ClickHouse/docs/en/operations/system-tables/information_schema.md

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INFORMATION_SCHEMA

INFORMATION_SCHEMA (or: information_schema) is a system database which provides a (somewhat) standardized, DBMS-agnostic view on metadata of database objects. The views in INFORMATION_SCHEMA are generally inferior to normal system tables but tools can use them to obtain basic information in a cross-DBMS manner. The structure and content of views in INFORMATION_SCHEMA is supposed to evolves in a backwards-compatible way, i.e. only new functionality is added but existing functionality is not changed or removed. In terms of internal implementation, views in INFORMATION_SCHEMA usually map to to normal system tables like system.columns, system.databases and system.tables.

SHOW TABLES FROM INFORMATION_SCHEMA;

-- or:
SHOW TABLES FROM information_schema;

┌─name────────────────────┐
│ COLUMNS                 │
│ KEY_COLUMN_USAGE        │
│ REFERENTIAL_CONSTRAINTS │
│ SCHEMATA                │
│ TABLES                  │
│ VIEWS                   │
│ columns                 │
│ key_column_usage        │
│ referential_constraints │
│ schemata                │
│ tables                  │
│ views                   │
└─────────────────────────┘

INFORMATION_SCHEMA contains the following views:

• COLUMNS
• SCHEMATA
• TABLES
• VIEWS
• KEY_COLUMN_USAGE
• REFERENTIAL_CONSTRAINTS

Case-insensitive equivalent views, e.g. INFORMATION_SCHEMA.columns are provided for reasons of compatibility with other databases. The same applies to all the columns in these views - both lowercase (for example, table_name) and uppercase (TABLE_NAME) variants are provided.

COLUMNS

Contains columns read from the system.columns system table and columns that are not supported in ClickHouse or do not make sense (always NULL), but must be by the standard.

Columns:

• table_catalog (String) — The name of the database in which the table is located.
• table_schema (String) — The name of the database in which the table is located.
• table_name (String) — Table name.
• column_name (String) — Column name.
• ordinal_position (UInt64) — Ordinal position of a column in a table starting with 1.
• column_default (String) — Expression for the default value, or an empty string if it is not defined.
• is_nullable (UInt8) — Flag that indicates whether the column type is Nullable.
• data_type (String) — Column type.
• character_maximum_length (Nullable(UInt64)) — Maximum length in bytes for binary data, character data, or text data and images. In ClickHouse makes sense only for FixedString data type. Otherwise, the NULL value is returned.
• character_octet_length (Nullable(UInt64)) — Maximum length in bytes for binary data, character data, or text data and images. In ClickHouse makes sense only for FixedString data type. Otherwise, the NULL value is returned.
• numeric_precision (Nullable(UInt64)) — Accuracy of approximate numeric data, exact numeric data, integer data, or monetary data. In ClickHouse it is bit width for integer types and decimal precision for Decimal types. Otherwise, the NULL value is returned.
• numeric_precision_radix (Nullable(UInt64)) — The base of the number system is the accuracy of approximate numeric data, exact numeric data, integer data or monetary data. In ClickHouse it's 2 for integer types and 10 for Decimal types. Otherwise, the NULL value is returned.
• numeric_scale (Nullable(UInt64)) — The scale of approximate numeric data, exact numeric data, integer data, or monetary data. In ClickHouse makes sense only for Decimal types. Otherwise, the NULL value is returned.
• datetime_precision (Nullable(UInt64)) — Decimal precision of DateTime64 data type. For other data types, the NULL value is returned.
• character_set_catalog (Nullable(String)) — NULL, not supported.
• character_set_schema (Nullable(String)) — NULL, not supported.
• character_set_name (Nullable(String)) — NULL, not supported.
• collation_catalog (Nullable(String)) — NULL, not supported.
• collation_schema (Nullable(String)) — NULL, not supported.
• collation_name (Nullable(String)) — NULL, not supported.
• domain_catalog (Nullable(String)) — NULL, not supported.
• domain_schema (Nullable(String)) — NULL, not supported.
• domain_name (Nullable(String)) — NULL, not supported.

Example

Query:

SELECT table_catalog,
       table_schema,
       table_name,
       column_name,
       ordinal_position,
       column_default,
       is_nullable,
       data_type,
       character_maximum_length,
       character_octet_length,
       numeric_precision,
       numeric_precision_radix,
       numeric_scale,
       datetime_precision,
       character_set_catalog,
       character_set_schema,
       character_set_name,
       collation_catalog,
       collation_schema,
       collation_name,
       domain_catalog,
       domain_schema,
       domain_name,
       column_comment,
       column_type
FROM INFORMATION_SCHEMA.COLUMNS
WHERE (table_schema = currentDatabase() OR table_schema = '')
  AND table_name NOT LIKE '%inner%' 
LIMIT 1 
FORMAT Vertical;

Result:

Row 1:
──────
table_catalog:            default
table_schema:             default
table_name:               describe_example
column_name:              id
ordinal_position:         1
column_default:
is_nullable:              0
data_type:                UInt64
character_maximum_length: ᴺᵁᴸᴸ
character_octet_length:   ᴺᵁᴸᴸ
numeric_precision:        64
numeric_precision_radix:  2
numeric_scale:            0
datetime_precision:       ᴺᵁᴸᴸ
character_set_catalog:    ᴺᵁᴸᴸ
character_set_schema:     ᴺᵁᴸᴸ
character_set_name:       ᴺᵁᴸᴸ
collation_catalog:        ᴺᵁᴸᴸ
collation_schema:         ᴺᵁᴸᴸ
collation_name:           ᴺᵁᴸᴸ
domain_catalog:           ᴺᵁᴸᴸ
domain_schema:            ᴺᵁᴸᴸ
domain_name:              ᴺᵁᴸᴸ

SCHEMATA

Contains columns read from the system.databases system table and columns that are not supported in ClickHouse or do not make sense (always NULL), but must be by the standard.

Columns:

• catalog_name (String) — The name of the database.
• schema_name (String) — The name of the database.
• schema_owner (String) — Schema owner name, always 'default'.
• default_character_set_catalog (Nullable(String)) — NULL, not supported.
• default_character_set_schema (Nullable(String)) — NULL, not supported.
• default_character_set_name (Nullable(String)) — NULL, not supported.
• sql_path (Nullable(String)) — NULL, not supported.

Example

Query:

SELECT catalog_name,
       schema_name,
       schema_owner,
       default_character_set_catalog,
       default_character_set_schema,
       default_character_set_name,
       sql_path
FROM information_schema.schemata
WHERE schema_name ilike 'information_schema' 
LIMIT 1 
FORMAT Vertical;

Result:

Row 1:
──────
catalog_name:                  INFORMATION_SCHEMA
schema_name:                   INFORMATION_SCHEMA
schema_owner:                  default
default_character_set_catalog: ᴺᵁᴸᴸ
default_character_set_schema:  ᴺᵁᴸᴸ
default_character_set_name:    ᴺᵁᴸᴸ
sql_path:                      ᴺᵁᴸᴸ

TABLES

Contains columns read from the system.tables system table.

Columns:

• table_catalog (String) — The name of the database in which the table is located.
• table_schema (String) — The name of the database in which the table is located.
• table_name (String) — Table name.
• table_type (String) — Table type. Possible values:
  • BASE TABLE
  • VIEW
  • FOREIGN TABLE
  • LOCAL TEMPORARY
  • SYSTEM VIEW
• table_comment (Nullable(String)) — The comment used when creating the table.
• table_collation (Nullable(String)) — The table default collation. Always utf8mb4.

Example

Query:

SELECT table_catalog, 
       table_schema, 
       table_name, 
       table_type, 
       table_collation, 
       table_comment
FROM INFORMATION_SCHEMA.TABLES
WHERE (table_schema = currentDatabase() OR table_schema = '')
  AND table_name NOT LIKE '%inner%'
LIMIT 1 
FORMAT Vertical;

Result:

Row 1:
──────
table_catalog:   default
table_schema:    default
table_name:      describe_example
table_type:      BASE TABLE
table_collation: utf8mb4_0900_ai_ci
table_comment:   

VIEWS

Contains columns read from the system.tables system table, when the table engine View is used.

Columns:

• table_catalog (String) — The name of the database in which the table is located.
• table_schema (String) — The name of the database in which the table is located.
• table_name (String) — Table name.
• view_definition (String) — SELECT query for view.
• check_option (String) — NONE, no checking.
• is_updatable (Enum8) — NO, the view is not updated.
• is_insertable_into (Enum8) — Shows whether the created view is materialized. Possible values:
  • NO — The created view is not materialized.
  • YES — The created view is materialized.
• is_trigger_updatable (Enum8) — NO, the trigger is not updated.
• is_trigger_deletable (Enum8) — NO, the trigger is not deleted.
• is_trigger_insertable_into (Enum8) — NO, no data is inserted into the trigger.

Example

Query:

CREATE VIEW v (n Nullable(Int32), f Float64) AS SELECT n, f FROM t;
CREATE MATERIALIZED VIEW mv ENGINE = Null AS SELECT * FROM system.one;
SELECT table_catalog,
       table_schema,
       table_name,
       view_definition,
       check_option,
       is_updatable,
       is_insertable_into,
       is_trigger_updatable,
       is_trigger_deletable,
       is_trigger_insertable_into
FROM information_schema.views
WHERE table_schema = currentDatabase() 
LIMIT 1
FORMAT Vertical;

Result:

Row 1:
──────
table_catalog:              default
table_schema:               default
table_name:                 mv
view_definition:            SELECT * FROM system.one
check_option:               NONE
is_updatable:               NO
is_insertable_into:         YES
is_trigger_updatable:       NO
is_trigger_deletable:       NO
is_trigger_insertable_into: NO

KEY_COLUMN_USAGE (#key_column_usage)

Contains columns from the system.tables system table which are restricted by constraints.

Columns:

• constraint_catalog (String) — Currently unused. Always def.
• constraint_schema (String) — The name of the schema (database) to which the constraint belongs.
• constraint_name (Nullable(String)) — The name of the constraint.
• table_catalog (String) — Currently unused. Always def.
• table_schema (String) — The name of the schema (database) to which the table belongs.
• table_name (String) — The name of the table that has the constraint.
• column_name (Nullable(String)) — The name of the column that has the constraint.
• ordinal_position (UInt32) — Currently unused. Always 1.
• position_in_unique_constraint (Nullable(UInt32)) — Currently unused. Always NULL.
• referenced_table_schema (Nullable(String)) — Currently unused. Always NULL.
• referenced_table_name (Nullable(String)) — Currently unused. Always NULL.
• referenced_column_name (Nullable(String)) — Currently unused. Always NULL.

Example

CREATE TABLE test (i UInt32, s String) ENGINE MergeTree ORDER BY i;
SELECT constraint_catalog,
       constraint_schema,
       constraint_name,
       table_catalog,
       table_schema,
       table_name,
       column_name,
       ordinal_position,
       position_in_unique_constraint,
       referenced_table_schema,
       referenced_table_name,
       referenced_column_name
FROM information_schema.key_column_usage 
WHERE table_name = 'test' 
FORMAT Vertical;

Result:

Row 1:
──────
constraint_catalog:            def
constraint_schema:             default
constraint_name:               PRIMARY
table_catalog:                 def
table_schema:                  default
table_name:                    test
column_name:                   i
ordinal_position:              1
position_in_unique_constraint: ᴺᵁᴸᴸ
referenced_table_schema:       ᴺᵁᴸᴸ
referenced_table_name:         ᴺᵁᴸᴸ
referenced_column_name:        ᴺᵁᴸᴸ

REFERENTIAL_CONSTRAINTS (#referential_constraints)

Contains information about foreign keys. Currently returns an empty result (no rows) which is just enough to provide compatibility with 3rd party tools like Tableau Online.

Columns:

• constraint_catalog (String) — Currently unused.
• constraint_schema (String) — Currently unused.
• constraint_name (Nullable(String)) — Currently unused.
• unique_constraint_catalog (String) — Currently unused.
• unique_constraint_schema (String) — Currently unused.
• unique_constraint_name (Nullable(String)) — Currently unused.
• match_option (String) — Currently unused.
• update_rule (String) — Currently unused.
• delete_rule (String) — Currently unused.
• table_name (String) — Currently unused.
• referenced_table_name (String) — Currently unused.