SQL ORDER BY before TABLESAMPLE BERNOULLI, or in Python - sql

Question

I am performing a query on PostrgreSQL using Python (pyscopg2)
The data is point geometries, stored in patched of 600 points per patch.
I am trying to streamline and speed up the process, previously I would do the following:
explode the geometry
order by x, y, z
save the result to a new table
Use TABLESAMPLE BERNOULLI(1) to sample the data to 1%
save back to the database
To speed things up I'm trying to reduce the amount of writing to the database and keep the data in python as much as possible.
The old code:
Exploding the patches
query = sql.SQL("""INSERT INTO {}.{} (x,y,z)
SELECT
st_x(PC_EXPLODE(pa)::geometry) as x,
st_y(PC_EXPLODE(pa)::geometry) as y,
st_z(PC_EXPLODE(pa)::geometry) as z
from "public".{} order by x,y,z;""").format(
*map(sql.Identifier, (schema_name, table_name2, table_name1)))
sampling the data:
query2 = ("CREATE TABLE {}.{} AS (SELECT * FROM {}.{} TABLESAMPLE BERNOULLI ({}))".format(
schema, table_name_base, schema, imported_table_name_base, sample_base))
This works, but I would like to either:
A) Perform this as a single query, so explode --> order by --> sample.
B) Perform the explode in SQL, then sample in python.
For A) I have attempted to nest/subquery but PostgreSQL will not allow TABLESAMPLE to work on anything that isn't a table or a view.
For B) I use data = gpd.read_postgis(query, con=conn) to get the data directly into a geopandas dataframe, so sorting is then easy, but how do I perform the equivalent of TABLESAMPLE BERNOULLI to a geopandas dataframe?
Option A is my preferred option, but it might be useful to test option B incase I end up allowing different sampling methods.
Edit:
This is the visual result of:
query = """
SELECT
PC_EXPLODE(pa)::geometry as geom,
st_x(PC_EXPLODE(pa)::geometry) as x,
st_y(PC_EXPLODE(pa)::geometry) as y,
st_z(PC_EXPLODE(pa)::geometry) as z
FROM {}.{}
TABLESAMPLE BERNOULLI ({})
ORDER BY x,y,z, geom
;
""".format(schema, pointcloud, sample)

Answer 1

I am a little lost. A random sample is a random sample and doesn't depend on the ordering. If you want a sample that depends on the ordering, then use an nth sample. That would be:
select t.*
from (select t.*,
row_number() over (order by x, y, z) as seqnum
from (select st_x(PC_EXPLODE(pa)::geometry) as x,
st_y(PC_EXPLODE(pa)::geometry) as y,
st_z(PC_EXPLODE(pa)::geometry) as z
from "public".{}
) t
) t
where seqnum % 100 = 1;
Or perhaps you just want to take the sample and then order afterwards, which you can also do with a subquery.