Postgresql Knowledge Patch

PostgreSQL Knowledge Patch

Use this skill when writing SQL, planning an upgrade, operating a cluster, building a client, or maintaining an extension. Start below, then open the matching reference.

Reference index

Check breaking changes before migration

Checksums now start enabled

initdb enables data checksums by default. If an unchecked cluster is intentional, say so explicitly:

initdb --no-data-checksums -D new-cluster

Source and destination checksum settings must match for pg_upgrade.

Generated columns now default to virtual

An omitted storage keyword means compute-on-read. Add STORED when the value must be materialized during writes:

CREATE TABLE line_item (
  quantity integer,
  unit_price numeric,
  total numeric GENERATED ALWAYS AS (quantity * unit_price) STORED
);

Parent maintenance includes children

VACUUM and ANALYZE on an inheritance parent process child relations. Use ONLY when parent-only behavior is required:

VACUUM (ONLY, ANALYZE) measurements;

Maintenance uses a safe search path

Functions invoked by expression indexes or materialized views must qualify non-default objects or set their own path:

ALTER FUNCTION app.normalize(text)
  SET search_path = pg_catalog, app;

Authentication and cluster definitions need review

• MD5 password authentication is deprecated; migrate password storage and pg_hba.conf rules.
• Unlogged partitioned tables are rejected.
• Session time-zone abbreviations take precedence over timezone_abbreviations entries.
• Deferred AFTER triggers run as the role active when the event was queued.
• Full-text-search and pg_trgm indexes may need reindexing after a default-collation-provider change.

Read Migration and Compatibility before running upgrade automation; it includes removed settings, catalog renames, constraint repair, and changed statistics preservation.

Use new schema primitives

Create time-ordered UUIDs

uuidv7() generates a time-ordered identifier. An optional interval shifts its embedded timestamp:

SELECT uuidv7(), uuidv7(interval '-1 hour');

Use uuid_extract_timestamp() for version 1 or 7 timestamps and uuid_extract_version() for RFC 9562 versions. Either returns NULL when the UUID form does not support the requested metadata.

Enforce temporal keys

WITHOUT OVERLAPS makes the final primary/unique-key range non-overlapping. PERIOD on the final foreign-key column requires referenced ranges to cover the referencing range:

CREATE TABLE room_prices (
  room_id bigint,
  valid_at daterange,
  UNIQUE (room_id, valid_at WITHOUT OVERLAPS)
);

CREATE TABLE bookings (
  room_id bigint,
  stay daterange,
  FOREIGN KEY (room_id, PERIOD stay)
    REFERENCES room_prices (room_id, PERIOD valid_at)
);

Return row images directly

Data-changing statements expose old and new in RETURNING:

UPDATE products
SET price = price * 1.05
RETURNING id, old.price AS before, new.price AS after;

MERGE also supports updatable views, WHEN NOT MATCHED BY SOURCE, RETURNING, and merge_action().

Ingest imperfect data deliberately

COPY FROM ... ON_ERROR ignore skips conversion failures. Add REJECT_LIMIT so a bad file cannot discard an unlimited number of rows:

COPY staging_orders FROM '/imports/orders.csv'
WITH (
  FORMAT csv,
  ON_ERROR ignore,
  REJECT_LIMIT 100,
  LOG_VERBOSITY silent
);

Monitor skipped rows through pg_stat_progress_copy.tuples_skipped. Be careful with \. in CSV: server-side files treat it as data, while psql recognizes an otherwise empty \. line as the end of STDIN.

Build incremental physical backups

Turn on WAL summaries, take an incremental relative to a prior manifest, then combine the chain into a synthetic full backup:

summarize_wal = on

pg_basebackup -D /backup/inc \
  --incremental=/backup/full/backup_manifest
pg_combinebackup /backup/full /backup/inc -o /backup/combined

Set wal_summary_keep_time for the backup cadence. Validate the result with pg_verifybackup; tar backups are accepted.

Query JSON with standard SQL

Use JSON_EXISTS for path existence, JSON_VALUE for one scalar, and JSON_QUERY for JSON results and wrappers:

SELECT JSON_EXISTS(
         payload,
         'strict $.items[*] ? (@.qty > $minimum)'
         PASSING 10 AS minimum
       ),
       JSON_VALUE(payload, '$.customer.id' RETURNING bigint)
FROM orders;

Use JSON_TABLE for lateral relational projection with typed columns, FOR ORDINALITY, EXISTS, and nested paths. Its sibling nested paths form a union rather than a cross product.

Keep logical replication available through failover

Create a failover-capable subscription and synchronize its slot to a standby:

CREATE SUBSCRIPTION orders_sub
  CONNECTION 'host=publisher dbname=app'
  PUBLICATION orders_pub
  WITH (failover = true);

Coordinate sync_replication_slots with synchronized_standby_slots so decoded changes do not outrun the required physical standby. Bound abandoned slots with idle_replication_slot_timeout.

Generated columns are published when named in a publication column list. Without a list, control them with publish_generated_columns.

Tune and observe asynchronous I/O

Select the implementation with io_method and inspect active handles through pg_aios. Tune combining with io_combine_limit and io_max_combine_limit.

EXPLAIN ANALYZE includes buffer statistics automatically and exposes richer memory, disk, WAL, window, bitmap-cache, and index-lookup details. Add MEMORY for planner memory and SERIALIZE to measure result conversion:

EXPLAIN (ANALYZE, MEMORY, SERIALIZE TEXT)
SELECT * FROM orders;

Statistics consumers must account for I/O and WAL data moving between views, new per-backend reset functions, one-based memory-context levels, and renamed pg_stat_statements timing columns.

Secure client connections

For immediate TLS negotiation, use sslnegotiation=direct only when ALPN and the server version support it:

host=db.example dbname=app sslmode=require sslnegotiation=direct

OAuth authentication uses oauth in pg_hba.conf and server validators from oauth_validator_libraries. Source builds that enable it need libcurl.

Wire protocol 3.2 supports 256-bit cancel keys. Bound acceptable protocol versions when compatibility matters, and use the current libpq cancellation API when cancellation must retain encrypted transport.

Follow task-specific references

Do not infer old defaults for generated-column storage, checksums, I/O concurrency, subscription streaming, or vacuum inheritance. Open the indexed reference when code, configuration, monitoring queries, or migration automation depends on these behaviors.