improve docs

README.postgres.md

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+# Postgres container
+
+## Update Postgres to new major version (UNTESTED!)
+
+_here 16 => 17_
+
+1. Stop services that use postgres or cut connection by change postgres port
+2. Login as postgres user
+```shell
+sudo -su postgres
+old=16
+cd /var/lib/postgresql/
+pg_old=$(nix-build --no-out-link -A postgresql_${old:?} '<nixpkgs>')
+pg_new=$(nix-build --no-out-link -A postgresql_$((old+1)) '<nixpkgs>')
+```
+3. Initialize new data directory
+```shell
+$pg_new/bin/initdb --encoding=UTF8 --locale=C $((old+1))
+```
+4. Run check
+```shell
+$pg_new/bin/pg_upgrade \
+    --old-bindir=$pg_old/bin \
+    --new-bindir=$pg_new/bin \
+    --old-datadir=/var/lib/postgresql/${old:?} \
+    --new-datadir=/var/lib/postgresql/$((old+1)) \
+    --clone \
+    --check
+```
+5. Stop the old Postgres
+```shell
+systemctl stop postgresql
+```
+6. Run the migration
+```shell
+$pg_new/bin/pg_upgrade \
+    --old-bindir=$pg_old/bin \
+    --new-bindir=$pg_new/bin \
+    --old-datadir=/var/lib/postgresql/${old:?} \
+    --new-datadir=/var/lib/postgresql/$((old+1)) \
+    --clone
+```
+7. Start the new Postgres
+```shell
+# Change 'services.postgres.packages = pkgs.postgresql_17;' in services/postgres.nix
+nixos-rebuild switch
+```
+
+Cleanup (after a few days):
+```shell
+sudo -su postgres
+vacuumdb --all --analyze-in-stages
+cd /var/lib/postgresql/
+./delete_old_cluster.sh
+rm delete_old_cluster.sh
+```
+
+---
+
+Build with ❤️ and ❄️.
+