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MySQL spatial functionality - points of interest around me
This week I was preparing the exercises for our MySQL/MariaDB for Beginners training. One of the exercises of the training is about MySQL spatial (GIS) features. I always tell customers: "With these features you can answer questions like: Give me all points of interest around me!"
Now I wanted to try out how it really works and if it is that easy at all...
To get myself an idea of what I want to do I did a little sketch first:
My position | |
Shops | |
Restaurants | |
Cafes |
To do this I needed a table and some data:
CREATE TABLE poi ( id INT UNSIGNED NOT NULL AUTO_INCREMENT , name VARCHAR(40) , type VARCHAR(20) , sub_type VARCHAR(20) , pt POINT NOT NULL , PRIMARY KEY (id) , SPATIAL INDEX(pt) ) ENGINE=InnoDB; INSERT INTO poi (name, type, sub_type, pt) VALUES ('Shop 1', 'Shop', 'Cloth', Point(2,2)) , ('Cafe 1', 'Cafe', '', Point(11,2)) , ('Shop 2', 'Shop', 'Cloth', Point(5,4)) , ('Restaurant 1', 'Restaurant', 'Portugies', Point(8,7)) , ('Cafe 2', 'Cafe', '', Point(3,9)) , ('Shop 3', 'Shop', 'Hardware', Point(11,9)) ;
This looks as follows:
SELECT id, CONCAT(ST_X(pt), '/', ST_Y(pt)) AS "X/Y", name, type, sub_type FROM poi; +----+-----------+--------------+------------+-----------+ | id | X/Y | name | type | sub_type | +----+-----------+--------------+------------+-----------+ | 1 | 2/2 | Shop 1 | Shop | Cloth | | 2 | 11/2 | Cafe 1 | Cafe | | | 3 | 5/4 | Shop 2 | Shop | Cloth | | 4 | 8/7 | Restaurant 1 | Restaurant | Portugies | | 5 | 3/9 | Cafe 2 | Cafe | | | 6 | 11/9 | Shop 3 | Shop | Hardware | +----+-----------+--------------+------------+-----------+
Now the question: "Give me all shops in a distance of 4.5 units around me":
SET @hereami = POINT(9,4); SELECT id, ST_AsText(pt) AS point, name, ROUND(ST_Distance(@hereami, pt), 2) AS distance FROM poi WHERE ST_Distance(@hereami, pt) < 4.5 AND type = 'Shop' ORDER BY distance ASC ; +----+------------+--------+----------+ | id | point | name | distance | +----+------------+--------+----------+ | 3 | POINT(5 4) | Shop 2 | 4.00 | +----+------------+--------+----------+ 1 row in set (0.37 sec)
The query execution plan looks like this:
id: 1 select_type: SIMPLE table: poi partitions: NULL type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 650361 filtered: 10.00 Extra: Using where; Using filesort
So no use of the spatial index yet. :-(
Reading the MySQL documentation Using Spatial Indexes provides some more information:
The optimizer investigates whether available spatial indexes can be involved in the search for queries that use a function such as MBRContains()
or MBRWithin()
in the WHERE
clause.
So it looks like the optimizer CAN evaluate function covered fields in this specific case. But not with the function ST_Distance
I have chosen.
So my WHERE
clause must look like: "Give me all points within a polygon spanned 4.5 units around my position..."
I did not find any such function in the short run. So I created a hexagon which is not too far from a circle...
With this hexagon I tried again:
SET @hereami = POINT(9,4); SET @hexagon = 'POLYGON((9 8.5, 12.897 6.25, 12.897 1.75, 9 -0.5, 5.103 1.75, 5.103 6.25, 9 8.5))'; SELECT id, ST_AsText(pt) AS point, name, ROUND(ST_Distance(@hereami, pt), 2) AS distance FROM poi WHERE MBRContains(ST_GeomFromText(@hexagon), pt) AND ST_Distance(@hereami, pt) < 4.5 AND type = 'Shop' ORDER BY distance ASC ; Empty set (0.03 sec)
And tadaaah: Damned fast, but the result is not the same! :-( When you look at the graph above it is obvious why: The missing shop is 0.103 units outside of our hexagon search range but within our circle range. So an octagon would have been the better approach...
At least the index is considered now! :-)
id: 1 select_type: SIMPLE table: poi partitions: NULL type: range possible_keys: pt key: pt key_len: 34 ref: NULL rows: 31356 filtered: 10.00 Extra: Using where; Using filesort
For specifying a an "outer" hexagon I was too lazy. So I was specifying a square:
SET @hereami = POINT(9,4); SET @square = 'POLYGON((4.5 8.5, 13.5 8.5, 13.5 -0.5, 4.5 -0.5, 4.5 8.5))'; SELECT id, ST_AsText(pt) AS point, name, ROUND(ST_Distance(@hereami, pt), 2) AS distance FROM poi WHERE MBRContains(ST_GeomFromText(@square), pt) AND ST_Distance(@hereami, pt) < 4.5 AND type = 'Shop' ORDER BY distance ASC ; +----+------------+--------+----------+ | id | point | name | distance | +----+------------+--------+----------+ | 3 | POINT(5 4) | Shop 2 | 4.00 | +----+------------+--------+----------+ 1 row in set (0.02 sec)
So, my shop is in the result again now. And even a bit faster!
Now I wanted to find out if this results are any faster than the conventional method with an index on (x) and (y) or (x, y):
SELECT id, ST_AsText(pt) AS point, name, ROUND(ST_Distance(@hereami, pt), 2) AS distance FROM poi WHERE x >= 4.5 AND x <= 13.5 AND y >= -0.5 AND y <= 8.5 AND ST_Distance(@hereami, pt) < 4.5 AND type = 'Shop' ORDER BY distance ASC ; 1 row in set (0.15 sec)
Here the optimizer chooses the index on x. But I think he could do better. So I forced to optimizer to use the index on (x, y):
SELECT id, ST_AsText(pt) AS point, name, ROUND(ST_Distance(@hereami, pt), 2) AS distance FROM poi FORCE INDEX (xy) WHERE x >= 4.5 AND x <= 13.5 AND y >= -0.5 AND y <= 8.5 AND ST_Distance(@hereami, pt) < 4.5 AND type = 'Shop' ORDER BY distance ASC ; 1 row in set (0.03 sec) id: 1 select_type: SIMPLE table: poi partitions: NULL type: range possible_keys: xy key: xy key_len: 10 ref: NULL rows: 115592 filtered: 1.11 Extra: Using index condition; Using where; Using filesort
Same performance than with the spatial index. So it looks like for this simple task with my data distribution conventional methods do well enough.
No I wanted to try a polygon which comes as close as possible to a circle. This I solved with a MySQL stored function which returns a polygon:/p>
DROP FUNCTION polygon_circle; delimiter // CREATE FUNCTION polygon_circle(pX DOUBLE, pY DOUBLE, pDiameter DOUBLE, pPoints SMALLINT UNSIGNED) -- RETURNS VARCHAR(4096) DETERMINISTIC RETURNS POLYGON DETERMINISTIC BEGIN DECLARE i SMALLINT UNSIGNED DEFAULT 0; DECLARE vSteps SMALLINT UNSIGNED; DECLARE vPolygon VARCHAR(4096) DEFAULT ''; -- Input validation IF pPoints < 3 THEN RETURN NULL; END IF; IF pPoints > 360 THEN RETURN NULL; END IF; IF pPoints > 90 THEN RETURN NULL; END IF; if (360 % pPoints) != 0 THEN RETURN NULL; END IF; -- Start SET vSteps = 360 / pPoints; WHILE i < 360 DO SET vPolygon = CONCAT(vPolygon, (pX + (SIN(i * 2 * PI() / 360) * pDiameter)), ' ', (pY + (COS(i * 2 * PI() / 360) * pDiameter)), ', '); SET i = i + vSteps; END WHILE; -- Add first point again SET vPolygon = CONCAT("POLYGON((", vPolygon, (pX + (SIN(0 * 2 * PI() / 360) * pDiameter)), " ", (pY + (COS(0 * 2 * PI() / 360) * pDiameter)), "))"); -- RETURN vPolygon; RETURN ST_GeomFromText(vPolygon); END; // delimiter ; SELECT ST_AsText(polygon_circle(9, 4, 4.5, 6)); -- SELECT polygon_circle(9, 4, 4.5, 8);
Then calling the query in the same way:
SET @hereami = POINT(9,4); SELECT id, ST_AsText(pt) AS point, name, ROUND(ST_Distance(@hereami, pt), 2) AS distance FROM poi WHERE MBRContains(polygon_circle(9, 4, 4.5, 90), pt) AND ST_Distance(@hereami, pt) < 4.5 AND type = 'Shop' ORDER BY distance ASC ; +----+------------+--------+----------+ | id | point | name | distance | +----+------------+--------+----------+ | 3 | POINT(5 4) | Shop 2 | 4.00 | +----+------------+--------+----------+ 1 row in set (0.03 sec)
This seems not to have any significant negative impact on performance.
Results
Test | #rows | operation | latency |
---|---|---|---|
Total | 655360 | FTS | 1300 ms |
Spatial exact Circle | 4128 | FTS | 520 ms |
Spatial inner Hexagon | 3916 | range (pt) | 20 ms |
Spatial outer Square | 4128 | range (pt) | 30 ms |
Conventional outer Square on (x) | 4128 | range (x) or (y) | 150 ms |
Conventional outer Square on (xy) | 4128 | range (x,y) | 30 ms |
Spatial good Polygon | 4128 | range (pt) | 30 ms |
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