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

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/en/operations/system-tables/information_schema

INFORMATION_SCHEMA

INFORMATION_SCHEMA (information_schema) is a system database that contains views. Using these views, you can get information about the metadata of database objects. These views read data from the columns of the system.columns, system.databases and system.tables system tables.

The structure and composition of system tables may change in different versions of the product, but the support of the information_schema makes it possible to change the structure of system tables without changing the method of access to metadata. Metadata requests do not depend on the DBMS used.

SHOW TABLES FROM INFORMATION_SCHEMA;

┌─name─────┐
│ COLUMNS  │
│ SCHEMATA │
│ TABLES   │
│ VIEWS    │
└──────────┘

INFORMATION_SCHEMA contains the following views:

• COLUMNS
• SCHEMATA
• TABLES
• VIEWS

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 * 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 * 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 (Enum8) — Table type. Possible values:
  • BASE TABLE
  • VIEW
  • FOREIGN TABLE
  • LOCAL TEMPORARY
  • SYSTEM VIEW

Example

Query:

SELECT * 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

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 * 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