PostgreSQL 18.6 Deep Dive: Async I/O, Skip Scan, UUIDv7 & More

What Is PostgreSQL

PostgreSQL (often called Postgres) is an open-source relational database with full ACID compliance, rich data types, and an extensible architecture. Its mascot is an elephant — long name, steady temperament. Version 18 focuses on two practical goals: faster large-table reads and less verbose SQL for schema changes and data modifications.

Six Major Features in PostgreSQL 18

1. Async I/O: Queued Disk Reads

Previously, backend processes read pages one by one — read a block, wait, request the next. Sequential scans, bitmap heap scans, and VACUUM often read many contiguous pages while the disk could keep streaming. PostgreSQL 18 adds an async I/O subsystem: the backend enqueues multiple read requests, and dedicated I/O workers (or the kernel's io_uring on Linux) fulfill them. The controlling parameter is io_method (default worker, portable; set to sync for 17-like behavior). io_workers sets the worker count (start around ¼ of CPU cores). Inspect in-flight async reads via the pg_aios system view. Note: this release covers reads only; index point lookups remain synchronous. Sequential scans of large tables and VACUUM on large tables benefit most.

-- Check current I/O method
SHOW io_method;
-- Worker count
SHOW io_workers;
-- Active async reads
SELECT * FROM pg_aios;

2. Skip Scan: Composite Indexes Without Leading Column

Composite B-tree indexes traditionally required the leading column in the query. An index on (status, created_at) would be ignored if the WHERE clause only filtered on created_at. PostgreSQL 18 implements skip scan: when the leading column has low cardinality (e.g., order status with only a few values), the planner can jump through each distinct leading-column value and apply the remaining column predicates. The planner decides automatically; check EXPLAIN output for "Skip Scan" to confirm it kicked in.

CREATE INDEX idx_order_status_time ON t_order (status, created_at);
-- No status in WHERE, but 18 may still use the index
EXPLAIN SELECT id, status, created_at
FROM t_order
WHERE created_at >= TIMESTAMPTZ '2026-08-01 00:00:00';

3. UUIDv7: Time-Ordered Primary Keys

Random UUIDs (v4) cause B-tree page splits because inserts scatter across the tree. The new built-in uuidv7() embeds a timestamp in the most significant bits, so new rows land near the right edge of the index. Pages stay denser, and primary-key order approximates creation order. For pure randomness, uuidv4() remains available.

4. Virtual Generated Columns (Default)

Generated columns now default to VIRTUAL: the value is computed on read, not stored on write. This saves space for derived fields like line totals ( price * qty) or display concatenations. If you need an index on the column or want the value persisted with the row, declare it STORED explicitly.

CREATE TABLE t_order_item (
  price  numeric(12,2) NOT NULL,
  qty    int NOT NULL CHECK (qty > 0),
  -- VIRTUAL by default: computed at query time
  line_total numeric(12,2) GENERATED ALWAYS AS (price * qty)
);

5. OAuth Authentication in pg_hba.conf

pg_hba.conf

now accepts oauth as an authentication method. The database forwards the client's token to an external validator; on success, the connection proceeds. Teams with single sign-on no longer need a separate password store in the database. MD5 passwords are deprecated in 18; SCRAM or OAuth are the recommended paths.

6. RETURNING old / new for Audit-Friendly DML

INSERT

, UPDATE, DELETE, and MERGE statements can now reference old and new in the RETURNING clause. A single statement captures both pre- and post-change values — ideal for audit logs without a prior SELECT.

-- Table with time-ordered PK and auto timestamp
CREATE TABLE t_order (
  id uuid PRIMARY KEY DEFAULT uuidv7(),
  user_name text NOT NULL,
  status text NOT NULL DEFAULT 'pending',
  amount numeric(12,2) NOT NULL,
  created_at timestamptz NOT NULL DEFAULT now()
);

INSERT INTO t_order (user_name, amount) VALUES ('林晓', 128.50)
RETURNING id, created_at;

-- Price change returns both old and new amounts
UPDATE t_order SET amount = amount - 20 WHERE user_name = '林晓'
RETURNING old.amount AS before_amount, new.amount AS after_amount;

Bonus: Exclusion Constraint WITHOUT OVERLAPS

For scheduling scenarios (e.g., meeting rooms), a primary key with WITHOUT OVERLAPS on a range column enforces non-overlapping periods per resource:

CREATE TABLE t_room_booking (
  room_id int,
  during  tstzrange,
  PRIMARY KEY (room_id, during WITHOUT OVERLAPS)
);

What 18.6 Adds on Top

The 18.6 release notes are lengthy, mostly crash fixes, wrong-result fixes, and security patches. Three operational items stand out:

Logical Decoding Plugins Require Allowlist

New parameter output_plugin_libraries restricts which output plugins can be loaded. Default: 'pgoutput, test_decoding'. Third-party decoding plugins (e.g., my_trusted_decoder) must be added, otherwise replication slots fail to start (CVE-2026-6471).

# postgresql.conf
output_plugin_libraries = 'pgoutput, test_decoding, my_trusted_decoder'

Legacy PGP Encryption Now Errors by Default

pgcrypto

previously silently failed to decrypt data encrypted with obsolete algorithms (Blowfish, CAST5, 3DES) when OpenSSL rejected them — effectively leaving data unencrypted (CVE-2026-14663). 18.6 refuses to decrypt such data. To recover already-corrupted rows, use the ignore-cipher-failure=1 option once, then re-encrypt with a modern algorithm:

-- One-time recovery of broken historical data
SELECT pgp_sym_decrypt(
  secret_data,
  'temp-passphrase',
  'ignore-cipher-failure=1'
) FROM t_legacy_secret;

Parallel GIN Index Build May Corrupt reltuples

During parallel CREATE INDEX ... USING GIN, a worker can report an uninitialized row count, setting reltuples to Infinity, NaN, or a wildly wrong number. Autovacuum and auto-analyze then skip the table, and the statistic never self-corrects. After upgrading, check GIN-indexed tables:

SELECT DISTINCT t.oid::regclass AS table_name,
       t.reltuples
FROM pg_class t
JOIN pg_index i ON t.oid = i.indrelid
JOIN pg_class ic ON i.indexrelid = ic.oid
JOIN pg_am am ON ic.relam = am.oid
WHERE am.amname = 'gin'
ORDER BY t.reltuples;

If reltuples is NaN, Infinity, or off by orders of magnitude, run ANALYZE on that table or rebuild an index to reset the estimate.

Other 18.6 Fixes Worth Knowing

Range partition pruning could miss the default partition under certain predicates. BEFORE UPDATE triggers: RETURNING old could return the row as it existed before a concurrent update, not the immediate pre-image.

Multi-column hash joins with many nulls could blow up the hash table with null keys.

Time zone data updated to tzdata 2026c.

Post-Upgrade Checklist (18.x → 18.6)

Minor-version upgrades don't require dump/restore. Verify three areas: output_plugin_libraries if you use logical decoding.

Legacy pgcrypto data — decrypt and re-encrypt if needed.

GIN-indexed tables — run the reltuples query above and ANALYZE outliers.

Also, if you use btree_gist on float/bit columns or ltree with very long labels, consult the release notes for potential index rebuild advice.