</refsection>
</refentry>
+ <refentry id="ST_QuantizeCoordinates">
+ <refnamediv>
+ <refname>
+ ST_QuantizeCoordinates
+ </refname>
+ <refpurpose>
+ Sets least significant bits of coordinates to zero
+ </refpurpose>
+ </refnamediv>
+
+ <refsynopsisdiv>
+ <funcsynopsis>
+ <funcprototype>
+ <funcdef>
+ geometry
+ <function>ST_QuantizeCoordinates</function>
+ </funcdef>
+ <paramdef>
+ <type>geometry</type>
+ <parameter>g</parameter>
+ </paramdef>
+ <paramdef>
+ <type>int</type>
+ <parameter>prec_x</parameter>
+ </paramdef>
+ <paramdef>
+ <type>int</type>
+ <parameter>prec_y</parameter>
+ </paramdef>
+ <paramdef>
+ <type>int</type>
+ <parameter>prec_z</parameter>
+ </paramdef>
+ <paramdef>
+ <type>int</type>
+ <parameter>prec_m</parameter>
+ </paramdef>
+ </funcprototype>
+ </funcsynopsis>
+ </refsynopsisdiv>
+
+ <refsection>
+ <title>Description</title>
+ <para>
+ <code>ST_QuantizeCoordinates</code> determines the number of bits
+ (<code>N</code>) required to represent a coordinate value with a
+ specified number of digits after the decimal point, and then sets
+ all but the <code>N</code> most significant bits to zero. The
+ resulting coordinate value will still round to the original value,
+ but will have improved compressiblity. This can result in a
+ significant disk usage reduction provided that the geometry column
+ is using a <ulink
+ url="https://www.postgresql.org/docs/current/static/storage-toast.html#STORAGE-TOAST-ONDISK">
+ compressible storage type</ulink>. The function allows
+ specification of a different number of digits after the decimal
+ point in each dimension; unspecified dimensions are assumed to have
+ the precsion of the <code>x</code> dimension. Negative digits are
+ interpreted to refer digits to the left of the decimal point, (i.e.,
+ <code>prec_x=-2</code> will preserve coordinate values to the
+ nearest 100.
+ </para>
+ <para>
+ The coordinates produced by <code>ST_QuantizeCoordinates</code> are
+ independent of the geometry that contains those coordinates and the
+ relative position of those coordinates within the geometry. As a result,
+ existing topological relationships between geometries are unaffected
+ by use of this function. The function may produce invalid geometry
+ when it is called with a number of digits lower than the intrinsic
+ precision of the geometry.
+ </para>
+ <para>Availability: 2.5.0</para>
+ </refsection>
+ <refsection>
+ <title>Technical Background</title>
+ <para>
+ PostGIS stores all coordinate values as double-precision floating
+ point integers, which can reliably represent 15 significant digits.
+ However, PostGIS may be used to manage data that intrinsically has
+ fewer than 15 significant digits. An example is TIGER data, which is
+ provided as geographic coordinates with six digits of precision
+ after the decimal point (thus requiring only nine significant digits
+ of longitude and eight significant digits of latitude.)
+ </para>
+ <para>
+ When 15 significant digits are available, there are many possible
+ representations of a number with 9 significant digits. A double
+ precision floating point number uses 52 explicit bits to represent
+ the significand (mantissa) of the coordinate. Only 30 bits are needed
+ to represent a mantissa with 9 significant digits, leaving 22
+ insignificant bits; we can set their value to anything we like and
+ still end up with a number that rounds to our input value. For
+ example, the value 100.123456 can be represented by the floating
+ point numbers closest to 100.123456000000, 100.123456000001, and
+ 100.123456432199. All are equally valid, in that
+ <code>ST_AsText(geom, 6)</code> will return the same result with
+ any of these inputs. As we can set these bits to any value,
+ <code>ST_QuantizeCoordinates</code> sets the 22 insignificant
+ bits to zero. For a long coordinate sequence this creates a
+ pattern of blocks of consecutive zeros that is compressed
+ by PostgreSQL more effeciently.
+ </para>
+
+ <note>
+ <para>
+ Only the on-disk size of the geometry is potentially affected by
+ <code>ST_QuantizeCoordinates</code>. <xref linkend="ST_MemSize"></xref>,
+ which reports the in-memory usage of the geometry, will return the
+ the same value regardless of the disk space used by a geometry.
+ </para>
+ </note>
+ </refsection>
+
+ <refsection>
+ <title>Examples</title>
+
+ <programlisting>SELECT ST_AsText(ST_QuantizeCoordinates('POINT (100.123456 0)'::geometry, 4));
+st_astext
+-------------------------
+POINT(100.123455047607 0)
+ </programlisting>
+
+ <programlisting>WITH test AS (SELECT 'POINT (123.456789123456 123.456789123456)'::geometry AS geom)
+SELECT
+ digits,
+ encode(ST_QuantizeCoordinates(geom, digits), 'hex'),
+ ST_AsText(ST_QuantizeCoordinates(geom, digits))
+FROM test, generate_series(15, -15, -1) AS digits;
+
+digits | encode | st_astext
+--------+--------------------------------------------+------------------------------------------
+15 | 01010000005f9a72083cdd5e405f9a72083cdd5e40 | POINT(123.456789123456 123.456789123456)
+14 | 01010000005f9a72083cdd5e405f9a72083cdd5e40 | POINT(123.456789123456 123.456789123456)
+13 | 01010000005f9a72083cdd5e405f9a72083cdd5e40 | POINT(123.456789123456 123.456789123456)
+12 | 01010000005c9a72083cdd5e405c9a72083cdd5e40 | POINT(123.456789123456 123.456789123456)
+11 | 0101000000409a72083cdd5e40409a72083cdd5e40 | POINT(123.456789123456 123.456789123456)
+10 | 0101000000009a72083cdd5e40009a72083cdd5e40 | POINT(123.456789123455 123.456789123455)
+9 | 0101000000009072083cdd5e40009072083cdd5e40 | POINT(123.456789123418 123.456789123418)
+8 | 0101000000008072083cdd5e40008072083cdd5e40 | POINT(123.45678912336 123.45678912336)
+7 | 0101000000000070083cdd5e40000070083cdd5e40 | POINT(123.456789121032 123.456789121032)
+6 | 0101000000000040083cdd5e40000040083cdd5e40 | POINT(123.456789076328 123.456789076328)
+5 | 0101000000000000083cdd5e40000000083cdd5e40 | POINT(123.456789016724 123.456789016724)
+4 | 0101000000000000003cdd5e40000000003cdd5e40 | POINT(123.456787109375 123.456787109375)
+3 | 0101000000000000003cdd5e40000000003cdd5e40 | POINT(123.456787109375 123.456787109375)
+2 | 01010000000000000038dd5e400000000038dd5e40 | POINT(123.45654296875 123.45654296875)
+1 | 01010000000000000000dd5e400000000000dd5e40 | POINT(123.453125 123.453125)
+0 | 01010000000000000000dc5e400000000000dc5e40 | POINT(123.4375 123.4375)
+-1 | 01010000000000000000c05e400000000000c05e40 | POINT(123 123)
+-2 | 01010000000000000000005e400000000000005e40 | POINT(120 120)
+-3 | 010100000000000000000058400000000000005840 | POINT(96 96)
+-4 | 010100000000000000000058400000000000005840 | POINT(96 96)
+-5 | 010100000000000000000058400000000000005840 | POINT(96 96)
+-6 | 010100000000000000000058400000000000005840 | POINT(96 96)
+-7 | 010100000000000000000058400000000000005840 | POINT(96 96)
+-8 | 010100000000000000000058400000000000005840 | POINT(96 96)
+-9 | 010100000000000000000058400000000000005840 | POINT(96 96)
+-10 | 010100000000000000000058400000000000005840 | POINT(96 96)
+-11 | 010100000000000000000058400000000000005840 | POINT(96 96)
+-12 | 010100000000000000000058400000000000005840 | POINT(96 96)
+-13 | 010100000000000000000058400000000000005840 | POINT(96 96)
+-14 | 010100000000000000000058400000000000005840 | POINT(96 96)
+-15 | 010100000000000000000058400000000000005840 | POINT(96 96)
+</programlisting>
+
+ </refsection>
+
+ <refsection>
+ <title>See Also</title>
+
+ <para><xref linkend="ST_SnapToGrid" /></para>
+ </refsection>
+
+ </refentry>
+
+
<refentry id="ST_RemovePoint">
<refnamediv>
<refname>ST_RemovePoint</refname>
</refsection>
</refentry>
- <refentry id="ST_QuantizeCoordinates">
- <refnamediv>
- <refname>
- ST_QuantizeCoordinates
- </refname>
- <refpurpose>
- Sets least significant bits of coordinates to zero
- </refpurpose>
- </refnamediv>
-
- <refsynopsisdiv>
- <funcsynopsis>
- <funcprototype>
- <funcdef>
- geometry
- <function>ST_QuantizeCoordinates</function>
- </funcdef>
- <paramdef>
- <type>geometry</type>
- <parameter>g</parameter>
- </paramdef>
- <paramdef>
- <type>int</type>
- <parameter>prec_x</parameter>
- </paramdef>
- <paramdef>
- <type>int</type>
- <parameter>prec_y</parameter>
- </paramdef>
- <paramdef>
- <type>int</type>
- <parameter>prec_z</parameter>
- </paramdef>
- <paramdef>
- <type>int</type>
- <parameter>prec_m</parameter>
- </paramdef>
- </funcprototype>
- </funcsynopsis>
- </refsynopsisdiv>
-
- <refsection>
- <title>Description</title>
- <para>
- <code>ST_QuantizeCoordinates</code> determines the number of bits
- (<code>N</code>) required to represent a coordinate value with a
- specified number of digits after the decimal point, and then sets
- all but the <code>N</code> most significant bits to zero. The
- resulting coordinate value will still round to the original value,
- but will have improved compressiblity. This can result in a
- significant disk usage reduction provided that the geometry column
- is using a <ulink
- url="https://www.postgresql.org/docs/current/static/storage-toast.html#STORAGE-TOAST-ONDISK">
- compressible storage type</ulink>. The function allows
- specification of a different number of digits after the decimal
- point in each dimension; unspecified dimensions are assumed to have
- the precsion of the <code>x</code> dimension. Negative digits are
- interpreted to refer digits to the left of the decimal point, (i.e.,
- <code>prec_x=-2</code> will preserve coordinate values to the
- nearest 100.
- </para>
- <para>
- The coordinates produced by <code>ST_QuantizeCoordinates</code> are
- independent of the geometry that contains those coordinates and the
- relative position of those coordinates within the geometry. As a result,
- existing topological relationships between geometries are unaffected
- by use of this function. The function may produce invalid geometry
- when it is called with a number of digits lower than the intrinsic
- precision of the geometry.
- </para>
- <para>Availability: 2.5.0</para>
- </refsection>
- <refsection>
- <title>Technical Background</title>
- <para>
- PostGIS stores all coordinate values as double-precision floating
- point integers, which can reliably represent 15 significant digits.
- However, PostGIS may be used to manage data that intrinsically has
- fewer than 15 significant digits. An example is TIGER data, which is
- provided as geographic coordinates with six digits of precision
- after the decimal point (thus requiring only nine significant digits
- of longitude and eight significant digits of latitude.)
- </para>
- <para>
- When 15 significant digits are available, there are many possible
- representations of a number with 9 significant digits. A double
- precision floating point number uses 52 explicit bits to represent
- the significand (mantissa) of the coordinate. Only 30 bits are needed
- to represent a mantissa with 9 significant digits, leaving 22
- insignificant bits; we can set their value to anything we like and
- still end up with a number that rounds to our input value. For
- example, the value 100.123456 can be represented by the floating
- point numbers closest to 100.123456000000, 100.123456000001, and
- 100.123456432199. All are equally valid, in that
- <code>ST_AsText(geom, 6)</code> will return the same result with
- any of these inputs. As we can set these bits to any value,
- <code>ST_QuantizeCoordinates</code> sets the 22 insignificant
- bits to zero. For a long coordinate sequence this creates a
- pattern of blocks of consecutive zeros that is compressed
- by PostgreSQL more effeciently.
- </para>
-
- <note>
- <para>
- Only the on-disk size of the geometry is potentially affected by
- <code>ST_QuantizeCoordinates</code>. <xref linkend="ST_MemSize"></xref>,
- which reports the in-memory usage of the geometry, will return the
- the same value regardless of the disk space used by a geometry.
- </para>
- </note>
- </refsection>
-
- <refsection>
- <title>Examples</title>
-
- <programlisting>SELECT ST_AsText(ST_QuantizeCoordinates('POINT (100.123456 0)'));
-st_astext
--------------------------
-POINT(100.123455941677 0)
- </programlisting>
-
- <programlisting>
-WITH test AS (SELECT 'POINT (123.456789123456 123.456789123456)'::geometry AS geom)
-SELECT
- digits,
- encode(ST_QuantizeCoordinates(geom, digits), 'hex'),
- ST_AsText(ST_QuantizeCoordinates(geom, digits))
-FROM test, generate_series(15, -15, -1) AS digits;
-
-digits | encode | st_astext
---------+--------------------------------------------+------------------------------------------
-15 | 01010000005f9a72083cdd5e405f9a72083cdd5e40 | POINT(123.456789123456 123.456789123456)
-14 | 01010000005f9a72083cdd5e405f9a72083cdd5e40 | POINT(123.456789123456 123.456789123456)
-13 | 01010000005f9a72083cdd5e405f9a72083cdd5e40 | POINT(123.456789123456 123.456789123456)
-12 | 01010000005c9a72083cdd5e405c9a72083cdd5e40 | POINT(123.456789123456 123.456789123456)
-11 | 0101000000409a72083cdd5e40409a72083cdd5e40 | POINT(123.456789123456 123.456789123456)
-10 | 0101000000009a72083cdd5e40009a72083cdd5e40 | POINT(123.456789123455 123.456789123455)
-9 | 0101000000009072083cdd5e40009072083cdd5e40 | POINT(123.456789123418 123.456789123418)
-8 | 0101000000008072083cdd5e40008072083cdd5e40 | POINT(123.45678912336 123.45678912336)
-7 | 0101000000000070083cdd5e40000070083cdd5e40 | POINT(123.456789121032 123.456789121032)
-6 | 0101000000000040083cdd5e40000040083cdd5e40 | POINT(123.456789076328 123.456789076328)
-5 | 0101000000000000083cdd5e40000000083cdd5e40 | POINT(123.456789016724 123.456789016724)
-4 | 0101000000000000003cdd5e40000000003cdd5e40 | POINT(123.456787109375 123.456787109375)
-3 | 0101000000000000003cdd5e40000000003cdd5e40 | POINT(123.456787109375 123.456787109375)
-2 | 01010000000000000038dd5e400000000038dd5e40 | POINT(123.45654296875 123.45654296875)
-1 | 01010000000000000000dd5e400000000000dd5e40 | POINT(123.453125 123.453125)
-0 | 01010000000000000000dc5e400000000000dc5e40 | POINT(123.4375 123.4375)
--1 | 01010000000000000000c05e400000000000c05e40 | POINT(123 123)
--2 | 01010000000000000000005e400000000000005e40 | POINT(120 120)
--3 | 010100000000000000000058400000000000005840 | POINT(96 96)
--4 | 010100000000000000000058400000000000005840 | POINT(96 96)
--5 | 010100000000000000000058400000000000005840 | POINT(96 96)
--6 | 010100000000000000000058400000000000005840 | POINT(96 96)
--7 | 010100000000000000000058400000000000005840 | POINT(96 96)
--8 | 010100000000000000000058400000000000005840 | POINT(96 96)
--9 | 010100000000000000000058400000000000005840 | POINT(96 96)
--10 | 010100000000000000000058400000000000005840 | POINT(96 96)
--11 | 010100000000000000000058400000000000005840 | POINT(96 96)
--12 | 010100000000000000000058400000000000005840 | POINT(96 96)
--13 | 010100000000000000000058400000000000005840 | POINT(96 96)
--14 | 010100000000000000000058400000000000005840 | POINT(96 96)
--15 | 010100000000000000000058400000000000005840 | POINT(96 96)
-</programlisting>
-
- </refsection>
-
- <refsection>
- <title>See Also</title>
-
- <para><xref linkend="ST_SnapToGrid" /></para>
- </refsection>
-
- </refentry>
</sect1>
projects / postgis / commitdiff