Querying Local Postgres Agent Skill

Querying local Postgres (READ-ONLY) — PostHog repo

User's query: $ARGUMENTS

Scope: This repo uses PostgreSQL for app metadata (teams, projects, flags, Django models, etc.). Analytics event data lives in ClickHouse, not Postgres — use HogQL / ClickHouse tools for events-style questions unless the user explicitly wants Postgres.

When to use

• User asks to query the database, inspect tables, or run SQL against Postgres
• Debugging: Row-level checks (e.g. why a team/project/flag row looks wrong), migrations, constraints, duplicate keys
• Performance: EXPLAIN / EXPLAIN (ANALYZE, …) on read-only SELECT against Django or app tables

Instructions

• Strictly forbid mutations — See "Mutations strictly forbidden" below. If the user asks for any write or mutation, refuse and explain the skill is read-only.
• Translate the user's question into one or more read-only SQL statements.
• Show the SQL in a code block before running.
• Run using the command pattern below (always with PGOPTIONS='-c default_transaction_read_only=on' to force a read-only connection).
• Show results and give a brief interpretation (especially when used for debugging or plan review).

Mutations strictly forbidden

Do not run, suggest, or generate any of the following. Refuse and state that this skill is read-only.

• DML: INSERT, UPDATE, DELETE, MERGE, TRUNCATE
• DDL: CREATE, DROP, ALTER, RENAME
• Other writes: COPY ... TO program, CALL (if it mutates), GRANT/REVOKE
• EXPLAIN ANALYZE on anything other than a read-only SELECT (including WITH … SELECT). Do not wrap DML in EXPLAIN ANALYZE — it would execute the write. The read-only connection below rejects writes, but the agent must not attempt this pattern.
• Any statement that modifies data, schema, or roles

Allowed:

• SELECT (including WITH … SELECT)
• EXPLAIN … SELECT (estimate-only plan; no execution)
• EXPLAIN (ANALYZE, …) SELECT — executes the SELECT once; use only for performance analysis. Must run on the read-only connection below.
• SHOW, SELECT from catalog views (pg_stat_*, information_schema, etc.) when read-only

If the user requests a write operation, say: "This skill is read-only. I can't run INSERT/UPDATE/DELETE or other mutations. Use a DB client or migration tool for writes."

EXPLAIN and EXPLAIN ANALYZE (performance)

| Goal | What to use |

| ----------------------------------------- | ------------------------------------------------------------------------------- |

| Plan shape, estimated costs, no execution | EXPLAIN (FORMAT TEXT, COSTS) or add VERBOSE |

| Actual timings, row counts, buffer hits | EXPLAIN (ANALYZE, BUFFERS, FORMAT TEXT) on the SELECT |

| Buffer + WAL stats | BUFFERS requires ANALYZE; WAL requires ANALYZE (PostgreSQL 13+) |

Safe pattern: the analyzed statement must be only a SELECT (or WITH … SELECT), run on the read-only connection (see Usage below). Example:

PGOPTIONS='-c default_transaction_read_only=on' psql "$DATABASE_URL" -v ON_ERROR_STOP=1 -c "
EXPLAIN (ANALYZE, BUFFERS, VERBOSE, FORMAT TEXT)
SELECT … LIMIT 100;"

Optional flags (when useful): SETTINGS (show non-default GUCs), WAL (with ANALYZE), TIMING (default on in recent versions for ANALYZE).

Caveats:

• EXPLAIN ANALYZE runs the query — can be slow or heavy on large scans; prefer a bounded SELECT (e.g. realistic WHERE, LIMIT matching production shape) when exploring.
• Production / shared DBs — analyzing hot or wide queries can add load; prefer staging, a replica, or off-peak when the user cares about impact.
• EXPLAIN without ANALYZE — does not execute the inner statement (except some special cases); still only wrap read-only SQL.

PostHog: connection and DATABASE_URL

Local Postgres (host machine) — default for this skill

Use this hardcoded URL for day-to-day local queries (matches typical Docker Compose + port 5432 on localhost, SSL off):

| Setting | Value |

| -------- | ----------- |

| Host | localhost |

| Port | 5432 |

| User | posthog |

| Password | posthog |

| Database | posthog |

| SSL | off |

# Prefer this unless the user says their local password/db differs
LOCAL_POSTGRES_URL='postgres://posthog:posthog@localhost:5432/posthog'

Equivalent: postgresql://posthog:posthog@localhost:5432/posthog

Other local DBs on the same server: swap the path only, e.g. ...5432/posthog_persons.

Configuration source of truth (app): posthog/settings/data_stores.py (Django DATABASES, optional replica POSTHOG_POSTGRES_READ_HOST, direct POSTHOG_POSTGRES_DIRECT_HOST, PERSONS_DB_WRITER_URL, product DB routing from products/db_routing.yaml).

When not using the hardcoded URL: Connecting from the host with the same credentials is documented in Developing locally (fe_sendauth troubleshooting). Ensure containers are running.

Default env when DEBUG is on: Django builds a default DATABASE_URL from PGHOST (default db), PGUSER / PGPASSWORD, PGPORT, PGDATABASE — matching in-container hostnames. From the host, use localhost and the same user/password/database name unless your shell already exports DATABASE_URL.

Multiple PostgreSQL databases (same server in local compose; separate logical DBs):

• Main app DB: usually posthog
• Persons DB: posthog_persons (PERSONS_DB_WRITER_URL / PERSONS_DB_READER_URL)
• Product-isolated DBs: posthog_<name> per products/db_routing.yaml (created by docker/postgres-init-scripts/create-product-dbs.sh)
• Other init scripts may create additional DBs (e.g. cyclotron) — inspect docker/postgres-init-scripts/ if needed

Point psql at the right database by changing the path in DATABASE_URL (e.g. .../posthog_persons).

Rust / sqlx: Some services use rust/.env for DATABASE_URL when working from posthog/rust — see rust/README.md.

Usage (command pattern)

Always force the connection read-only via PGOPTIONS='-c default_transaction_read_only=on' so Postgres rejects writes even if the generated SQL is wrong.

> Why PGOPTIONS, not SET SESSION CHARACTERISTICS AS TRANSACTION READ ONLY?

> A psql -c "..." string with multiple statements runs as a single implicit transaction.

> SET SESSION CHARACTERISTICS AS TRANSACTION READ ONLY only sets the default for _subsequent_

> transactions — the in-progress one keeps the read-write mode it was given at BEGIN, so a write

> in the same -c would not be rejected. PGOPTIONS='-c default_transaction_read_only=on' sets

> the GUC at connection startup, so every transaction (including the implicit -c one) starts

> read-only. The inline equivalent is SET TRANSACTION READ ONLY; as the first statement of the

> -c string (it affects the current transaction, unlike SET SESSION CHARACTERISTICS).

Run from the PostHog repo root so relative env paths resolve.

Default — local hardcoded URL (posthog / posthog @ localhost:5432 / db posthog):

PGOPTIONS='-c default_transaction_read_only=on' psql "postgres://posthog:posthog@localhost:5432/posthog" -v ON_ERROR_STOP=1 -c "SELECT 1;"

Option A — DATABASE_URL already in the shell (e.g. after flox activate or manual export):

PGOPTIONS='-c default_transaction_read_only=on' psql "$DATABASE_URL" -v ON_ERROR_STOP=1 -c "SELECT 1;"

Option B — load from a gitignored env file at repo root (if DATABASE_URL is set there):

npx dotenv -e .env -- bash -c "PGOPTIONS='-c default_transaction_read_only=on' psql \"\$DATABASE_URL\" -c 'SELECT ...'"

• Use single quotes for string literals in SQL inside the shell as usual; escape carefully when nesting quotes in -c.
• Default LIMIT 100 unless the user specifies otherwise.
• For wide rows use -x: psql ... -x -c "...".

Schema reference (PostHog)

• Django models → tables: see posthog/models/ (and product packages under products/). Table names are usually prefixed with posthog_ and snake-cased (e.g. posthog_team, posthog_user). Confirm with \dt posthog_* in psql, or check the model's Meta.db_table if nonstandard.
• Migrations: posthog/migrations/ (and product migration paths) define the authoritative DDL over time.
• Person table name: configurable via PERSON_TABLE_NAME (see data_stores.py); default posthog_person.

Debugging with the query runner (PostHog-flavored)

• Confirm a row exists for a team, project, user, or feature-flag linkage; check soft-delete / deleted fields where applicable.
• Compare counts and joins to what the app assumes (e.g. membership, project access).
• Validate replica vs primary read differences only if the user is connected to the right host (replica: POSTHOG_POSTGRES_READ_HOST).
• Use EXPLAIN ANALYZE on SELECT for slow Django queries replicated as SQL — mind loading production-sized data.

Cross-reference

• Local setup and DB gotchas: docs/published/handbook/engineering/developing-locally.md
• Repo CLI: hogli (see .agents/skills/hogli/SKILL.md)