ALTER TABLE

ALTER TABLE in Krishiv is supported primarily for Iceberg tables. The operations are metadata-only for the most part — no data rewrite required.

Supported operations

Operation	SQL form	What it does
Add column	ALTER TABLE t ADD COLUMN c TYPE	Metadata-only. New column is NULL for existing rows.
Drop column	ALTER TABLE t DROP COLUMN c	Metadata-only. Readers return NULL.
Rename column	ALTER TABLE t RENAME COLUMN old TO new	Metadata-only. Existing queries break.
Reorder column	ALTER TABLE t ALTER COLUMN c AFTER other	Metadata-only. Storage layout unchanged.
Widen type	ALTER TABLE t ALTER COLUMN c TYPE NEW	Metadata-only if the new type is a superset (e.g. INT → BIGINT, FLOAT → DOUBLE).
Set comment	ALTER TABLE t ALTER COLUMN c COMMENT '...'	Metadata-only.
Add partition spec	ALTER TABLE t ADD PARTITION FIELD bucket(16, id)	Metadata-only. New writes use the new spec; old data is not rewritten.

Examples

-- Add a new column (NULL for existing rows)
ALTER TABLE orders ADD COLUMN region VARCHAR;

-- Rename safely (will break old queries — deploy in lockstep)
ALTER TABLE orders RENAME COLUMN amt TO amount;

-- Widen a type (INT → BIGINT is metadata-only)
ALTER TABLE orders ALTER COLUMN user_id TYPE BIGINT;

-- Drop a column
ALTER TABLE orders DROP COLUMN legacy_col;

Compatibility modes

Set the compatibility mode on the table to control which operations are allowed:

Mode	Description
backward (default)	Old readers can read new data. Add/drop/widen are OK.
forward	New readers can read old data. Drop/widen are OK.
full	Both. Most restrictive.
none	No checks. Use only for development.

Branches and tags for risky changes

For destructive experiments, branch first:

-- Create a branch at the current main snapshot
CALL system.create_branch('orders', 'experiment', main_ref);

-- Run the migration on the branch
ALTER TABLE orders_branch ADD COLUMN region VARCHAR;

-- Inspect via time-travel
SELECT * FROM orders FOR SYSTEM_TIME AS OF 'experiment';

-- Promote or drop
CALL system.fast_forward('orders', 'experiment', main_ref);
CALL system.drop_branch('orders', 'experiment');

Unsupported operations

• Narrowing a column type (DOUBLE → FLOAT).
• Renaming a partition column.
• Changing the partition spec on existing data without a re-partition job.

See also

• Iceberg
• AS-OF Queries
• Iceberg schema migration recipe