# Database URLs and dev databases

Accepted database URL formats, and the difference between the target, dev, shadow, and throwaway databases.

Source: https://docs.ptah.run/latest/concepts/database-urls-and-dev-databases/

Every Ptah command that touches a database takes a URL, and the URL's scheme
selects the engine. The same syntax names databases in four roles: the target
you are changing, and up to three kinds of disposable databases that exist so
mistakes happen somewhere harmless. This page defines all four; other pages
link here instead of redefining them.

## URL formats

| Engine | Example |
| --- | --- |
| PostgreSQL | `postgres://user:pass@localhost:5432/app` |
| MySQL | `mysql://user:pass@localhost:3306/app` (Go-driver form `mysql://user:pass@tcp(localhost:3306)/app` is also accepted) |
| MariaDB | `mariadb://user:pass@localhost:3306/app` (the Atlas CLI spelling `maria://user:pass@localhost:3306/app` is also accepted) |
| SQLite | `sqlite://relative.db`, `sqlite:///absolute/path/app.db`, `sqlite:file:C:/absolute/windows/path/app.db`, `sqlite:///:memory:`, `sqlite:file:memdb1?mode=memory&cache=shared` |
| SQL Server | `sqlserver://sa:pass@localhost:1433?database=app` (plus a Ptah-only `schema` parameter — see [SQL Server](../../databases/sqlserver/)) |
| ClickHouse | `clickhouse://user:pass@localhost:9000/app` |
| CockroachDB | `cockroachdb://user:pass@localhost:26257/app` |
| YugabyteDB | `yugabytedb://user:pass@localhost:5433/app` |
| Spanner (PostgreSQL interface) | `spanner://user:pass@localhost:5432/app` |
| Oracle | `oracle://user:pass@localhost:1521/service` (renders, plans, reads a live catalog, and runs versioned migrations) |

Scheme aliases normalize to the canonical dialect (`postgresql://`,
`sqlite3://`, `mssql://`, `crdb://`, `ysql://`, `ch://`, and more) — the full
alias list is on the
[Database support matrix](../../databases/support-matrix/). A URL with an
unrecognized scheme fails with `unsupported database dialect`.

A MySQL or MariaDB server behind a Unix socket takes the Atlas CLI socket
form, `mysql+unix://user:pass@/run/mysqld/mysqld.sock?database=app`, or
`mariadb+unix://` and `maria+unix://`: the path is the socket, `database`
names the database, and a host is ignored. The Go-driver form
`mysql://user:pass@unix(/run/mysqld/mysqld.sock)/app` reaches the same socket.

A MySQL or MariaDB URL naming no database is the whole server, as in the
Atlas CLI; as a dev database it is a
[dev server](../../atlas/migrate-commands/#a-whole-dev-server).

## The four database roles

**The target database** is the one a command reads or changes: `--db-url` on
native commands, `--url` on Atlas-compatible ones. It is the only database
whose state matters after the command exits.

**A dev database** (`--dev-url`) is a disposable replay target used for
validation:

- `ptah migrations validate` and `ptah migrations lint` clean it and replay the
  migration directory on it to prove the SQL executes;
- `schema apply` rehearses its plan on it before touching the target;
- `schema diff` replays a migration directory on it, and creates a `--from`
  schema file on it when `--to` is a database or migration directory;
- Atlas-compatible verbs use it for planning, linting, and rollback
  verification.

A dev database must hold nothing the reset would drop when a command starts.
Ptah refuses a table before anything is dropped, as the Atlas CLI does, and
any other such object too. Ptah then cleans the
replay realm before migration execution and after the replay, failed or not.
Commands read the replayed state before the final cleanup, on the same session.
No fixed time limit applies to a cleanup: it takes as long as emptying the
realm takes. After an interrupt, the cleanup has 30 seconds to finish; a second
interrupt stops the process at once.

Everything a dev database runs stays inside it. Where a schema names a database
rather than a namespace — MySQL, MariaDB, and ClickHouse — a plan carrying the
target's schema name is re-scoped onto the dev database before it is rehearsed,
and a statement naming some third database is refused instead of run.

`mysql://`, `mariadb://` and `maria://` open one driver, and the server's
version banner, not the scheme, says which engine answers. A dev URL may spell
the family differently from its target, and two spellings of one database are
one database. `schema apply` and `migrate diff` refuse a MySQL dev database for
a MariaDB target, and the reverse.

**A shadow database** (`--shadow-db`) is a disposable verification target for
commands that write or record migrations: `ptah migrations generate` replays
the directory — including the new migration, up, down, and up again — before
keeping any files, and `ptah migrations checkpoint` and
`ptah migrations baseline` use it to verify that migrations reproduce the
expected schema before anything is recorded. `ptah migrations down` uses it to
verify the rollback plan before changing the target.

The shadow database must meet the same rule and must not be the target's live
database realm. Ptah checks both before changing either database, and empties
the shadow database again after each run.

**A throwaway test database** is what `ptah migrations test` and
`ptah schema test` run cases against: by default a fresh ephemeral SQLite
database per case, or the database passed with `--db-url` when tests must
exercise a real server dialect — see
[Test migrations and schemas](../../testing/migrations-and-schema/).

## Consequences

- **Disposable means Ptah may drop everything it supports.** Dev and shadow
  database workflows clean user objects, and test seed steps bypass the
  seeder's protected-environment guards. Point these flags at scratch databases
  only, never at a real environment. Rollback verification, `schema apply`,
  `schema diff`, and `migrate diff` refuse, before any reset, a dev or shadow
  URL naming a database they read, by URL or by the realm each server reports.
  Equal network database names fail closed across different endpoints because
  DNS aliases and replicated members cannot be proven independent before
  destructive cleanup. Cleanup rejects known system, template, metadata, and
  administrative database names.
- **Migration-diff scope does not reduce the replay realm.** Repeated
  `--schema` values select which schemas `migrate diff` compares and emits. They do not
  limit which schemas a migration may create or which user schemas final
  cleanup removes. PostgreSQL extensions remain in both scoped projections:
  an extension is a database-wide identity, and its schema records installation
  placement rather than ownership by that schema. An extension declared on
  both sides remains synced even when installed outside the named schemas; an
  extension omitted from desired state remains an explicit global removal.
- **The replay realm follows the database engine.** PostgreSQL, CockroachDB,
  and YugabyteDB cleanup treats all user schemas and user-installed extensions
  in the selected database as one dependency graph. Extensions the dev
  database held when the run started stay installed, with everything they own,
  so a migration may use a type from one without creating it, as under Atlas;
  a schema one is installed in is emptied in place. A dev URL that selects a
  schema with `search_path` also keeps the database's other schemas as they
  were, `public` included. An extension the run created is dropped with the
  schemas and tables it owns; TimescaleDB's catalog schemas are this shape.
  MySQL, MariaDB, and ClickHouse cleanup owns the selected database. SQL
  Server cleanup owns all supported user schemas in the selected database.
  SQLite cleanup owns `main` on one pinned session.
- **PostgreSQL cleanup gives back the schema it empties.** The schema the dev
  URL selects, `public` when it selects none, comes back after the cleanup
  with the owner, grants and comment it had. A role created on the server
  afterwards can use `public` as it could before.
- **MySQL-family cleanup needs global catalog visibility.** MySQL cleanup
  credentials require global `SELECT`, `DROP`, `ALTER`, `ALTER ROUTINE`,
  `EVENT`, `LOCK TABLES`, and `PROCESS`. MySQL also requires global `TRIGGER`
  and, on MySQL 8.0.20 and newer, `SHOW_ROUTINE`; MariaDB requires global
  `SHOW VIEW`. Ptah checks these privileges before destructive DDL. It fails
  closed when another user database contains a routine, event, or trigger
  because stored-program bodies can reference the cleanup realm without a
  catalog dependency. Use dedicated server instances and credentials only for
  disposable dev databases.
- **ClickHouse realm cleanup requires 24.11 or newer.** Ptah uses `CHECK GRANT`
  with global `SHOW DATABASES` and `SHOW TABLES` to prove complete catalog
  visibility before dropping objects. ClickHouse does not expose ordinary-view
  dependencies, so Ptah fails closed when another user database contains a
  view, materialized view, live/window view, dictionary, or `Buffer`,
  `Distributed`, or `Merge` table. Older servers fail before cleanup because
  role-aware visibility cannot be proven safely.
- **PostgreSQL-family cleanup rejects database-scoped artifacts.** PostgreSQL
  and YugabyteDB reject publications, subscriptions, logical replication
  slots, event triggers, and non-extension foreign-data wrappers, servers, or
  user mappings before DDL. An object the server created when it built the
  database template is not counted: YugabyteDB 2026.1.2 puts the foreign
  server `yb_global_views_server` into every database, and the cleanup leaves
  it, and the extensions it depends on, in place. PostgreSQL also removes and
  verifies database large objects transactionally. YugabyteDB does not run
  that PostgreSQL-specific large-object operation.
- **SQL Server cleanup rejects replication state.** A replication-enabled
  database or replicated table fails before DDL, along with other unsupported
  database-scoped artifacts. Remove replication configuration or use a
  dedicated disposable database before replay.
- **Cross-realm operations fail before execution.** Ptah rejects direct
  statements that switch or mutate another database, protected namespace,
  server, cluster, temporary namespace, external file, or attached SQLite
  database. It also rejects statement forms whose nested SQL cannot be
  confined safely during replay.
- **Replay cleanup is serialized by realm.** PostgreSQL, YugabyteDB, MySQL,
  MariaDB, and SQL Server use database advisory locks. SQLite, ClickHouse, and
  CockroachDB use an operating-system file lock keyed by the normalized
  database identity. That file lock coordinates only Ptah processes that
  resolve the same temporary lock path and can access it, normally processes
  running as the same operating-system user with the same temporary-directory
  configuration. Different users, different temporary directories, other
  hosts, and non-Ptah clients are not coordinated. Cross-host ClickHouse and
  CockroachDB replay is unsupported because neither engine provides the
  required effective session advisory lock.
- **Match the target engine.** Replay verification proves the SQL executes on
  the engine it ran against, so a dev or shadow database should run the same
  engine (and ideally the same version) as the target.
- **Every flag has an environment variable.** `PTAH_DB_URL`, `PTAH_DEV_URL`,
  and `PTAH_SHADOW_DB` set the corresponding flags, which keeps credentials
  out of CI command lines. A malformed non-empty value fails before command
  execution. See [Configuration](../../reference/configuration/).

### Statement forms that fail closed

Replay rejects SQL sublanguages and storage mechanisms whose effects cannot be
proven to stay inside the disposable realm:

- PostgreSQL-family `DO`, `CALL`, routine creation or alteration, foreign-table
  creation or alteration, foreign servers, `IMPORT FOREIGN SCHEMA`,
  `SET search_path`, `SELECT INTO` protected namespaces, and CockroachDB's
  `ALTER DEFAULT PRIVILEGES FOR ALL ROLES` without `IN SCHEMA`.
- MySQL and MariaDB executable comments, `CALL`, events, triggers, routines,
  `LOAD DATA`/`LOAD XML`, and externally backed `FEDERATED` or `CONNECT`
  tables.
- SQL Server `EXEC`/`EXECUTE`, procedure/function/trigger creation or
  alteration, synonym/external-table creation, `BULK INSERT`, `BACKUP`,
  `RESTORE`, and server-level maintenance statements.
- ClickHouse remote, distributed, replicated, and unknown table engines,
  external dictionary sources, and `FREEZE`/`UNFREEZE`. Tables and standalone
  materialized views must select an explicitly allowlisted engine.
- SQLite `ATTACH`, `DETACH`, temporary objects, and non-restorable pragmas.

Replay runs every migration of a directory on one session, so a PostgreSQL
`SET` or `RESET` that changes the session would carry into the migrations after
it, and is refused. A setting that ends with its transaction is accepted:

- `SET LOCAL` and `SET CONSTRAINTS`;
- `set_config(name, value, true)`, with the name written as a string and `true`
  written as the keyword.

Replay runs each migration the way `migrations up` does, in one transaction
unless the file opts out with `-- +ptah no_transaction` or
`-- atlas:txmode none`. Such a setting holds for the rest of its migration and
is gone before the next. In a file that opts out, PostgreSQL ignores a
`SET LOCAL` with a warning, on replay and on apply alike.

Three parameters stay refused even for one transaction. `search_path` (and its
alias `SET SCHEMA`) decides where an unqualified name lands, which would let a
`CREATE FUNCTION` reach `pg_catalog`. `role` and `session_authorization` change
who the rest of the migration runs as.

The rejection happens while Ptah validates the whole migration, before its
first statement executes. Realm-local removals Ptah can classify without
reading a routine body, such as `DROP FUNCTION`, `DROP FOREIGN TABLE`,
`DROP SYNONYM` and `DROP EXTERNAL TABLE`, stay allowed.

### A server Ptah provisions

A `docker://` dev URL starts a server for one command and removes it
afterwards, so the server holds only what the run put there. On such a server,
replay also runs the statements it refuses elsewhere only because their effect
reaches past the dev database:

- PostgreSQL-family `DO`, `CALL`, routine creation and alteration,
  `CREATE` and `DROP` of a role, user, group or database, `ALTER ROLE`
  without `SET` or `RESET`, role membership, privileges on a role, database,
  schema, language or parameter, CockroachDB's
  `ALTER DEFAULT PRIVILEGES FOR ALL ROLES` without `IN SCHEMA`, `DROP OWNED`,
  `REASSIGN OWNED`, and `COMMENT ON ROLE` or `DATABASE`.
- MySQL and MariaDB routines, triggers, `CALL`, `GRANT`, `REVOKE`, `CREATE`
  and `DROP` of a user, role or database, and writes to another database.

So a migration directory that creates its role in a `DO` block or defines a
function replays on `docker://postgres/18/dev`.

The rest of the list above still fails closed on a provisioned server. Those
statements change the replay session or the sessions the rest of the command
opens (`SET search_path`, `ALTER ROLE ... SET`, any `ALTER DATABASE`,
transaction control), change how the command's later SQL runs (`ALTER SYSTEM`,
event triggers, casts, languages), reach past the container (foreign servers,
subscriptions, `dblink`, an external `COPY`), or mutate a protected namespace
such as `pg_catalog`. A MySQL or MariaDB event stays refused because the
scheduler runs it during the rest of the command.

The decision reads what Ptah recorded when it started the server, not the
URL's spelling. A server URL keeps the whole list, because such a server may
hold databases and roles that are not the run's: a role a replay created there
would outlive the command.

### A server declared disposable

A server Ptah did not start can be as disposable: a CI service container
the job throws away, or a container running an image `docker://` cannot name,
such as TimescaleDB. `PTAH_DEV_SERVER_DISPOSABLE=1` declares that the server
`--dev-url` names is the run's own. Replay then runs the statements listed
above on it, as on a server Ptah provisions, and refuses the same rest.

```bash
PTAH_DEV_SERVER_DISPOSABLE=1 ptah migrations validate --dir migrations --dev-url "$DEV_URL"
```

The declaration covers the whole server, and Ptah cleans only the dev
database after a replay. A role or database a replay creates stays on the
server until the container is removed. A later command that replays the same
directory on the same server meets it, so a migration that creates a role
without checking for it first fails the second time. Declare only a server
that nothing else uses.

Every command that replays a migration directory on a dev database reads the
variable, on both binaries. A value that is not a boolean fails the command
before it does any work, whether or not that run replays. Strict Atlas
compatibility keeps the variable, because the pinned community binary runs
these statements on any dev database.

## Where it appears

- Replay validation with a dev database: [Integrity and safety](../../versioned/integrity-and-safety/).
- Linting against a dev database: [Lint and gate unsafe SQL](../../versioned/lint/).
- Shadow-verified generation and baselining: [Generate migrations](../../versioned/generate/) and [Adopt an existing database](../../start/adopt-an-existing-database/).
- Engine-specific URL behavior: [Database support matrix](../../databases/support-matrix/).