Migration Guard › sample report 3

ALTER COLUMN TYPE

Illustrative: written by hand to show the report format. The Action is not built yet, and this statement is a minimal synthetic example, not taken from a real repository.

Statement	ALTER TABLE orders ALTER COLUMN total TYPE bigint;
Lock taken	ACCESS EXCLUSIVE on `orders`. It blocks all reads and writes while held.
Rewrites the table	Yes for int to bigint: the types are not binary-coercible, so the table and its indexes on that column are rewritten under the lock. Some other type changes avoid a rewrite; see the ALTER TABLE docs.
Verdict	BLOCK-RISK

Why

The rewrite runs while all reads and writes on the table are blocked, so the outage lasts at least as long as the rewrite.

Lower-risk rewrite

A common lower-risk pattern is to add a new column, backfill in batches, then swap over in a later release. Review each step for its own lock:

ALTER TABLE orders ADD COLUMN total_new bigint;
-- backfill in batches outside this migration
-- deploy application code that writes both columns, then reads the new one
-- later: drop the old column and rename the new one (each takes ACCESS EXCLUSIVE briefly)

Sources: PostgreSQL docs: explicit locking; PostgreSQL docs: ALTER TABLE (notes on ALTER COLUMN TYPE).

Table size is unknown to this report. Statements outside the fixed rule list would be marked REVIEW.