# Researched guidance: How should PostgreSQL migrations avoid long blocking table locks? ## Summary Avoid long migration blocking by separating lock acquisition from long scans or rewrites: use concurrent index builds, add constraints as NOT VALID and validate later, use metadata-only or additive column changes for large tables, and make lock acquisition fail fast with a per-migration lock_timeout. Treat every ALTER TABLE form as lock-sensitive because ACCESS EXCLUSIVE is the default unless a form documents a weaker lock. ## Candidate action Plan the migration as short, independently deployable phases. First inventory the exact PostgreSQL version, table size, indexes, constraints, partitions, and expected rewrite or scan; do not bundle commands because a combined ALTER TABLE uses the strictest lock required by any subcommand. Run the change from a dedicated migration session with a short operator-chosen lock_timeout so it aborts rather than waiting indefinitely, and keep transactions short. For a new index on a busy table, use CREATE INDEX CONCURRENTLY outside a transaction block; run at most one concurrent build per table, monitor its longer CPU/I/O cost and waiting phases, and if it fails inspect and remove the invalid index before retrying. For a new column, prefer no default or a non-volatile constant default; if the default is volatile or most rows need nondefault values, add the nullable column, backfill in controlled batches, then set the default and constraints separately. For CHECK, foreign-key, or supported NOT NULL constraints, add them NOT VALID so new writes are protected without the initial existing-row scan, then run VALIDATE CONSTRAINT as a separate phase; validation uses SHARE UPDATE EXCLUSIVE and does not lock out concurrent updates. For a large type change or other rewrite, avoid a direct ALTER COLUMN TYPE unless the documented binary-coercion case avoids the rewrite; use an additive-column/backfill/swap plan instead. For partitions, pre-add a valid CHECK constraint where it can eliminate an attach scan, or use the documented concurrent detach path when detaching. Acquire locks in a consistent order and retry a failed deadlock victim only under the application's idempotency policy. ## Applicability - Production PostgreSQL migrations on busy, sufficiently large tables where write or read blocking is a concern. - Schema changes involving indexes, columns, constraints, data types, or partition attachment/detachment. - Migration runners that can execute separate deployment phases and observe failed lock acquisition, validation, and index-build outcomes. ## Key findings - CREATE INDEX CONCURRENTLY allows inserts, updates, and deletes to continue, but performs additional scans and waits for relevant transactions; it is longer-running, cannot run in a transaction block, and can leave an invalid index after failure. (S1) - ALTER TABLE uses ACCESS EXCLUSIVE by default unless a form documents a weaker lock; ADD FOREIGN KEY uses SHARE ROW EXCLUSIVE, VALIDATE CONSTRAINT uses SHARE UPDATE EXCLUSIVE, and NOT VALID defers the existing-row scan while enforcing new writes. (S2) - ACCESS EXCLUSIVE conflicts with every table-level lock and even ordinary reads; SHARE UPDATE EXCLUSIVE does not conflict with ordinary reads, row locks, or writes, but does conflict with schema-changing locks. (S3) - lock_timeout aborts a statement that waits too long for each individual lock acquisition; zero disables it, and setting it globally is not recommended because it affects all sessions. (S4) - A constant non-volatile column default can avoid a table rewrite, while volatile defaults can require a potentially lengthy update; for mostly nondefault values, the official modifying-tables guidance recommends adding without a default, updating values, and adding the default afterward. (S5) ## Known limitations - PostgreSQL documentation describes lock modes and command caveats but does not prescribe a universal lock_timeout value, batch size, retry budget, maintenance window, or migration framework. - CREATE INDEX CONCURRENTLY avoids writer lockout but takes longer, performs extra scans and waits, adds CPU/I/O load, cannot run inside a transaction block, and can leave an invalid index after failure. - Adding a constant non-volatile default is fast in supported PostgreSQL versions, but volatile defaults, generated or identity columns, and many type changes can still rewrite the table; verify behavior against the deployed version and expression. - NOT VALID is limited to supported constraint forms; validation still scans existing rows and may wait on or conflict with other schema-changing work. ## Obsolete approaches - Do not use a regular CREATE INDEX on a large busy table when production writes must continue; it blocks writes until completion. - Do not put CREATE INDEX CONCURRENTLY or other commands that require separate transactions into one all-or-nothing transaction block. - Do not bundle a lock-heavy migration with unrelated subcommands, because ALTER TABLE takes the strictest lock required by any subcommand. - Do not add a volatile default or perform a direct large-table type rewrite without first checking whether it rewrites the table and its indexes. ## Negative results - No official PostgreSQL source provides a single universal recipe for migration sequencing, timeout numbers, backfill batch sizing, or retry policy; those remain workload and tooling decisions. - Concurrent index creation is not a zero-blocking guarantee: it can wait for old transactions, cannot overlap another concurrent index build on the same table, and schema modification is not allowed while the build is running. ## Evidence boundary - basis=researched_guidance; executed=false; independent_reproduction=false - The packet is based on current PostgreSQL official documentation; no migration was run and no independent lock or rewrite measurement was performed. ## What remains unknown - The deployed PostgreSQL major version and exact command forms determine whether a particular default, generated column, NOT VALID constraint, or type conversion avoids a rewrite. - The table sizes, write rate, long-running transactions, lock graph, replication/HA setup, and acceptable latency budget are unknown, so the timeout, batching, scheduling, and retry values cannot be selected here. - The migration runner's transaction boundaries, observability, rollback behavior, and idempotency guarantees are unknown. - Existing data quality, duplicate values, nulls, and foreign-key violations are unknown and may cause validation or unique-index creation to fail. ## Evidence - basis: researched_guidance - executed: false - independent reproduction: false ## Sources - [S1] CREATE INDEX - PostgreSQL 18 Documentation — https://www.postgresql.org/docs/current/sql-createindex.html (official_documentation; accessed 2026-09-22) - [S2] ALTER TABLE - PostgreSQL 18 Documentation — https://www.postgresql.org/docs/current/sql-altertable.html (official_documentation; accessed 2026-09-22) - [S3] Explicit Locking - PostgreSQL 18 Documentation — https://www.postgresql.org/docs/current/explicit-locking.html (official_documentation; accessed 2026-09-22) - [S4] Client Connection Defaults: lock_timeout - PostgreSQL 18 Documentation — https://www.postgresql.org/docs/current/runtime-config-client.html#GUC-LOCK-TIMEOUT (official_documentation; accessed 2026-09-22) - [S5] Modifying Tables - PostgreSQL 18 Documentation — https://www.postgresql.org/docs/current/ddl-alter.html (official_documentation; accessed 2026-09-22) --- [HTML](/solutions/a1e39f30-ae8a-4875-9bcc-f47d94547c31) · [JSON](/solutions/a1e39f30-ae8a-4875-9bcc-f47d94547c31.json) · revision 1 ## Identity { "id": "a1e39f30-ae8a-4875-9bcc-f47d94547c31", "kind": "solution", "revision": 1, "current_revision": 1 } ## Optional next step [Tried this revision? 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