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ALTER TABLE

Update a table’s definition or schema.

Syntax 
ALTER TABLE <table_name>
{ ADD PRIMARY KEY ( <column_name> [ , ... ] )
  | DROP PRIMARY KEY
  | ADD COLUMNS ( <column_name> <data_type> [ NULL ] [ , ... ] )
  | DROP COLUMN <column_name>
  | { ALTER | MODIFY | CHANGE } COLUMN <old_name> <new_name> <data_type> 
    [ { NULL | NOT NULL | DROP NOT NULL } ]
  | MODIFY COLUMN <column_name>
    { SET MASKING POLICY <function_name> ( <column_name> [, ... ] )
  | UNSET MASKING POLICY <function_name> }
  | { ADD | DROP } ROW ACCESS POLICY <function_name> ( <column_name> [, ... ] )
  | CLUSTER BY ( <column_name> [ , ... ]  )
  | DROP CLUSTERING KEY
  | LOCALSORT BY ( <column_name> [ , ... ] )
  | REFRESH METADATA [ IN <catalog_name> ]
    [ FOR PARTITIONS ( <partition_name> = '<value>') ]
    [ { AVOID | AUTO } PROMOTION ]
    [ { FORCE | LAZY } UPDATE ]
    [ { MAINTAIN | DELETE } WHEN MISSING ]
  | FORGET METADATA
  | SET TBLPROPERTIES ( '<property_name>' = '<property_value>' [ , ... ] )
  | UNSET TBLPROPERTIES ( '<property_name>' [ , ... ] )
  | CREATE AGGREGATE REFLECTION <reflection_name> USING  
    { DIMENSIONS ( <column_name> [ , ... ] )
    | MEASURES ( <column_name> ( <aggregation_type> ) [ , ... ] ) )
    | DIMENSIONS ( <column_name> [ , ... ] ) MEASURES ( <column_name> ( <aggregation_type> ) [ , ...  ] ) }
    [ PARTITION BY ( { <column_name> | <partition_transform> } [ , ... ] ) ]
    [ LOCALSORT BY ( <column_name> [ , ... ] ) ]
  | CREATE EXTERNAL REFLECTION <reflection_name> USING <table_name>
  | CREATE RAW REFLECTION <reflection_name> USING DISPLAY ( <column_name> [ , ... ] )
    [ PARTITION BY ( { <column_name> | <partition_transform> } [ , ... ] ) ]
    [ LOCALSORT BY ( <column_name> [ , ... ] ) ]
  | DROP REFLECTION <reflection_name>
  | REFRESH REFLECTIONS
  | ROUTE REFLECTIONS TO { DEFAULT ENGINE | ENGINE { <engine_name> | <engine_uuid> } }
  | { ADD | DROP } PARTITION FIELD { <column_name> | <partition_transform> }
}

Parameters​

<table_name> String

The name of the table that you want to alter.

ADD PRIMARY KEY ( <column_name> [ , ... ] ) String

Specifies to use one or more existing columns as the primary key of a table. Primary keys provide hints to the query planning during join planning. They can be added to Apache Iceberg tables only. Uniqueness of the values in a primary key is not enforced.

DROP PRIMARY KEY String

Removes a table's primary key. The columns that make up the primary key remain in the table.

ADD COLUMNS ( <column_name> <data_type> [ NULL ] [ , ... ] ) String

Creates one or more columns that have the specified names, data types, character limits, and nullability properties. The size is an attribute of the data type.

These are the supported primitive types:

  • BOOLEAN
  • VARBINARY
  • DATE
  • FLOAT
  • DECIMAL
  • DOUBLE
  • INTERVAL
  • INT
  • BIGINT
  • TIME
  • TIMESTAMP
  • VARCHAR (the length is always 65536 bytes, and any specified value is ignored)

You can define complex types by using either of these two sets of syntax:

Set 1

  • struct_type: ROW( name primitive_or_complex_type, .. )
  • list_type: ARRAY(primitive_or_complex_type)

Examples:

ROW(innerfield INT, anotherinnerfield DOUBLE)
ARRAY(INT)
ROW(innerfield ARRAY(INT))
ARRAY(ROW(innerfield INT))

Set 2

  • struct_type: STRUCT <name : primitive_or_complex_type, ... >
  • list_type: { LIST | ARRAY } < primitive_or_complex_type >

Examples:

STRUCT<innerfield : INT, anotherinnerfield : DOUBLE>
LIST<INT>
ARRAY<INT>
STRUCT<innerfield : LIST<INT>>
LIST<STRUCT<innerfield : INT>>

Use the NULL keyword to specify that a column can contain NULL values. If you don't specify a keyword, the column allows NULL values by default. However, you cannot add a new NOT NULL column to an existing table.

DROP COLUMN <column_name> String

Drops the specified column. This action cannot be undone.

{ ALTER | MODIFY | CHANGE } COLUMN <old_column_name> <new_column_name> <data_type> String

Changes the data type for a column, and gives you the option to rename the column. Renaming columns is supported except for Parquet, JSON, or BSON (Binary JSON used in MongoDB) formats.

Only three types of changes to primitive types are allowed:

  • INT to BIGINT
  • FLOAT to DOUBLE
  • DECIMAL(p, s) to DECIMAL(p', s), if you are widening the precision

You can alter columns that use complex types by using either of these two sets of syntax:

Set 1

  • struct_type: ROW( name primitive_or_complex_type, .. )
  • list_type: ARRAY(primitive_or_complex_type)

Examples:

ROW(innerfield INT, anotherinnerfield DOUBLE)
ARRAY(INT)
ROW(innerfield ARRAY(INT))
ARRAY(ROW(innerfield INT))

Set 2

  • struct_type: STRUCT <name : primitive_or_complex_type, ...>
  • list_type: { LIST | ARRAY } < primitive_or_complex_type >

Examples:

STRUCT<innerfield : INT, anotherinnerfield : DOUBLE>
LIST<INT>
ARRAY<INT>
STRUCT<innerfield : LIST<INT>>
LIST<STRUCT<innerfield : INT>>

[ { NULL | NOT NULL | DROP NOT NULL } ] String   Optional

Uses one of the following keywords to specify whether the column can contain NULL values:

  • NULL: Allow the column to contain NULL values.
  • NOT NULL: Prevent the column from containing NULL values.
  • DROP NOT NULL: Change the column from one that prevents NULL values to one that allows them.

If you don't specify a keyword, the column allows NULL values by default.

You cannot change a column that allows NULL values to one that prevents them, nor can you add a new NOT NULL column to an existing table. However, you can add a new NULL column to an existing table.

MODIFY COLUMN <column_name> String

Specifies the column on which to either set a masking policy or unset one.

SET MASKING POLICY <function_name> ( <column_name> [, ... ] ) String

Sets a policy for masking data. The UDF serving as the masking policy must accept and return the same data type as the column it is masking. If a UDF with the given name does not exist, then the affected table will not be reachable until the policy is dropped or a UDF created. Available only in the Enterprise edition.

UNSET MASKING POLICY <function_name> String

Removes a policy for masking data. Available only in the Enterprise edition.

{ ADD | DROP } ROW ACCESS POLICY <function_name> ( <column_name> [, ... ] ) String

Adds a row-access policy to the table, or removes one from the table. The UDF serving as the row-access policy must return BOOLEAN. If the UDF specified in the policy does not exist, then the Dremio engine gives an error when attempting to query the table. Available only in the Enterprise edition.

CLUSTER BY ( <column_name> [ , ... ] ) String   Optional

The columns on which to cluster the table by. When columns are added or removed from the table using the CLUSTER BY parameter, future OPTIMIZE TABLE commands will follow the updated clustering scheme. Available only in the Enterprise edition.

DROP CLUSTERING KEY String   Optional

Removes the clustering keys on existing tables. The underlying data will remain clustered. However, future OPTIMIZE TABLE commands will not cluster the data. Available only in the Enterprise edition.

LOCALSORT BY ( <column_name> [ , ... ] ) String   Optional

The columns on which to sort new data that is added to the table after the ALTER TABLE command is run. When used in the Reflection parameters, sorts the data that is in the Reflection.

REFRESH METADATA

Refreshes the metadata associated with the specified table. You can choose to either do a full or partial metadata refresh. A full refresh will update the metadata for the entire table. A partial refresh will enable you to only update specified partitions. Optional clauses are available for refreshing a table's metadata. If you choose to use two or more of these clauses, they must be entered in a specified priority order:

  1. Promotion option: either AVOID PROMOTION or AUTO PROMOTION

  2. Update option: either FORCE UPDATE or LAZY UPDATE

  3. Missing option: either MAINTAIN WHEN MISSING or DELETE WHEN MISSING

    note

    If the table is not partitioned, you will receive an error when you attempt to refresh the metadata using the FOR PARTITIONS clause.

FOR PARTITIONS ( <partition_name> = '<value>' ) String   Optional

Use this clause to do a partial refresh of the table's metadata. The <partition_name> identifies the name of the partition to be refreshed. The <value> identifies the specific partition that should be refreshed. Must be contained in single quotes.

{ AVOID | AUTO } PROMOTION Optional

Clauses that determine whether files and folders are promoted to datasets when you run a query. The AVOID PROMOTION prevents queries from promoting files/folders to datasets. The AUTO PROMOTION allows queries to promote files/folders to datasets. This is the default option when you do not include a promotion clause.

{ FORCE | LAZY } UPDATE Optional

Clauses that determine whether metadata is updated when you run a query. The FORCE UPDATE forces a full update of metadata. The LAZY UPDATE does not perform a full update of metadata. This is the default option when you do not include an update clause.

{ MAINTAIN | DELETE } WHEN MISSING Optional

Clauses that determine how missing metadata is handled when you run a query. The MAINTAIN WHEN MISSING Prevents missing metadata from being deleted during refresh. DELETE WHEN MISSING deletes missing metadata during refresh. This is the default option when you do not include a clause.

FORGET METADATA

Deletes the metadata information stored in Dremio for the specified table until the next metadata refresh. The dataset can still be queried using SQL.

SET TBLPROPERTIES ( '<property_name>' = '<property_value>' [ , ... ] )

Sets one or more table properties for controlling the behavior of the table. See Properties of Apache Iceberg Tables for a list of the supported properties.

UNSET TBLPROPERTIES ( '<property_name>' [ , ... ] )

Removes one or more table properties that control the behavior of the table. See Properties of Apache Iceberg Tables for a list of the supported properties.

CREATE AGGREGATE REFLECTION <reflection_name> String

Creates an aggregation Reflection with the specified name.

DIMENSIONS ( <column_name> [ , ... ] ) String

The columns to include as dimensions in the Reflection.

MEASURES ( <column_name> ( <aggregation_type> ) [ , ... ] ) String

The columns to include as measures in the Reflection, and the type of aggregation to perform on them. The possible types are COUNT, MIN, MAX, SUM, and APPROXIMATE COUNT DISTINCT.

PARTITION BY ( { <column_name> | <partition_transform> } [ , ... ] ) String   Optional

The columns or partition-transformation functions on which to partition the data horizontally in the Reflection.

  • <column_name>: The unique name of the column. Multiple columns may be specified as long as the column names include their data type. Use commas to separate each column name and data-type pair. If a column is listed as a partition column, it cannot also be listed as a sort column for the same Reflection. Each column specified must also be listed as a dimension column.

  • <partition_transform>: Applies only to Iceberg tables. Use one of these partition-transformation functions:

    Type of Partition TransformDescription
    identity(<column_name>)Creates one partition per value. This is the default transform. The column can use any supported data type.
    year(<column_name>)Partitions by year. The column must use the DATE or TIMESTAMP data type.
    month(<column_name>)Partitions by month. The column must use the DATE or TIMESTAMP data type.
    day(<column_name>)Partitions on the equivalent of dateint. The column must use the DATE or TIMESTAMP data type.
    hour(<column_name>)Partitions on the equivalent of dateint and hour. The column must use the TIMESTAMP data type.
    bucket(<integer>, <column_name>)Partitions data into the number of partitions specified by an integer. For example, if the integer value N is specified, the data is partitioned into N, or (0 to (N-1)), partitions. The partition in which an individual row is stored is determined by hashing the column value and then calculating <hash_value> mod N. If the result is 0, the row is placed in partition 0; if the result is 1, the row is placed in partition 1; and so on.

    The column can use the DECIMAL, INT, BIGINT, VARCHAR, VARBINARY, DATE, or TIMESTAMP data type.
    truncate(<integer>, <column_name>)If the specified column uses the string data type, truncates strings to a maximum of the number of characters specified by an integer. For example, suppose the specified transform is truncate(1, stateUS). A value of CA is truncated to C, and the row is placed in partition C. A value of CO is also truncated to C, and the row is also placed in partition C.

    If the specified column uses the integer or long data type, truncates column values in the following way: For any truncate(L, col), truncates the column value to the biggest multiple of L that is smaller than the column value. For example, suppose the specified transform is truncate(10, intColumn). A value of 1 is truncated to 0 and the row is placed in the partition 0. A value of 247 is truncated to 240 and the row is placed in partition 240. If the transform is truncate(3, intColumn), a value of 13 is truncated to 12 and the row is placed in partition 12. A value of 255 is not truncated, because it is divisble by 3, and the row is placed in partition 255.

    The column can use the DECIMAL, INT, BIGINT, VARCHAR, or VARBINARY data type.

    Note: The truncate transform does not change column values. It uses column values to calculate the correct partitions in which to place rows.

CREATE EXTERNAL REFLECTION <reflection_name> String

Creates an external Reflection with the specified name.

USING <table_name> String

The path of the derived table to use with the external Reflection.

CREATE RAW REFLECTION <reflection_name> String

Creates a raw Reflection with the specified name.

USING DISPLAY ( <column_name> [ , ... ] ) String

The columns to include in the Reflection.

DROP REFLECTION String   Optional

Drops the specified Reflection defined on the table.

REFRESH REFLECTIONS String   Optional

For information about the refresh action performed, see Trigger Reflection Refreshes.

ROUTE REFLECTIONS TO { DEFAULT ENGINE | ENGINE { <engine_name> | <engine_uuid> } } String

Specify the engine on which to run jobs that create and refresh Reflections that are defined on the table. Use either the name or UUID of the engine. If neither a name nor a UUID is specified, the default engine is used. You can also directly specify the default engine.

{ ADD | DROP } PARTITION FIELD { <column_name> | <partition_transform> } String   Optional

ADD partitions the table data on the values in a single column or by using one of the partition-transformation functions. DROP drops the partition definition.

  • <partition_transform>: Applies only to Iceberg tables. Use one of these partition-transformation functions:

    TransformDescription
    identity( <col> )Explicitly specified identity transform.
    year( <col> )Partition by year. The column uses either the TIMESTAMP or DATE data type.
    month( <ts_col> )Partition by month. The column uses either the TIMESTAMP or DATE data type.
    day( <ts_col> )Partition by day. The column uses either the TIMESTAMP or DATE data type.
    hour( <ts_col> )Partition by hour. The column uses the TIMESTAMP data type.
    bucket( <count>, <col> )Partition by hashed value into <count> buckets.
    truncate( <length>, <col> )Partition by truncated value.
    • Strings are truncated to the specified length.
    • Integer and biginteger values are truncated to bins.
      Example: truncate(10, i) produces 0, 10, 20, and so on.

Examples​

Add Primary Key 
ALTER TABLE services
  ADD PRIMARY KEY (Country_ID);
Add a column 
ALTER TABLE services
  ADD COLUMNS (county varchar);
Add a column to a table 
ALTER TABLE services
 ADD COLUMNS (county varchar);
Modify a column 
ALTER TABLE services
  MODIFY COLUMN tip_amount tip_amount DECIMAL;
Modify a column for a struct table 
ALTER TABLE struct_type
  MODIFY COLUMN a  a struct<x: varchar, y: bigint>;
Rename a column 
ALTER TABLE services
  MODIFY COLUMN tip_amount gratuity_amount DECIMAL;
Change a column to allow NULL values 
ALTER TABLE age_table 
  CHANGE age age INT DROP NOT NULL;
Add a new NULL column to an existing table 
ALTER TABLE my_table 
  ADD COLUMNS ( email VARCHAR NULL, date_of_birth DATE );
Refresh all the metadata for a table 
ALTER TABLE services
  REFRESH METADATA;
Refresh all the metadata for a table using optional clauses 
ALTER TABLE services
  REFRESH METADATA
    AUTO PROMOTION
    LAZY UPDATE
    MAINTAIN WHEN MISSING;
Refresh the metadata for a single partition 
ALTER TABLE Samples."samples.dremio.com"."zips.json"  
  REFRESH METADATA FOR PARTITIONS (state = 'TX');
Refresh the metadata for a single partition using optional clauses 
ALTER TABLE Samples."samples.dremio.com"."zips.json"  
  REFRESH METADATA FOR PARTITIONS (state = 'TX')
    AUTO PROMOTION
    LAZY UPDATE
    MAINTAIN WHEN MISSING;
Forget the metadata for a table 
ALTER TABLE Samples."samples.dremio.com"."zips.json"
  FORGET METADATA;
Create a raw Reflection that sorts customers by last name and partitions them by country 
ALTER TABLE Sales."customers"
  CREATE RAW REFLECTION customers_by_country USING DISPLAY (id,lastName,firstName,address,country)
    PARTITION BY (country)
    LOCALSORT BY (lastName);
Create an aggregation Reflection that counts the cities per state and sorts by state 
ALTER TABLE Samples."samples.dremio.com"."zips.json"
  CREATE AGGREGATE REFLECTION per_state USING
  DIMENSIONS (state) MEASURES (city (COUNT))
  LOCALSORT BY (state);
Create an aggregation Reflection that counts p1 and p2 as measures in the Reflection 
ALTER TABLE BI."p1_table"
  CREATE AGGREGATE REFLECTION p1_table_agg USING
  DIMENSIONS (c1,c2) MEASURES (p1(COUNT, MIN), p2(SUM, MAX, APPROXIMATE COUNT DISTINCT))
    PARTITION BY (c1)
    LOCALSORT BY (c2)
Route Reflections to Engine 1 
ALTER TABLE "Table 1" ROUTE REFLECTIONS TO ENGINE "Engine 1";
ALTER VIEW "View 1" ROUTE REFLECTIONS TO ENGINE "Engine 1";
ALTER VIEW "View 3" ROUTE REFLECTIONS TO ENGINE "Engine 1";
Add a column for an Apache Iceberg table 
ALTER TABLE myTable
  ADD COLUMNS (address VARCHAR);
Change the data type of a column to BIGINT for an Apache Iceberg table 
ALTER TABLE myTable
  ALTER COLUMN id id BIGINT;
Modify clustering keys 
ALTER TABLE clustered_table
  CLUSTER BY (Col_one, Col_two, Col_three);
Drop clustering keys 
ALTER TABLE clustered_table
  DROP CLUSTERING KEY;