A guide to building applications with PostgreSQL, a relational database system. It covers tenant separation, database structure, schema-change migrations, asynchronous Python database access, and query speed.

Use it to design tenant-aware tables with row-level security, write Alembic migrations, use async SQLAlchemy, and improve PostgreSQL queries.

It helps prevent one customer, or tenant, from seeing another customer's data and avoids common mistakes when changing or tuning a database.

postgresql — Skill by itechmeat

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods

npx agentmods add skills/itechmeat/llm-code/postgresql

Any agent

npx skills add itechmeat/llm-code --skill postgresql

Clone the repo

git clone --depth 1 https://github.com/itechmeat/llm-code

Made for: Claude Code, Codex.

Per session 81 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 1,462 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. Scan, not verified.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5	$0.00081	$0.01462
Opus 5	$0.00041	$0.00731
Sonnet 5	$0.00016	$0.00292
Haiku 4.5	$0.00008	$0.00146

Measured 2d ago against content hash debf84a8b7cf, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

postgresql scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

skills/postgresql/SKILL.md · 145 lines

How it starts

The opening of the file, as written. The whole thing — 145 lines — stays where its author put it; the contents beside it link to each section on GitHub.

PostgreSQL

RLS Multi-tenancy Pattern

Non-negotiables

RLS context is mandatory for any tenant-scoped query
Context must be set inside the same transaction as the queries
No fallbacks for tenant ID (fail fast if missing)
Async-only DB access when using async frameworks

Setting RLS Context

RLS works only if the current transaction has the context set:

SET LOCAL app.current_tenant_id = '<tenant_uuid>';

Must run before the first tenant-scoped query in that transaction.

Common Failure Modes

Setting SET LOCAL ... after the first select()
Setting the context in one session, then querying in another
Running queries outside the expected transaction scope

Typical RLS Policy

ALTER TABLE some_table ENABLE ROW LEVEL SECURITY;

CREATE POLICY some_table_tenant_isolation
ON some_table
USING (tenant_id = current_setting('app.current_tenant_id', true)::uuid);

Multi-tenant Table Checklist

Tenant ID column is UUID
FK to tenants table with ON DELETE CASCADE
Indexes aligned with access patterns (usually tenant_id first)
- PostgreSQL does not auto-index FK columns — add explicit indexes
- UNIQUE allows multiple NULLs unless using NULLS NOT DISTINCT (PG15+)
RLS is enabled and policies exist
Application code sets RLS context at transaction start

Alembic Migrations Checklist

Add/modify schema (columns, constraints, FKs)
Create/update indexes
Enable RLS and create/adjust policies
Add verification (tests) for isolation
Provide a real downgrade (no stubs)

Version Notes

PostgreSQL 19 (beta)

19 is at Beta 2 (2026-07-16); GA is expected September/October 2026 and details may still change. Latest stable line is 18.4.
Headline changes: REPACK / REPACK CONCURRENTLY replacing VACUUM FULL and CLUSTER, parallel autovacuum with a scoring system, logical replication of sequences, SQL/PGQ property graphs, GROUP BY ALL, FOR PORTION OF, and online checksum enable/disable.
18 incompatible changes to plan for, including forced standard_conforming_strings, RADIUS removal, jit off by default, default_toast_compression switching to lz4, and max_locks_per_transaction defaulting to 128 with changed sizing.
Full detail and the upgrade checklist: postgresql-19.md

Read the full file on GitHub · 145 lines

Files

What ships with it

13 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

2d ago First seen · 145 lines · 81 tokens per session scan A debf84a8b7cf

Subscribe to this mod's changes

postgresql is a skill published in the GitHub repository itechmeat/llm-code (22 stars, last pushed 1mo ago), licensed MIT. It adds 81 tokens to every session and 1,462 once invoked, about $0.0004 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

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