Try optimize PostgreSQL function

function.sql

CREATE OR REPLACE FUNCTION point_to_tiles(
    _point geometry
) RETURNS SETOF tile
AS $$
DECLARE
    d2r CONSTANT DOUBLE PRECISION := pi() / 180;
    lon CONSTANT DOUBLE PRECISION := st_x(ST_Transform(_point, 4326));
    lat CONSTANT DOUBLE PRECISION := st_y(ST_Transform(_point, 4326));
    _sin CONSTANT DOUBLE PRECISION := sin(lat * d2r);
    z1 CONSTANT DOUBLE PRECISION := pow(2, 1);
    z2 CONSTANT DOUBLE PRECISION := pow(2, 2);
    z3 CONSTANT DOUBLE PRECISION := pow(2, 3);
    z4 CONSTANT DOUBLE PRECISION := pow(2, 4);
    z5 CONSTANT DOUBLE PRECISION := pow(2, 5);
    z6 CONSTANT DOUBLE PRECISION := pow(2, 6);
    z7 CONSTANT DOUBLE PRECISION := pow(2, 7);
    z8 CONSTANT DOUBLE PRECISION := pow(2, 8);
    z9 CONSTANT DOUBLE PRECISION := pow(2, 9);
    z10 CONSTANT DOUBLE PRECISION := pow(2, 10);
    z11 CONSTANT DOUBLE PRECISION := pow(2, 11);
    z12 CONSTANT DOUBLE PRECISION := pow(2, 12);
    z13 CONSTANT DOUBLE PRECISION := pow(2, 13);
    z14 CONSTANT DOUBLE PRECISION := pow(2, 14);
BEGIN
    RETURN QUERY VALUES
    	(0, 0, 0),
    	(floor(z1 * (lon / 360 + 0.5)), floor(z1 * (0.5 - 0.25 * log((1 + _sin) / (1 - _sin)) / pi())), 1),
    	(floor(z2 * (lon / 360 + 0.5)), floor(z2 * (0.5 - 0.25 * log((1 + _sin) / (1 - _sin)) / pi())), 2),
    	(floor(z3 * (lon / 360 + 0.5)), floor(z3 * (0.5 - 0.25 * log((1 + _sin) / (1 - _sin)) / pi())), 3),
    	(floor(z4 * (lon / 360 + 0.5)), floor(z4 * (0.5 - 0.25 * log((1 + _sin) / (1 - _sin)) / pi())), 4),
    	(floor(z5 * (lon / 360 + 0.5)), floor(z5 * (0.5 - 0.25 * log((1 + _sin) / (1 - _sin)) / pi())), 5),
    	(floor(z6 * (lon / 360 + 0.5)), floor(z6 * (0.5 - 0.25 * log((1 + _sin) / (1 - _sin)) / pi())), 6),
    	(floor(z7 * (lon / 360 + 0.5)), floor(z7 * (0.5 - 0.25 * log((1 + _sin) / (1 - _sin)) / pi())), 7),
    	(floor(z8 * (lon / 360 + 0.5)), floor(z8 * (0.5 - 0.25 * log((1 + _sin) / (1 - _sin)) / pi())), 8),
    	(floor(z9 * (lon / 360 + 0.5)), floor(z9 * (0.5 - 0.25 * log((1 + _sin) / (1 - _sin)) / pi())), 9),
    	(floor(z10 * (lon / 360 + 0.5)), floor(z10 * (0.5 - 0.25 * log((1 + _sin) / (1 - _sin)) / pi())), 10),
    	(floor(z11 * (lon / 360 + 0.5)), floor(z10 * (0.5 - 0.25 * log((1 + _sin) / (1 - _sin)) / pi())), 11),
    	(floor(z12 * (lon / 360 + 0.5)), floor(z10 * (0.5 - 0.25 * log((1 + _sin) / (1 - _sin)) / pi())), 12),
    	(floor(z13 * (lon / 360 + 0.5)), floor(z10 * (0.5 - 0.25 * log((1 + _sin) / (1 - _sin)) / pi())), 13),
    	(floor(z14 * (lon / 360 + 0.5)), floor(z10 * (0.5 - 0.25 * log((1 + _sin) / (1 - _sin)) / pi())), 14);
END;
$$ LANGUAGE plpgsql IMMUTABLE;