Spatial queries with PostGIS

types_session.py

"""PostGIS 3.6.4 on PostgreSQL 18.6: versions, geography and geometry for the same two cities, and the longitude-first mistake."""

from pg_extensions import extension_settings
from pg_session import Server

# PostGIS may sit next to PostgreSQL or in a prefix of its own: pg_extensions.py finds such a prefix and returns
# the two settings PostgreSQL 18 needs for it (none when PostGIS is installed with PostgreSQL).
SETTINGS = {"autovacuum": "off", "max_parallel_workers_per_gather": 0, "jit": "off", "timezone": "'UTC'"}
SETTINGS.update(extension_settings())

with Server(settings=SETTINGS) as pg:
    pg.up()
    pg.psql("types.sql", errors=1)

$ psql -X -a -f types.sql
-- PostGIS is an extension: it is installed in this database by one statement.
CREATE EXTENSION postgis;
CREATE EXTENSION
SELECT postgis_lib_version() AS postgis, postgis_geos_version() AS geos,
       split_part(postgis_proj_version(), ' ', 1) AS proj;
 postgis |        geos        | proj  
---------+--------------------+-------
 3.6.4   | 3.14.1-CAPI-1.20.5 | 9.9.0
(1 row)

-- Two cities as plain numbers: longitude first, then latitude.
CREATE TABLE city (name text PRIMARY KEY, lon float8, lat float8);
CREATE TABLE
INSERT INTO city VALUES ('Pune', 73.8567, 18.5204), ('Mumbai', 72.8777, 19.0760);
INSERT 0 2
-- The same two points as geography (metres on the earth) and as geometry (degrees on a flat plane).
SELECT round(ST_Distance(p.pt::geography, m.pt::geography)::numeric / 1000, 1) AS km_geography,
       round(ST_Distance(p.pt, m.pt)::numeric, 4) AS degrees_geometry
FROM (SELECT ST_Point(lon, lat, 4326) AS pt FROM city WHERE name = 'Pune') p,
     (SELECT ST_Point(lon, lat, 4326) AS pt FROM city WHERE name = 'Mumbai') m;
 km_geography | degrees_geometry 
--------------+------------------
        120.1 |           1.1257
(1 row)

-- The mistake: latitude first. Pune lands at 18.52 E, 73.86 N, in the open sea north of Norway.
SELECT ST_AsText(ST_Point(lat, lon, 4326)) AS swapped_pune,
       round(ST_Distance(ST_Point(lat, lon, 4326)::geography, ST_Point(lon, lat, 4326)::geography)::numeric / 1000, 1) AS km_from_pune
FROM city WHERE name = 'Pune';
      swapped_pune      | km_from_pune 
------------------------+--------------
 POINT(18.5204 73.8567) |       6999.6
(1 row)

-- Both cities swapped: the distance between them still looks believable.
SELECT round(ST_Distance(ST_Point(p.lat, p.lon, 4326)::geography, ST_Point(m.lat, m.lon, 4326)::geography)::numeric / 1000, 1) AS km_swapped_pair
FROM city p, city m WHERE p.name = 'Pune' AND m.name = 'Mumbai';
 km_swapped_pair 
-----------------
           110.7
(1 row)

-- A guard: a loose box around India is 68 to 98 degrees east and 6 to 38 north, so a swapped pair breaks it.
ALTER TABLE city ADD CONSTRAINT city_in_india CHECK (lon BETWEEN 68 AND 98 AND lat BETWEEN 6 AND 38);
ALTER TABLE
INSERT INTO city VALUES ('Swapped Pune', 18.5204, 73.8567);
psql:types.sql:27: ERROR:  new row for relation "city" violates check constraint "city_in_india"
DETAIL:  Failing row contains (Swapped Pune, 18.5204, 73.8567).

types.sql

-- PostGIS is an extension: it is installed in this database by one statement.
CREATE EXTENSION postgis;
SELECT postgis_lib_version() AS postgis, postgis_geos_version() AS geos,
       split_part(postgis_proj_version(), ' ', 1) AS proj;

-- Two cities as plain numbers: longitude first, then latitude.
CREATE TABLE city (name text PRIMARY KEY, lon float8, lat float8);
INSERT INTO city VALUES ('Pune', 73.8567, 18.5204), ('Mumbai', 72.8777, 19.0760);

-- The same two points as geography (metres on the earth) and as geometry (degrees on a flat plane).
SELECT round(ST_Distance(p.pt::geography, m.pt::geography)::numeric / 1000, 1) AS km_geography,
       round(ST_Distance(p.pt, m.pt)::numeric, 4) AS degrees_geometry
FROM (SELECT ST_Point(lon, lat, 4326) AS pt FROM city WHERE name = 'Pune') p,
     (SELECT ST_Point(lon, lat, 4326) AS pt FROM city WHERE name = 'Mumbai') m;

-- The mistake: latitude first. Pune lands at 18.52 E, 73.86 N, in the open sea north of Norway.
SELECT ST_AsText(ST_Point(lat, lon, 4326)) AS swapped_pune,
       round(ST_Distance(ST_Point(lat, lon, 4326)::geography, ST_Point(lon, lat, 4326)::geography)::numeric / 1000, 1) AS km_from_pune
FROM city WHERE name = 'Pune';

-- Both cities swapped: the distance between them still looks believable.
SELECT round(ST_Distance(ST_Point(p.lat, p.lon, 4326)::geography, ST_Point(m.lat, m.lon, 4326)::geography)::numeric / 1000, 1) AS km_swapped_pair
FROM city p, city m WHERE p.name = 'Pune' AND m.name = 'Mumbai';

-- A guard: a loose box around India is 68 to 98 degrees east and 6 to 38 north, so a swapped pair breaks it.
ALTER TABLE city ADD CONSTRAINT city_in_india CHECK (lon BETWEEN 68 AND 98 AND lat BETWEEN 6 AND 38);
INSERT INTO city VALUES ('Swapped Pune', 18.5204, 73.8567);

pg_session.py

"""Throwaway PostgreSQL 18.6 servers for the examples of this lesson.

Every session creates a cluster of its own with initdb in a new folder inside the examples folder, starts it, runs
psql scripts and server programs on it, prints what they printed, and stops and deletes it again, also when something
fails. The server is the pinned PostgreSQL 18.6 from the mise installations next to the Python that runs this file.
It listens only on 127.0.0.1, an address no other computer can reach, on a port the operating system reports as
unused, with no Unix socket, and it asks for a password (scram-sha-256) made up for the run and never printed.
Besides PostgreSQL's own programs, the sessions read only this folder (and an extension's own folder where a lesson
needs one), and they connect to nothing but 127.0.0.1.

The output is exactly what the programs printed, with four substitutions so that a record never shows the computer
it ran on and is the same on every run: the run's folder is shown as ".", the folder of PostgreSQL's programs is left
out of their paths, the operating-system user's name is shown as "learner", and the dots pg_ctl prints while it waits
(one about every tenth of a second) as three. A session may name more patterns (mask=) for numbers that change on
every run, such as the rates and times ANALYZE VERBOSE prints or the random key of pg_dump's \\restrict line; the
session's own file says which, and its record shows a fixed word in their place. The sessions never print the port.
A session stops with status 1 when a command fails that should have worked, or reports a different number of errors
than the session expects, so a run that went wrong is never recorded as if it had worked.
"""

import os
import pwd
import re
import secrets
import shlex
import shutil
import socket
import subprocess
import sys
import tempfile
import textwrap
import time
from pathlib import Path

PG_VERSION = "18.6"
# .../installs/python/3.14.8/bin/python3 -> .../installs
INSTALLS = Path(sys.executable).resolve().parents[3]
BIN = INSTALLS / "postgres" / PG_VERSION / "bin"
OS_USER = pwd.getpwuid(os.geteuid()).pw_name


def free_port():
    """A TCP port on 127.0.0.1 that nothing listens on now."""
    with socket.socket() as probe:
        probe.bind(("127.0.0.1", 0))
        return probe.getsockname()[1]


class Server:
    """One throwaway cluster: initdb, settings, start; stopped and deleted when the with block ends."""

    def __init__(self, settings=None, initdb=(), name="data"):
        self.settings = dict(settings or {})
        self.initdb_args = list(initdb)
        self.name = name
        self.running = []

    def __enter__(self):
        if not (BIN / "postgres").exists():
            sys.exit(f"PostgreSQL {PG_VERSION} was not found next to this Python")
        self.base = Path(tempfile.mkdtemp(prefix="pg", dir=".")).resolve()
        self.port = free_port()
        self.password = secrets.token_urlsafe(24)
        (self.base / "pwfile").write_text(self.password + "\n")
        self.env = {**os.environ, "PGHOST": "127.0.0.1", "PGPORT": str(self.port), "PGUSER": "postgres",
                    "PGPASSWORD": self.password, "PGDATABASE": "postgres", "PGCONNECT_TIMEOUT": "60"}
        for key in ("PGSERVICE", "PGSERVICEFILE", "PGOPTIONS", "PSQLRC"):
            self.env.pop(key, None)
        for script in Path(".").glob("*.sql"):
            shutil.copy(script, self.base / script.name)
        for data in Path(".").glob("*.csv"):
            shutil.copy(data, self.base / data.name)
        return self

    def __exit__(self, *exc):
        for folder, _port in reversed(self.running):
            self._call(["pg_ctl", "-D", folder, "-m", "fast", "-w", "-t", "60", "stop"])
        self.running = []
        shutil.rmtree(self.base, ignore_errors=True)

    # ---- the cluster ---------------------------------------------------------------------------------------------
    def initdb(self, show=False):
        args = ["initdb", "-D", self.name, "-U", "postgres", "--auth=scram-sha-256", "--pwfile=pwfile",
                *self.initdb_args]
        if not show:
            args.append("--no-instructions")
        self._expect(args, show=show)
        with open(self.base / self.name / "postgresql.conf", "a") as conf:
            conf.write("\n# Added by the session: 127.0.0.1 only, no Unix socket.\n")
            conf.write(f"listen_addresses = '127.0.0.1'\nport = {self.port}\nunix_socket_directories = ''\n")
            for key, value in self.settings.items():
                conf.write(f"{key} = {value}\n")

    def start(self, show=False, folder=None, port=None):
        folder = folder or self.name
        self._expect(["pg_ctl", "-D", folder, "-l", f"{folder}.log", "-w", "-t", "60", "start"], show=show)
        self.running.append((folder, port or self.port))

    def up(self):
        """initdb and start, quietly."""
        self.initdb()
        self.start()
        return self

    def standby(self, folder="standby"):
        """A streaming standby of this server, made with pg_basebackup -R, started on a port of its own."""
        self._expect(["pg_basebackup", "-D", folder, "-R", "-X", "stream", "-c", "fast"])
        port = free_port()
        with open(self.base / folder / "postgresql.conf", "a") as conf:
            conf.write(f"port = {port}\n")
        self.start(folder=folder, port=port)
        for _ in range(300):
            if self.query("SELECT count(*) FROM pg_stat_replication WHERE state = 'streaming'") == "1":
                return port
            time.sleep(0.1)
        sys.exit("the standby did not start streaming")

    # ---- running things ------------------------------------------------------------------------------------------
    def _call(self, args, env=None):
        args = [str(a) for a in args]
        done = subprocess.run([str(BIN / args[0]), *args[1:]], stdout=subprocess.PIPE, stderr=subprocess.STDOUT,
                              text=True, cwd=self.base, env=env or self.env)
        return done.returncode, self.clean(done.stdout)

    def _expect(self, args, show=False, status=0, mask=()):
        code, out = self._call(args)
        out = _masked(out, mask)
        if show:
            _show(args, out)
        if code != status:
            if not show:
                print(out)
            sys.exit(f"{args[0]} ended with status {code}, not {status}")
        return out

    def clean(self, text):
        """The run's folder as ".", the user as learner, pg_ctl's waiting dots as three (see the docstring)."""
        text = text.replace(str(BIN) + "/", "").replace(str(self.base) + "/", "").replace(str(self.base), ".")
        text = re.sub(r"(waiting for server to (?:start|shut down))\.+", r"\1...", text)
        return text.replace(f'"{OS_USER}"', '"learner"')

    def wait_for(self, sql, value):
        """Waits (unprinted) until a query returns the value, for example after a reload."""
        for _ in range(300):
            if self.query(sql) == value:
                return
            time.sleep(0.1)
        sys.exit(f"{sql} never returned {value}")

    def show_file(self, path):
        """Prints a text file of the cluster, as `cat` would."""
        print(f"$ cat {path}")
        print((self.base / path).read_text().rstrip("\n"))
        print()

    def query(self, sql, port=None):
        """One value from one query, unprinted (the session uses it to wait or to decide)."""
        env = {**self.env, "PGPORT": str(port or self.port)}
        code, out = self._call(["psql", "-X", "-At", "-c", sql], env=env)
        if code != 0:
            sys.exit(f"the query failed: {out}")
        return out.strip()

    def silent(self, sql):
        """Runs a statement without printing it (used only to give a role the run's password)."""
        self.query(sql)

    def psql(self, script, errors=0, status=None, user=None, db=None, port=None, flags=("-a",), note=None, mask=()):
        """Runs a psql script with -X (no ~/.psqlrc) and the given flags, and prints the command and its output.
        `errors` is how many ERROR lines the script must print; `status` the exit status it must end with."""
        env = {**self.env, "PGPORT": str(port or self.port)}
        if user:
            env["PGUSER"] = user
        if db:
            env["PGDATABASE"] = db
        args = ["psql", "-X", *flags, "-f", script]
        code, out = self._call(args, env=env)
        out = _masked(out, mask)
        shown = ["psql", "-X", *flags, "-f", script]
        if user:
            shown[1:1] = ["-U", user]
        if db:
            shown[1:1] = ["-d", db]
        if note:
            print(f"-- {note}")
        _show(shown, out)
        found = sum(1 for line in out.splitlines() if "ERROR:" in line or ": error:" in line)
        expected_status = 0 if status is None else status
        if found != errors or code != expected_status:
            sys.exit(f"{script}: {found} errors and status {code}; the session expects {errors} and {expected_status}")

    def sessions(self, count):
        """Opens `count` idle psql connections (each one a client backend) until the with block ends."""
        return _Idle(self, count)

    def shell(self, *args, status=0, mask=()):
        """Runs one of PostgreSQL's programs and prints the command and its output."""
        return self._expect(list(args), show=True, status=status, mask=mask)

    def listing(self, folder, prefix=""):
        """Prints the names in a folder of the cluster (sorted, folders with a slash), as `ls -p` would list them."""
        path = self.base / folder
        names = sorted(n + ("/" if (path / n).is_dir() else "") for n in os.listdir(path) if n.startswith(prefix))
        print(f"-- in {folder}" + (f", the names that start with {prefix}" if prefix else "") + ":")
        print(textwrap.fill("  ".join(names), width=78, initial_indent="   ", subsequent_indent="   ",
                            break_on_hyphens=False))
        print()
        return names


def _masked(text, mask):
    """Replaces what differs on every run (a rate, a time, a random key) with fixed text (see the docstring)."""
    for pattern, replacement in mask:
        text = re.sub(pattern, replacement, text, flags=re.M)
    return text


def _arg(a):
    """A command-line argument as a reader would type it: double quotes around text with single quotes in it."""
    a = str(a)
    if re.fullmatch(r"[\w@%+=:,./-]+", a):
        return a
    if not re.search(r"[\"$`\\!]", a):
        return f'"{a}"'
    return shlex.quote(a)


def _show(args, out):
    print("$ " + " ".join(_arg(a) for a in args))
    if out.strip():
        print(out.rstrip("\n"))
    print()


class _Idle:
    """Idle psql connections, closed again when the with block ends."""

    def __init__(self, server, count):
        self.server, self.count, self.procs = server, count, []

    def __enter__(self):
        before = int(self.server.query("SELECT count(*) FROM pg_stat_activity WHERE backend_type = 'client backend'"))
        for _ in range(self.count):
            self.procs.append(subprocess.Popen([str(BIN / "psql"), "-X", "-q"], stdin=subprocess.PIPE,
                                               stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL,
                                               env=self.server.env, cwd=self.server.base))
        sql = "SELECT count(*) FROM pg_stat_activity WHERE backend_type = 'client backend'"
        for _ in range(300):
            # the query's own connection is one more client backend
            if int(self.server.query(sql)) == before + self.count:
                return self
            time.sleep(0.1)
        sys.exit("the idle connections did not open")

    def __exit__(self, *exc):
        for p in self.procs:
            p.stdin.close()
            p.wait(timeout=30)

pg_extensions.py

"""Where the sessions of this lesson find an extension that is not installed next to PostgreSQL.

An extension installed together with PostgreSQL (by a package manager, for example) needs nothing from this file:
extension_settings() then returns no settings. An extension built into a prefix of its own, a folder that holds
share/extension/ and lib/, is found by PostgreSQL 18 through two settings, extension_control_path and
dynamic_library_path. This file builds them from the prefixes it finds, in this order:

1. PG_EXTENSION_DIR, a colon-separated list of such prefixes, when it is set;
2. otherwise the entries of a folder named pg-extensions in the session's folder or in a folder above it: each entry
   is a prefix, or a link to one.

The settings go into the throwaway cluster's configuration only. The sessions never print them, because they hold
paths of the computer that ran them.
"""

import os
from pathlib import Path


def extension_prefixes():
    """The prefixes to add, or [] when the extension sits next to PostgreSQL."""
    listed = [p for p in os.environ.get("PG_EXTENSION_DIR", "").split(":") if p]
    if listed:
        return listed
    here = Path.cwd()
    for folder in (here, *here.parents):
        holder = folder / "pg-extensions"
        if holder.is_dir():
            return sorted(str(p.resolve()) for p in holder.iterdir() if (p / "share" / "extension").is_dir())
    return []


def extension_settings():
    """Settings for Server(settings=...): empty when no prefix is needed."""
    prefixes = extension_prefixes()
    if not prefixes:
        return {}
    return {
        "extension_control_path": "'" + ":".join(f"{p}/share" for p in prefixes) + ":$system'",
        "dynamic_library_path": "'" + ":".join(f"{p}/lib" for p in prefixes) + ":$libdir'",
    }