Name Resolution

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This page documents name resolution in CockroachDB.

To reference an object (e.g., a table) in a query, you can specify a database, a schema, both, or neither. To resolve which object a query references, CockroachDB scans the appropriate namespaces, following a set of rules outlined below.

Naming hierarchy

For compatibility with PostgreSQL, CockroachDB supports a three-level structure for names. This is called the "naming hierarchy".

In the naming hierarchy, the path to a stored object has three components:

• database name
• schema name
• object name

A CockroachDB cluster can store multiple databases. Each database can store multiple schemas, and each schema can store multiple tables, views, sequences, and user-defined types.

When you first start a cluster, a number of preloaded databases and schemas are included, including the defaultdb database and the public schema. By default, objects (e.g., tables) are stored in the preloaded public schema, in the current database (defaultdb, by default).

In addition to the public schema, CockroachDB supports a fixed set of system catalog schemas, available in every database, that provide ancillary, non-stored data to client applications. For example, information_schema is provided for compatibility with the SQL standard, and pg_catalog and pg_extension are provided for compatibility with PostgreSQL.

To create a new database, use a CREATE DATABASE statement. To create a new schema, use a CREATE SCHEMA statement. The list of all databases can be obtained with SHOW DATABASES. The list of all schemas for a given database can be obtained with SHOW SCHEMAS. The list of all objects for a given schema can be obtained with other SHOW statements.

Migrating namespaces from previous versions of CockroachDB

In CockroachDB versions < v20.2, user-defined schemas are not supported, and all objects created in a given database use the public schema. To provide a multi-level structure for stored objects in earlier versions of CockroachDB, we have recommended using database namespaces instead of schema namespaces.

In CockroachDB versions >= v20.2, we recommend using schema namespaces, not database namespaces, to create a naming structure that is more similar to PostgreSQL.

If you are upgrading to v20.2, take any combination of the following actions after the upgrade is complete:

• Use the ALTER DATABASE ... CONVERT TO SCHEMA statement to convert databases into schemas. This statement is particularly useful if you created databases for the sole purpose of creating a multi-level naming structure. When you convert a database to a schema, all tables, sequences, and user-defined types in the database become child objects of a new schema, and the database is deleted. Note that you cannot convert databases that contain user-defined schemas or views.

• Create new schemas in databases on your cluster. After the schemas are created, use ALTER TABLE ... RENAME, ALTER SEQUENCE ... RENAME, ALTER TYPE ... RENAME, or ALTER VIEW ... RENAME statements to move objects between databases as needed. To move objects between schemas, use ALTER TABLE ... SET SCHEMA, ALTER SEQUENCE ... SET SCHEMA, or ALTER VIEW ... SET SCHEMA.

• If your cluster contains cross-database references (e.g., a cross-database foreign key reference, or a cross-database view reference), use the relevant ALTER TABLE, ALTER SEQUENCE, ALTER TYPE, or ALTER VIEW statements to move any cross-referencing objects to the same database, but different schemas. Cross-database object references were allowed in earlier versions of CockroachDB to make database-object naming hierarchies more flexible for users. In v20.2, creating cross-database references are disabled for foreign keys, views, and sequence ownership. For details, see tracking issue.

How name resolution works

Name resolution occurs separately to look up existing objects and to decide the full name of a new object.

The rules to look up an existing object are as follows:

1. If the name already fully specifies the database and schema, use that information.
2. If the name has a single-component prefix (e.g., a schema name), try to find a schema with the prefix name in the current database and current schema. If that fails, try to find the object in the public schema of a database with the prefix name.
3. If the name has no prefix, use the search path with the current database.

Similarly, the rules to decide the full name of a new object are as follows:

1. If the name already fully specifies the database and schema, use that.
2. If the name has a single-component prefix (e.g., a schema name), try to find a schema with that name. If no such schema exists, use the public schema in the database with the prefix name.
3. If the name has no prefix, use the current schema in the current database.

Parameters for name resolution

Current database

The current database is used when a name is unqualified or has only one component prefix. It is the current value of the database session variable.

• You can view the current value of the database session variable with SHOW database and change it with SET database.

• You can inspect the list of valid database names that can be specified in database with SHOW DATABASES.

• For client apps that connect to CockroachDB using a URL of the form postgres://..., the initial value of the database session variable can be set using the path component of the URL. For example, postgres://node/mydb sets database to mydb when the connection is established.

Search path

The search path is used when a name is unqualified (i.e., has no prefix). It lists the schemas where objects are looked up. Its first element is also the current schema where new objects are created.

• You can set the current search path with SET search_path and inspected it with SHOW search_path.
• You can inspect the list of valid schemas that can be listed in search_path with SHOW SCHEMAS.
• By default, the search path contains $user, public, pg_catalog, and pg_extension. For compatibility with PostgreSQL, pg_catalog is forced to be present in search_path at all times, even when not specified with SET search_path.
• To mimic the behavior in PostgreSQL, CockroachDB will attempt a resolution to pg_extension prior to attempting public.

Current schema

The current schema is used as target schema when creating a new object if the name is unqualified (has no prefix).

• The current schema is always the first value of search_path, for compatibility with PostgreSQL.

• You can inspect the current schema using the special built-in function/identifier current_schema().

Index name resolution

CockroachDB supports the following ways to specify an index name for statements that require one (e.g., DROP INDEX, ALTER INDEX ... RENAME, etc.):

1. Index names are resolved relative to a table name using the @ character, e.g., DROP INDEX tbl@idx;. This is the default and most common syntax.
2. Index names are resolved by searching all tables in the current schema to find a table with an index named idx, e.g., DROP INDEX idx; or (with optional schema prefix) DROP INDEX public.idx;. This syntax is necessary for Postgres compatibility because Postgres index names live in the schema namespace such that e.g., public.idx will resolve to the index idx of some table in the public schema. This capability is used by some ORMs.

The name resolution algorithm for index names supports both partial and complete qualification, using the same name resolution rules as other objects.

Examples

The examples below use the following logical schema as a starting point:

> CREATE DATABASE mydb;

> CREATE TABLE mydb.mytable(x INT);

> SET database = mydb;

Lookup with unqualified names

An unqualified name is a name with no prefix, that is, a simple identifier.

> SELECT * FROM mytable;

This uses the search path over the current database. The search path is $user by default, in the current database. If a $user schema does not exist, the search path resolves to the public schema. In this case, there is no $user schema, and the resolved name is mydb.public.mytable.

> SET database = system;

> SELECT * FROM mytable;

pq: relation "mytable" does not exist

This uses the search path over the current database, which is now system. No schema in the search path contain table mytable, so the look up fails with an error.

Lookup with fully qualified names

A fully qualified name is a name with two prefix components, that is, three identifiers separated by periods.

> SELECT * FROM mydb.public.mytable;

Both the database and schema components are specified. The lookup succeeds if and only if the object exists at that specific location.

Lookup with partially qualified names

A partially qualified name is a name with one prefix component, that is, two identifiers separated by a period. When a name is partially qualified, CockroachDB will try to use the prefix as a schema name first; and if that fails, use it as a database name.

For example:

> SELECT * FROM public.mytable;

This looks up mytable in the public schema of the current database. If the current database is mydb, the lookup succeeds.

To ease development in multi-database scenarios, CockroachDB also allows queries to specify a database name in a partially qualified name. For example:

> SELECT * FROM mydb.mytable;

In that case, CockroachDB will first attempt to find a schema called mydb in the current database. When no such schema exists (which is the case with the starting point in this section), it then tries to find a database called mydb and uses the public schema in that. In this example, this rule applies and the fully resolved name is mydb.public.mytable.

Using the search path to use tables across schemas

Suppose that a client frequently accesses a stored table as well as a virtual table in the Information Schema. Because information_schema is not in the search path by default, all queries that need to access it must mention it explicitly.

For example:

> SELECT * FROM mydb.information_schema.schemata; -- valid

> SELECT * FROM information_schema.schemata; -- valid; uses mydb implicitly

> SELECT * FROM schemata; -- invalid; information_schema not in search_path

For clients that use information_schema often, you can add it to the search path to simplify queries. For example:

> SET search_path = public, information_schema;

> SELECT * FROM schemata; -- now valid, uses search_path

Databases with special names

When resolving a partially qualified name with just one component prefix, CockroachDB will look up a schema with the given prefix name first, and only look up a database with that name if the schema lookup fails. This matters in the (likely uncommon) case where you wish your database to be called information_schema, public, pg_catalog, pg_extension, or crdb_internal.

For example:

> CREATE DATABASE public;

> SET database = mydb;

> CREATE TABLE public.mypublictable (x INT);

The CREATE TABLE statement in this example uses a partially qualified name. Because the public prefix designates a valid schema in the current database, the full name of mypublictable becomes mydb.public.mypublictable. The table is created in database mydb.

To create the table in database public, one would instead use a fully qualified name, as follows:

> CREATE DATABASE public;

> CREATE TABLE public.public.mypublictable (x INT);

Preloaded databases

New clusters and existing clusters upgraded to v21.1 or later will include auto-generated databases, with the following purposes:

• The empty defaultdb database is used if a client does not specify a database in the connection parameters.
• The movr database contains data about users, vehicles, and rides for the vehicle-sharing app MovR.
• The empty postgres database is provided for compatibility with PostgreSQL client applications that require it.
• The startrek database contains quotes from episodes.
• The system database contains CockroachDB metadata and is read-only.

All databases except for the system database can be deleted if they are not needed.

See also

• CREATE SCHEMA
• SET
• SHOW
• SHOW DATABASES
• SHOW SCHEMAS
• Information Schema

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