Vertica Analytics Platform Version 9.2.x Documentation

Dropping Table Columns

ALTER TABLE…DROP COLUMN drops the specified table column and the ROS containers that correspond to the dropped column:

ALTER TABLE [schema.]table DROP [ COLUMN ] [IF EXISTS] column [CASCADE | RESTRICT]

After the drop operation completes, data backed up from the current epoch onward recovers without the column. Data recovered from a backup that precedes the current epoch re-add the table column. Because drop operations physically purge object storage and catalog definitions (table history) from the table, AT EPOCH (historical) queries return nothing for the dropped column.

The altered table retains its object ID.

Drop column operations can be fast because these catalog-level changes do not require data reorganization, so Vertica can quickly reclaim disk storage.

Restrictions

  • You cannot drop or alter a primary key column or a column that participates in the table partitioning clause.
  • You cannot drop the first column of any projection sort order, or columns that participate in a projection segmentation expression.
  • All nodes must be up.
  • You cannot drop a column associated with an access policy. Attempts to do so produce the following error:
    ERROR 6482: Failed to parse Access Policies for table "t1"

Using CASCADE to Force a Drop

If the table column to drop has dependencies, you must qualify the DROP COLUMN clause with the CASCADE option. For example, the target column might be specified in a projection sort order. In this and other cases, DROP COLUMN…CASCADE handles the dependency by reorganizing catalog definitions or dropping a projection. In all cases, CASCADE performs the minimal reorganization required to drop the column.

Use CASCADE to drop a column with the following dependencies:

Dropped column dependency CASCADE behavior

Any constraint

Vertica drops the column when a FOREIGN KEY constraint depends on a UNIQUE or PRIMARY KEY constraint on the referenced columns.

Specified in projection sort order

Vertica truncates projection sort order up to and including the projection that is dropped without impact on physical storage for other columns and then drops the specified column. For example if a projection's columns are in sort order (a,b,c), dropping column b causes the projection's sort order to be just (a), omitting column (c).

Specified in a projection segmentation expression

The column to drop is integral to the projection definition. If possible, Vertica drops the projection as long as doing so does not compromise K-safety; otherwise, the transaction rolls back.

Referenced as default value of another column

See Dropping a Column Referenced as Default, below.


Dropping a Column Referenced as Default

You might want to drop a table column that is referenced by another column as its default value. For example, the following table is defined with two columns, a and b:, where b gets its default value from column a:

=> CREATE TABLE x (a int) UNSEGMENTED ALL NODES;
CREATE TABLE
=> ALTER TABLE x ADD COLUMN b int DEFAULT a;
ALTER TABLE

In this case, dropping column a requires the following procedure:

  1. Remove the default dependency through ALTER COLUMN..DROP DEFAULT:
    => ALTER TABLE x ALTER COLUMN b DROP DEFAULT;
  2. Create a replacement superprojection for the target table if one or both of the following conditions is true:
    • The target column is the table's first sort order column. If the table has no explicit sort order, the default table sort order specifies the first table column as the first sort order column. In this case, the new superprojection must specify a sort order that excludes the target column.
    • If the table is segmented, the target column is specified in the segmentation expression. In this case, the new superprojection must specify a segmentation expression that excludes the target column.

    Given the previous example, table x has a default sort order of (a,b). Because column a is the table's first sort order column, you must create a replacement superprojection that is sorted on column b:

    => CREATE PROJECTION x_p1 as select * FROM x ORDER BY b UNSEGMENTED ALL NODES;
  3. Run START_REFRESH:
    => SELECT START_REFRESH();
                  START_REFRESH
    ----------------------------------------
     Starting refresh background process.
    
    (1 row)
  4. Run MAKE_AHM_NOW:
    => SELECT MAKE_AHM_NOW();
             MAKE_AHM_NOW
    -------------------------------
     AHM set (New AHM Epoch: 1231)
    (1 row)
  5. Drop the column:
    => ALTER TABLE x DROP COLUMN a CASCADE;

Vertica implements the CASCADE directive as follows:

  • Drops the original superprojection for table x (x_super).
  • Updates the replacement superprojection x_p1 by dropping column a.

Examples

The following series of commands successfully drops a BYTEA data type column:

=> CREATE TABLE t (x BYTEA(65000), y BYTEA, z BYTEA(1));       
CREATE TABLE
=> ALTER TABLE t DROP COLUMN y;
ALTER TABLE
=> SELECT y FROM t;
ERROR 2624:  Column "y" does not exist
=> ALTER TABLE t DROP COLUMN x RESTRICT;
ALTER TABLE
=> SELECT x FROM t;
ERROR 2624:  Column "x" does not exist
=> SELECT * FROM t;
 z
---
(0 rows)
=> DROP TABLE t CASCADE;
DROP TABLE

The following series of commands tries to drop a FLOAT(8) column and fails because there are not enough projections to maintain K-safety.

=> CREATE TABLE t (x FLOAT(8),y FLOAT(08));
CREATE TABLE
=> ALTER TABLE t DROP COLUMN y RESTRICT;
ALTER TABLE
=> SELECT y FROM t;
ERROR 2624:  Column "y" does not exist
=> ALTER TABLE t DROP x CASCADE;
ROLLBACK 2409:  Cannot drop any more columns in t
=> DROP TABLE t CASCADE;