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)
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. 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: schema inspect describes every user database, and other commands refuse it (stokaro/ptah#3789).

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.

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.

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.

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.
• Linting against a dev database: Lint and gate unsafe SQL.
• Shadow-verified generation and baselining: Generate migrations and Adopt an existing database.
• Engine-specific URL behavior: Database support matrix.