Geospatial data

QuestDB adds support for working with geospatial data through a geohash type. This page describes how to use geohashes, with an overview of the syntax, including hints on converting from latitude and longitude, inserting via SQL, InfluxDB line protocol, and via Java embedded API.

To facilitate working with this data type, spatial functions and operators have been added to help with filtering and generating data.

Geohash description#

A geohash is a convenient way of expressing a location using a short alphanumeric string, with greater precision obtained with longer strings. The basic idea is that the Earth is divided into grids of defined size, and each area is assigned a unique id called its Geohash. For a given location on Earth, we can convert latitude and longitude as the approximate center point of a grid represented by a geohash string. This string is the Geohash and will determine which of the predefined regions the point belongs to.

In order to be compact, base32 is used as a representation of Geohashes, and are therefore comprised of:

  • all decimal digits (0-9) and
  • almost all of the alphabet (case-insensitive) except "a", "i", "l", "o".

The followng figure illustrates how increasing the length of a geohash results in a higher-precision grid size:

An illustration showing three maps with different geohash precision levels applied

QuestDB geohash type#

Geohash column types are represented in QuestDB as geohash(<precision>). Precision is specified in the format n{units} where n is a numeric multiplier and units may be either c for char or b for bits (c being shorthand for 5 x b).

The following example shows basic usage of geohashes by creating a column of 5 char precision, 29 bits of precision, and inserting geohash values into these columns:

CREATE TABLE geo_data (g5c geohash(5c), g29b geohash(29b));
INSERT INTO geo_data VALUES(#u33d8, ##10101111100101111111101101101)
-- Querying by geohash
SELECT * FROM geo_data WHERE g5c = #u33d8;

It's not possible to store variable size geohashes within a column, therefore the size and precision of a geohash must be known beforehand. Shorter-precision geohashes cannot be inserted into longer-precision columns as all bits are significant. Details on the size of geohashes is described in the geohash precision section below. Additionally, NULL is supported as a separate value for geohash columns of all precision.

Geohash literals#

Geohashes have a literal syntax which starts with the hash # symbol followed by up to 12 chars, i.e.:

INSERT INTO my_geo_data VALUES(#u33, #u33d8b12)

Geohash literals with a single hash (#) may include a suffix in the format /{bits} where bits is the number of bits from 1-60 to allow for further granularity of the geohash size. This is useful if a specific precision is desired on the column size, but the values being inserted are using a char notation:

-- insert a 5-bit geohash into a 4 bit column
INSERT INTO my_geo_data VALUES(#a/4)
-- insert a 20-bit geohash into an 18 bit column
INSERT INTO my_geo_data VALUES(#u33d/18)

The binary equivalent of geohashes may be expressed with two hash symbols (##) followed by up to 60 bits:

INSERT INTO my_geo_data VALUES(##0111001001001001000111000110)

Implicit casts from strings to literal geohashes is possible, but less efficient as string conversion to geohash must be performed:

INSERT INTO my_geo_data VALUES(#u33, #u33d8b12)
-- equivalent to
INSERT INTO my_geo_data VALUES('u33', 'u33d8b12')

NULL values reserve 1 bit which means 8-bit geohashes are stored in 9-bits as shorts internally.

Specifying geohash precision#

The size of the geohash type may be:

  • 1 to 12 chars or
  • 1 to 60 bits

The following table shows all options for geohash precision using chars and the calculated area of the grid the geohash refers to:

TypeExampleArea
geohash(1c)#u5,000km × 5,000km
geohash(2c)#u31,250km × 625km
geohash(3c)#u33156km × 156km
geohash(4c)#u33d39.1km × 19.5km
geohash(5c)#u33d84.89km × 4.89km
geohash(6c)#u33d8b1.22km × 0.61km
geohash(7c)#u33d8b1153m × 153m
geohash(8c)#u33d8b1238.2m × 19.1m
geohash(9c)#u33d8b1214.77m × 4.77m
geohash(10c)#u33d8b12121.19m × 0.596m
geohash(11c)#u33d8b12123149mm × 149mm
geohash(12c)#u33d8b12123437.2mm × 18.6mm

For geohashes with size determined by b for bits, the following table compares the precision of some geohashes with units expressed in bits compared to chars:

Type (char)Equivalent to
geohash(1c)geohash(5b)
geohash(6c)geohash(30b)
geohash(12c)geohash(60b)

Casting geohashes#

Explicit casts are not necessary, but given certain constraints, it may be required to cast from strings to geohashes. Empty strings are cast as null for geohash values which are stored in the column with all bits set:

INSERT INTO my_geo_data VALUES(cast({my_string} as geohash(8c))

It may be desirable to cast as geohashes in the circumstance where a table with a desired schema should be created such as the following query. Note that the use of WHERE 1 != 1 means that no rows are inserted, only the table schema is prepared:

CREATE TABLE new_table AS
(SELECT cast(null AS geohash(4c)) gh4c)
FROM source_table WHERE 1 != 1

Geohash types can be cast from higher to lower precision, but not from lower to higher precision:

-- The following cast is valid:
CAST(#123 as geohash(1c))
-- Invalid (low-to-high precision):
CAST(#123 as geohash(4c))

SQL examples#

The following queries create a table with two geohash type columns of varying precision and insert geohashes as string values:

CREATE TABLE my_geo_data (g1c geohash(1c), g8c geohash(8c));
INSERT INTO my_geo_data values(#u, #u33d8b12);

Larger-precision geohashes are truncated when inserted into smaller-precision columns, and inserting smaller-precision geohases into larger-precision columns produces an error, i.e.:

-- SQL will execute successfully with 'u33d8b12' truncated to 'u'
INSERT INTO my_geo_data values(#u33d8b12, #eet531sq);
-- ERROR as '#e' is too short to insert into 8c_geohash column
INSERT INTO my_geo_data values(#u, #e);

Performing geospatial queries is done by checking if geohash values are equal to or within other geohashes. Consider the following table:

CREATE TABLE geo_data
(ts timestamp,
device_id symbol,
g1c geohash(1c),
g8c geohash(8c))
index(device_id) timestamp(ts);
info

The within operator may only be used when all symbol columns in the query are indexed.

This creates a table with a symbol type column as an identifier and we can insert values as follows:

INSERT INTO geo_data values(now(), 'device_1', #u, #u33d8b12);
INSERT INTO geo_data values(now(), 'device_1', #u, #u33d8b18);
INSERT INTO geo_data values(now(), 'device_2', #e, #ezzn5kxb);
INSERT INTO geo_data values(now(), 'device_1', #u, #u33d8b1b);
INSERT INTO geo_data values(now(), 'device_2', #e, #ezzn5kxc);
INSERT INTO geo_data values(now(), 'device_3', #e, #u33dr01d);

This table contains the following values:

tsdevice_idg1cg8c
2021-09-02T14:20:04.669312Zdevice_1uu33d8b12
2021-09-02T14:20:06.553721Zdevice_1uu33d8b12
2021-09-02T14:20:07.095639Zdevice_1uu33d8b18
2021-09-02T14:20:07.721444Zdevice_2eezzn5kxb
2021-09-02T14:20:08.241489Zdevice_1uu33d8b1b
2021-09-02T14:20:08.807707Zdevice_2eezzn5kxc
2021-09-02T14:20:09.280980Zdevice_3eu33dr01d

We can check if the last-known location of a device is a specific geohash with the following query which will return an exact match based on geohash:

SELECT * FROM geo_data LATEST BY device_id WHERE g8c = #u33dr01d
tsdevice_idg1cg8c
2021-09-02T14:20:09.280980Zdevice_3eu33dr01d

First and last functions#

The use of first() and last() functions within geospatial queries has been significantly optimized so that common types of queries relating to location are improved. This means that queries such as "last-known location" by indexed column for a given time range or sample bucket is specifically optimized for query speed over large datasets:

SELECT ts, last(g8c) FROM geo_data WHERE device_id = 'device_3';
-- first and last locations sample by 1 hour:
SELECT ts, last(g8c), first(g8c) FROM geo_data
WHERE device_id = 'device_3' sample by 1h;

Within operator#

The within operator can be used as a prefix match to evaluate if a geohash is equal to or is within a larger grid. The following query will return the most recent entries by device ID if the g8c column contains a geohash within u33d:

LATEST BY usage
SELECT * FROM geo_data LATEST BY device_id
WHERE g8c within(#u33d);
info

The within operator can only be applied in LATEST BY queries and all symbol columns within the query must be indexed.

tsdevice_idg1cg8c
2021-09-02T14:20:08.241489Zdevice_1uu33d8b1b
2021-09-02T14:20:09.280980Zdevice_3eu33dr01d

For more information on the use of this operator, see the spatial operators reference.

Java embedded usage#

Inserting geohashes into tables via Java (embedded) QuestDB instance through the TableWriter's putGeoHash method which accepts LONG values natively with the destination precision. Additionally, GeoHashes.fromString may be used for string conversion, but comes with some performance overhead as opposed to long values directly:

try (TableWriter writer = engine.getWriter(ctx.getCairoSecurityContext(), "geohash_table")) {
for(int i = 0; i < 10; i++) {
TableWriter.Row row = writer.newRow();
row.putSym(0, "my_device");
// putGeoStr(columnIndex, hash)
row.putGeoStr(1, "u33d8b1b");
// putGeoHashDeg(columnIndex, latitude, longitude)
row.putGeoHashDeg(2, 48.669, -4.329)
row.append();
}
writer.commit();
}

Reading geohashes via Java is done by means of the following methods:

  • Record.getGeoByte(columnIndex)
  • Record.getGeoShort(columnIndex)
  • Record.getGeoInt(columnIndex)
  • Record.getGeoLong(columnIndex)

Therefore it's necessary to know the type of the column beforehand through column metadata by index:

ColumnType.tagOf(TableWriter.getMetadata().getColumnType(columnIndex));

Invoking the method above will return one of the following:

  • ColumnType.GEOBYTE
  • ColumnType.GEOSHORT
  • ColumnType.GEOINT
  • ColumnType.GEOLONG

For more information and detailed examples of using table readers and writers, see the Java API documentation.

InfluxDB line protocol#

Geohashes may also be inserted via InfluxDB line protocol. In order to perform inserts in this way;

  1. Create table with columns of geohash type beforehand:
CREATE TABLE tracking (ts timestamp, geohash geohash(8c));
  1. Inserts via InfluxDB line protocol using the geohash field:
tracking geohash="46swgj10"
info

The InfluxDB Line Protocol parser does not support geohash literals, only strings. This means that table columns of type geohash type with the desired precision must exist before inserting rows with this protocol.

If a value cannot be converted or is omitted it will be set as NULL

Inserting geohashes with larger precision than the column it is being inserted into will result in the value being truncated, for instance, given a column with 8c precision:

# Inserting the following line
geo_data geohash="46swgj10r88k"
# Equivalent to truncating to this value:
geo_data geohash="46swgj10"

Postgres#

Geohashes may also be used over Postgres wire protocol as other data types. The Python example below demonstrates how to connect to QuestDB over postgres wire, insert and query geohashes:

info

When querying geohash values over Postgres wire protocol, QuestDB always returns geohashes in text mode (i.e. as strings) as opposed to binary

import psycopg2 as pg
import datetime as dt
try:
connection = pg.connect(user="admin",
password="quest",
host="127.0.0.1",
port="8812",
database="qdb")
cursor = connection.cursor()
cursor.execute("""CREATE TABLE IF NOT EXISTS geo_data
(ts timestamp, device_id symbol index, g1c geohash(1c), g8c geohash(8c))
timestamp(ts);""")
cursor.execute("INSERT INTO geo_data values(now(), 'device_1', 'u', 'u33d8b12');")
cursor.execute("INSERT INTO geo_data values(now(), 'device_1', 'u', 'u33d8b18');")
cursor.execute("INSERT INTO geo_data values(now(), 'device_2', 'e', 'ezzn5kxb');")
cursor.execute("INSERT INTO geo_data values(now(), 'device_3', 'e', 'u33dr01d');")
# commit records
connection.commit()
print("Data in geo_data table:")
cursor.execute("SELECT * FROM geo_data;")
records = cursor.fetchall()
for row in records:
print(row)
print("Records within 'u33d' geohash:")
cursor.execute("SELECT * FROM geo_data LATEST BY device_id WHERE g8c within(#u33d);")
records = cursor.fetchall()
for row in records:
print(row)
finally:
if (connection):
cursor.close()
connection.close()
print("QuestDB connection closed")