Can I find distances between arrays in PostgreSQL using <->? - sql

Question

As far as I understood from this article, you can find nearest neighbors using <-> distance operator when working with geometric data types:
SELECT name, location --location is point
FROM geonames
ORDER BY location <-> '(29.9691,-95.6972)'
LIMIT 5;
You can also get some optimizations using SP-GiST indexes:
CREATE INDEX idx_spgist_geonames_location ON geonames USING spgist(location);
But I can't find anything about using <-> operator with arrays in the documentation. If I were to perform same queries using double precision[] instead of point, for example, would that work?

Answer 1

Apparently, we can't. For example, I've got a simple table:
CREATE TABLE test (
id SERIAL PRIMARY KEY,
loc double precision[]
);
And I want to query documents from it, ordering by distance,
SELECT loc FROM test ORDER BY loc <-> ARRAY[0, 0, 0, 0]::double precision[];
It doesn't work:
Query Error: error: operator does not exist: double precision[] <-> double precision[]
Documentation has no mention of <-> for arrays as well. I found a workaround in accepted answer for this question, but it poses some limitations, especially on array's length. Although there is an article (written in Russian), which suggests a workaround on array size limitation. Creation of sample table:
import postgresql
def setup_db():
db = postgresql.open('pq://user:pass#localhost:5434/db')
db.execute("create extension if not exists cube;")
db.execute("drop table if exists vectors")
db.execute("create table vectors (id serial, file varchar, vec_low cube, vec_high cube);")
db.execute("create index vectors_vec_idx on vectors (vec_low, vec_high);")
Element insertion:
query = "INSERT INTO vectors (file, vec_low, vec_high) VALUES ('{}', CUBE(array[{}]), CUBE(array[{}]))".format(
file_name,
','.join(str(s) for s in encodings[0][0:64]),
','.join(str(s) for s in encodings[0][64:128]),
)
db.execute(query)
Element querying:
import time
import postgresql
import random
db = postgresql.open('pq://user:pass#localhost:5434/db')
for i in range(100):
t = time.time()
encodings = [random.random() for i in range(128)]
threshold = 0.6
query = "SELECT file FROM vectors WHERE sqrt(power(CUBE(array[{}]) <-> vec_low, 2) + power(CUBE(array[{}]) <-> vec_high, 2)) <= {} ".format(
','.join(str(s) for s in encodings[0:64]),
','.join(str(s) for s in encodings[64:128]),
threshold,
) + \
"ORDER BY sqrt(power(CUBE(array[{}]) <-> vec_low, 2) + power(CUBE(array[{}]) <-> vec_high, 2)) ASC LIMIT 1".format(
','.join(str(s) for s in encodings[0:64]),
','.join(str(s) for s in encodings[64:128]),
)
print(db.query(query))
print('inset time', time.time() - t, 'ind', i)