| Bug #81129 | Geohash decoding returns the same result for different geohash | ||
|---|---|---|---|
| Submitted: | 18 Apr 2016 14:41 | Modified: | 19 Apr 2016 8:39 |
| Reporter: | Goncharov Vitalii | Email Updates: | |
| Status: | Not a Bug | Impact on me: | |
| Category: | MySQL Server: GIS | Severity: | S3 (Non-critical) |
| Version: | 5.7.11 Community Server | OS: | Ubuntu |
| Assigned to: | CPU Architecture: | Any | |
[18 Apr 2016 15:17]
MySQL Verification Team
Thank you for the bug report.
C:\dbs>c:\dbs\5.7\bin\mysql -uroot -p --port=3570 --prompt="mysql 5.7 > "
Enter password: ******
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.7.13 Source distribution PULL: 2016-APR-07
Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql 5.7 > SELECT st_longfromgeohash('tn1xn1'),st_longfromgeohash('tn1xn4');
+------------------------------+------------------------------+
| st_longfromgeohash('tn1xn1') | st_longfromgeohash('tn1xn4') |
+------------------------------+------------------------------+
| 47.38 | 47.38 |
+------------------------------+------------------------------+
1 row in set (0.00 sec)
---------------------------------------------------------------------------
c:\dbs\mysql-5.7.9>bin\mysql -uroot -p --port=3309
Enter password: ******
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.7.9 MySQL Community Server (GPL)
Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> SELECT st_longfromgeohash('tn1xn1'),st_longfromgeohash('tn1xn4');
+------------------------------+------------------------------+
| st_longfromgeohash('tn1xn1') | st_longfromgeohash('tn1xn4') |
+------------------------------+------------------------------+
| 47.4 | 47.4 |
+------------------------------+------------------------------+
1 row in set (0.00 sec)
[19 Apr 2016 8:39]
Erik Frøseth
Posted by developer: Hi Goncharov, The result you are getting is as expected and correct, so I'm closing this as "not a bug". If you convert those two geohashes to their binary representation, you will get the following binary values: tn1xn1: 110011010000001111011010000001 tn1xn4: 110011010000001111011010000100 As the "standard" describes (https://en.wikipedia.org/wiki/Geohash), you shall only use the even bits (counting the leftmost bit as 0) when decoding a geohash value to it's longitude value. So, removing the odd bits gives us the following binary values for the longitude value: tn1xn1: 1 0 1 0 0 0 0 1 1 0 1 1 0 0 0 tn1xn4: 1 0 1 0 0 0 0 1 1 0 1 1 0 0 0 Since they both are equal, the two geohashes should indeed produce the exact same longitude value. I investigated this a bit further, and it turns out that geohash.org returns the wrong results. For instance, the geohash value 'tn1xn1' gives us the following bounding box: Lower longitude: 47.373046875 Upper longitude: 47.384033203125 Lower latitude: 34.9859619140625 Upper latitude: 34.991455078125 Geohash.org returns [34.99 47.4] for this geohash value, and the longitude value is outside the bounding box. The same goes for the geohash value 'tn1xn4': Lower longitude: 47.373046875 Upper longitude: 47.384033203125 Lower latitude: 34.991455078125 Upper latitude: 34.9969482421875 Geohash.org returns [34.99 47.4] for this geohash as well, and in this case both the longitude value and the latitude value is outside the bounding box. Luckily, MySQL is smart enough to return [34.994 47.38] here :-)
Description: In "MySQL Community Server 5.7.11 has been released" description was wrote - * Geohash decoding (for example, for ST_LongFromGeoHash(), ST_LatFromGeoHash(), and ST_PointFromGeoHash()) could yield incorrect results due to the rounding algorithm being too aggressive. (Bug #22165582) but the error can be still reproduced How to repeat: SELECT st_longfromgeohash('tn1xn1'),st_longfromgeohash('tn1xn4')