BigQuery GIS table to GeoJSON type Feature Collection with SQL and/or Python - sql

Question

I have some BigQuery tables with spatial data (lon, lat, point, and linestring) representing boat tracks.
I'm trying to get them into a GeoJSON for an API output.
I have one method that uses Python to query my table into a pandas dataframe, which I can then use a function to create a feature collection.
But, there's so much data/day that it'd be ideal if I could perform some kind of efficient linestring union and simplify.
My current method:
# Query BigQuery table
sql = """SELECT
lon, lat, lead_lon, lead_lat,
CAST(CAST(timestamp AS DATE) as string) as date,
UNIX_SECONDS(timestamp) as unix_secs,
CAST(ship_num AS STRING) AS ship_num,
op,
CAST(knots AS FLOAT64) AS knots,
point,
linestring
FROM `ship_segments`
WHERE timestamp BETWEEN '2020-04-16' AND '2020-04-17';"""
# Make into pandas dataframe
df = client.query(sql).to_dataframe()
#df to geojson fn
def data2geojson(df):
features = []
insert_features = lambda X: features.append(
geojson.Feature(geometry=geojson.LineString(([X["lead_lon"], X["lead_lat"], X["knots"], X["unix_secs"]],
[X["lon"], X["lat"], X["knots"], X["unix_secs"]])),
properties=dict(date=X["date"],
mmsi=X["ship_num"],
operator=X["op"]
)))
df.apply(insert_features, axis=1)
geojson_obj = geojson.dumps(geojson.FeatureCollection(features, indent=2, sort_keys=True), sort_keys=True, ensure_ascii=False)
return(geojson_obj)
results = data2geojson(df)
This returns a GeoJSON:
{"features": [{"geometry": {"coordinates": [[-119.049945, 33.983277, 10.5502, 1587104709], [-119.034677, 33.975823, 10.5502, 1587104709]], "type": "LineString"}, "properties": {"date": "2020-04-17", "mmsi": "235098383", "operator": "Evergreen Marine Corp"}, "type": "Feature"}, {"geometry": {"coordinates": [[-120.176933, 34.282107, 22.7005, 1587114969], [-120.144453, 34.275147, 22.7005, 1587114969]], "type": "LineString"}, "properties": {"date": "2020-04-17", "mmsi": "235098383", "operator": "Evergreen Marine Corp"}, "type": "Feature"}, {"geometry": {"coordinates": [[-118.361737, 33.64647, 11.3283, 1587096305], [-118.356308, 33.643713, 11.3283, 1587096305]], "type": "LineString"}, "properties": {"date": "2020-04-17", "mmsi": "538005412", "operator": "Scorpio MR Pool Ltd"}, "type": "Feature"}, {"geometry": {"coordinates": [[-118.414667, 33.673013, 12.7684, 1587097278], [-118.411707, 33.671493, 12.7684, 1587097278]], "type": "LineString"}, "properties": {"date": "2020-04-17", "mmsi": "538005412", "operator": "Scorpio MR Pool Ltd"}, "type": "Feature"}, {"geometry": {"coordinates": [[-119.377783, 34.062612, 10.5456, 1587102119], [-119.384212, 34.064217, 10.5456, 1587102119]], "type": "LineString"}, "properties": {"date": "2020-04-17", "mmsi": "636018225", "operator": "Ocean Network Express Pte Ltd"}, "type": "Feature"}], "indent": 2, "sort_keys": true, "type": "FeatureCollection"}
But I'm trying something like:
select
ship_num,
date(timestamp) as date,
AVG(speed_knots) as avg_speed_knots,
st_union_agg(linestring) as multiline
from(
SELECT
*,
row_number() OVER w AS num,
ST_GeogPoint(lon,lat) as geom,
LEAD(ST_GeogPoint(lon,lat)) OVER w AS geom2,
ST_MAKELINE((ST_GeogPoint(lon,lat)), (LEAD(ST_GeogPoint(lon,lat)) OVER w)) AS linestring,
LEAD(STRING(timestamp), 0) OVER w AS t1,
LEAD(STRING(timestamp), 1) OVER w AS t2,
FROM
`ship_data`
where timestamp >= '2020-04-10'
WINDOW w AS (PARTITION BY ship_num ORDER BY timestamp)) AS q
group by
ship_num, date(timestamp);
This gives me multilinestrings in a table, but then I need to simplify and get them in a GeoJSON FeatureCollection output.
Any ideas that don't use PostGIS?