+++ /dev/null
-Welcome to the Initial version of LWGEOM.
-
-More information is available on the PostGIS user's mailing list and
-the PostGIS developer's mailing list.
-
-See "Installing', below, on how to build.
-
-
-Differences
------------
-
-The LWGEOM is very much like the original PostGIS GEOMETRY type. The
-main differences are:
-
-a) LWGEOMs are much smaller than the PostGIS GEOMETRY
-b) LWGEOMs natively support 2d, 3d, and 4d points
-c) LWGEOMs are indexed using single-precision bounding boxes. This
- make the indexes significantly smaller than PostGIS GEOMETRY
- indexes.
-d) LWGEOMs are internally very similar to OGC WKB
-e) The "normal" view of LWGEOMs is OGC WKB - PostGIS GEOMETRY is OGC WKT
-f) PostGIS geometries have a built-in BOX3D. LWGEOMs have an *optional*
- BOX2D (see below).
-
-
-Also included with the LWGEOMs is a type called 'box2d'. This is
-very similar to PostGIS BOX3D type:
-
-a) BOX2Ds are 2D - BOX3D is 3D
-b) BOX2Ds are represented by single-precision floating point numbers,
- while BOX3Ds are double-precision floating point numbers.
-c) BOX2Ds will sometimes be slightly larger than you might expect.
- This is because the conversion from double-precision to
- single-precision is inexact. When this happens, the BOX2D will
- automatically expand itself.
-
-
-Installing
-----------
-1. Build PostGIS normally
-2. Go into the "lwgeom/" directory underneath your PostGIS installation
-3. type 'make'
-4. Install PostGIS normally into your database
-5. Install LWGEOM into your database using the produced "lwgeom.sql" file.
- \i lwgeom.sql
-6. If you don't get any errors, LWGEOM is installed
-
-Testing
--------
-1. type this in your database:
- SELECT asText('POINT(0 0)'::lwgeom);
-2. it should respond with
- POINT(0 0)
- if it didn't, LWGEOM did not install properly.
-3. There are three regression tests to run. They are located
- in the "lwgeom/regress/" directory.
- ./run_regress
- ./run_regress2
- ./run_regress3
- When you run these, it should be obvious if it passes or
- fails.
-
-
-Usage
------
-
-There are not many actual LWGEOM functions available. Currently, there
-are:
-
-1. all the functions necessary to support the BOX2D and LWGEOM types
-2. functions required to build indexes on LWGEOMs
-3. conversion and casting functions to move between LWGEOM and GEOMETRYs
-4. OGC Well Known Text (WKT) and OGC Well Known Binary (WKB) parsing and
- writing functions (Thanks to Ralph Mason).
-
-This means you can do most normal things with LWGEOMs.
-
-
--- create a simple table. Notice that the_geom column is of
--- type 'lwgeom'
-CREATE TABLE lwgeom_test (the_geom lwgeom, id int);
-
---insert a geometry into the table (WKT version)
-INSERT INTO lwgeom_test VALUES ('POINT(1 1)', 1);
-
--- insert a geometry into the table (WKB version)
--- WKB is usually used by programs NOT humans
-INSERT INTO lwgeom_test VALUES ('0101000000000000000000F03F000000000000F03F', 2);
-
-
--- See whats in the table:
--- NOTICE that the 'normal' view of LWGEOMs is WKB - NOT WKT
-SELECT * FROM lwgeom_test;
- the_geom | id
---------------------------------------------+----
- 0101000000000000000000F03F000000000000F03F | 1
- 0101000000000000000000F03F000000000000F03F | 2
-(2 rows)
-
--- View it as WKT
-SELECT asText(the_Geom),id FROM lwgeom_test;
- astext | id
-------------+----
- POINT(1 1) | 1
- POINT(1 1) | 2
-(2 rows)
-
-
-
--- explicit conversions.
-
--- PostGIS -> LWGEOM
-SELECT lwgeom('POINT(0 0)'::geometry);
-
--- LWGEOM --> PostGIS
-SELECT geometry('POINT(0 0)'::lwgeom);
-
--- You can run PostGIS functions on LWGEOMs. All PostGIS functions
--- are runnable this way.
--- NOTE: this is doing a hidden LWGEOM->PostGIS GEOMETRY step
--- This is actually running the length(GEOMETRY) function.
-SELECT length('LINESTRING(0 0, 0 10)'::lwgeom);
- length
- --------
- 10
- (1 row)
-
-
--- Find the BOX2D bounding box of a geometry
-SELECT box2d('LINESTRING(0 0, 0 10)'::lwgeom);
- box2d
----------------
- BOX(0 0,0 10)
-(1 row)
-
-
-
-
-Building Indexes
-----------------
-
-This is very similar to PostGIS
-(use "GIST_LWGEOM_OPS" instead of GIST_GEOMETRY_OPS):
-
-CREATE INDEX <name> ON <table> USING GIST (<lwgeom column> GIST_LWGEOM_OPS);
-
-ie. CREATE INDEX lwgeom_test_lwgeom_idx ON lwgeom_test
- USING GIST ( the_geom GIST_LWGEOM_OPS);
-
-Don't forget to VACUUM ANALYSE your table:
-VACUUM ANALYSE <table>;
-
-Bounding Boxes
---------------
-
-<this section for advanced users. Ignore it if you don't understand
- what its saying.>
-
-Bounding boxes are optional in LWGEOMs. By not having one, you are
-saving a small amount of space per LWGEOM geometry (16 bytes).
-
-If you are cross-joining tables, you might want to explicitly put a
-bounding box inside the LWGEOM to make it faster.
-
-DROP INDEX <lwgeom index name>;
-UPDATE <table> SET <lwgeom column> = AddBBOX(<lwgeom column>);
-CREATE INDEX <lwgeom index name> ON <table> USING GIST
- (<lwgeom column> GIST_LWGEOM_OPS);
-VACUUM ANALYSE <table>;
-
-
-A cross-joining query looks like this:
-
-SELECT nodes1.id as node_id1,
- nodes2.id as node_id2
-FROM nodes nodes1,
- nodes nodes2
-WHERE nodes1.the_geom && nodes2.the_geom;
-
-For every row in nodes, postgresql searches all the rows in nodes
-for ones that overlap that lwgeom . If nodes has 10,000 rows,
-this query will do 10,000 index searches - THAT A LOT OF WORK.
-
-Pre-computing the bounding boxes makes this type of query much faster.
-
-We have not currently decided if bounding boxes should default to
-being inside the LWGEOM or not. Currently, its set to not being
-automatically there. This may change in the future. If you have
-an opinion, post it to the mailing list.
-
-
-Space Saving
-------------
-
-LWGEOM indexes are approximately 40% smaller than PostGIS indexes.
-
-A LWGEOM 'POINT(0 0)' takes up 21 bytes, a POSTGIS 'POINT(0 0)' takes
-up 140 bytes. This shows that LWGEOMs have a much smaller overhead.
-
-LWGEOMs will store 2D points as 2 double precision numbers (16 bytes) -
-PostGIS will store 2D points with 3 numbers (24 bytes). This can be
-another big savings.
-
-
-Future Development
--------------------
-
-Testing Testing Testing
-Native LWGEOM -> GEOS converter
-Move most of the PostGIS functions so they run directly on LWGEOMs (with
-out conversion)
-
-Stay tuned to the PostGIS mailing lists.
-
-
projects / postgis / commitdiff