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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchA short-looking ALTER TABLE can wait behind a long-running query if it needs a lock that conflicts with the query’s lock. The practical response is to check the exact DDL and PostgreSQL version, bound lock waiting with a migration-scoped lock_timeout, and roll out schema and application changes in compatible stages. These steps can reduce risk, but no migration recipe guarantees literal zero downtime.
Why an ALTER TABLE can wait behind a SELECT
In PostgreSQL, an ordinary read-only SELECT takes an ACCESS SHARE lock on each referenced table. That lock conflicts only with ACCESS EXCLUSIVE. An ACCESS EXCLUSIVE lock conflicts with every table-level lock mode, so a reader holding ACCESS SHARE can prevent DDL that needs ACCESS EXCLUSIVE from proceeding until the reader releases its lock.
PostgreSQL 18’s ALTER TABLE documentation says, “An ACCESS EXCLUSIVE lock is acquired unless explicitly noted.” That is a default, not a claim that every form of ALTER TABLE uses the same lock: some subforms document weaker modes. When one statement combines multiple subcommands, it needs the strictest lock required by any of them. For example, ADD FOREIGN KEY requires SHARE ROW EXCLUSIVE.
A lock wait can become an availability concern on a busy system. A queued DDL request may affect how later lock requests proceed, but the presence of one waiting ALTER TABLE does not mean every later query will necessarily be blocked. Check the actual outstanding locks and workload rather than assuming the queue’s effects.
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Check what the exact DDL will do
“Safe ALTER” is not a useful category on its own. Before deploying a change, check the command reference for the PostgreSQL major version you run and inspect every subform in the migration. Lock mode tells you which concurrent operations can conflict; scans and rewrites tell you about execution time and resource use.
| Change or operation | What PostgreSQL 18 documents | Operational implication |
|---|---|---|
ALTER TABLE subform with no documented exception |
ACCESS EXCLUSIVE is the default; a combined statement uses its strictest required lock. |
Check which active operations hold conflicting locks before treating the change as low risk. |
| Add a column with a non-volatile default | Does not require a table rewrite. | This avoids that particular rewrite cost; still check the required lock and application compatibility. |
| Add a column with a volatile default, or change many column types | May rewrite the table and its indexes. | Plan for work, disk headroom, and the duration of restrictive phases; the exact effect depends on the operation. |
| Verify a constraint against existing rows | Constraint verification can scan a large table. | Separate installation from verification where the constraint supports NOT VALID. |
CREATE INDEX CONCURRENTLY |
Uses two scans, waits for relevant transactions, and consumes more work and resources than an ordinary build. | It avoids locking out normal writes during the build, but is not cost-free; it cannot run inside a transaction block. |
These points are from the PostgreSQL 18 documentation. Lock behavior and optimizations can vary by major version, so verify against the version actually deployed.
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Use lock_timeout to bound waiting, not to speed up the migration
lock_timeout aborts a statement if it waits longer than the configured interval for an individual lock acquisition. Its default is zero, which disables the timeout. It is distinct from statement_timeout: a statement can acquire its locks promptly and still take a long time to scan or rewrite data. If a nonzero statement_timeout is at or below lock_timeout, the statement timeout can fire first.
Set the timeout only for the migration session, not globally in postgresql.conf; a global setting affects every session. Choose an interval that fits the service’s latency budget and the deployment’s retry or abort plan. For example, in a migration session:
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SET lock_timeout = '250ms';The value above is an example, not a universal recommendation. Choose the actual interval deliberately. If the migration cannot obtain a needed lock within it, the statement aborts; the timeout does not make the operation’s remaining work faster.
Stage schema and application changes with expand/contract
Expand/contract is a rollout pattern, not a PostgreSQL feature or a guarantee that each step is harmless. Its purpose is to keep intermediate application versions compatible with the schema while deployment proceeds.
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- Expand: Add the new schema in a form compatible with the currently deployed application. Confirm the exact DDL’s lock requirements and whether it scans or rewrites data.
- Deploy compatible code: Roll out code that can tolerate both the old and new schema representations. For a column replacement, intermediate versions may need to work with both columns.
- Backfill in bounded work where needed: Populate or reconcile the new representation without treating the backfill as one unbounded operation. Verify that it is complete before relying on the new data.
- Switch reads or writes: Move application behavior to the new representation only after deployed versions and data are ready for that transition.
- Contract later: Remove the old schema only after the running code no longer depends on it and the compatibility window has passed.
The ordering reduces the chance that old code encounters a schema it cannot use, or that new code depends on data that has not been populated. The lock and runtime risks of each DDL step still depend on its exact form and PostgreSQL version.
Install and validate supported constraints separately
For supported constraints, ADD CONSTRAINT ... NOT VALID installs the constraint without checking all existing rows at that moment. A later VALIDATE CONSTRAINT checks existing data and takes a SHARE UPDATE EXCLUSIVE lock; PostgreSQL documents that this validation does not lock out concurrent updates. This separates adding the rule from the potentially substantial work of verifying old rows.
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Use concurrent index creation with a recovery plan
CREATE INDEX CONCURRENTLY lets normal table operations, including writes, continue during the build. The trade-off is two scans, waits for relevant transactions, and more resource use. It also cannot run inside a transaction block. If it fails, it may leave an invalid index, so decide how the migration will detect and clean up that state before relying on the index.
Plan for timeout, failure, and recovery
Before running a migration, know what your deployment system does when a lock timeout aborts a statement. In particular, confirm that a failed migration is recorded safely, retries are serialized, and retries have bounds and backoff rather than looping without limit. PostgreSQL’s explicit locking documentation identifies pg_locks as a way to examine outstanding locks. Use it as part of diagnosing a wait, alongside your service’s normal operational signals; do not assume a particular blocker-identification query or dashboard without validating it for your environment.
- Record the exact PostgreSQL major version and DDL being deployed.
- Know which lock mode the operation requests and what concurrent work conflicts with it.
- Estimate whether the operation scans or rewrites data and confirm adequate disk and resource headroom.
- Scope
lock_timeoutto the migration session and define whether a timeout means abort, investigate, or retry. - For concurrent index builds, include invalid-index detection and cleanup in the failure path.
- Keep the old schema until the application rollout and any backfill have been verified.
For authoritative details, consult the PostgreSQL 18 documentation sections “ALTER TABLE,” “Explicit Locking,” “Constraints,” “Client Connection Defaults,” “ALTER TABLE,” and “CREATE INDEX.”
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