns a simplified copy of the input geometry using the RamerDouglasPeucker algorithm with the given tolerance.
The topology of the input geometry might not be preserved.
For more information about the algorithm, see Ramer–Douglas–Peucker algorithm
When ST_Simplify calculates distances to simplify a geometry, ST_Simplify operates on the 2D projection of the input geometry.
Syntax
ST_Simplify(geom, tolerance)
Arguments
 geom

A value of data type
GEOMETRY
or an expression that evaluates to aGEOMETRY
type.  tolerance

A value of data type
DOUBLE PRECISION
that represents the tolerance level of the RamerDouglasPeucker algorithm. If tolerance is a negative number, then zero is used.
Return type
GEOMETRY
.
The spatial reference system identifier (SRID) value of the returned geometry is the SRID value of the input geometry.
The dimension of the returned geometry is the same as that of the input geometry.
If geom is null, then null is returned.
Examples
The following SQL simplifies the input linestring using a Euclidean distance tolerance of 1 with the RamerDouglasPeucker algorithm. The units of the distance are the same as those of the coordinates of the geometry.
SELECT ST_AsEWKT(ST_Simplify(ST_GeomFromText('LINESTRING(0 0,1 2,1 1,2 2,2 1)'), 1));
st_asewkt

LINESTRING(0 0,1 2,2 1)