default_transaction_isolation

default_transaction_isolation is the PostgreSQL setting that defines the transaction isolation level of each new transaction.

Note

Fact — official short description: “Sets the transaction isolation level of each new transaction.”

Identity

Type , Valueenum: Upstream pg_settings type
Context , Valueuser: Settable by an ordinary user
Unit , Value—: Raw unit
Range , Value—: Raw limits in the last observed version
Enum values , Valueserializable, repeatable read, read committed, read uncommitted: — for non-enum types
Category , ValueClient Connection Defaults / Statement Behavior: Upstream classification
Latest boot value , Valueread committed: read committed

Lifecycle

Fact	Value
First observed	PG9.0 (research boundary)
Present in	PG9.0–19 Beta 3
Removed in	No
Introduction commit	Not asserted: predates the PG9.0 research boundary
Commit date	—
Discussion	—

Default history

Measured PG9.0–19 Beta 3 boot defaults
Versions	Raw `boot_val`	Unit	Human value
PG9.0–19 Beta 3	`read committed`	—	read committed

How it works

default_transaction_isolation sets the transaction isolation level of each new transaction. PostgreSQL copies this value into transaction_isolation when each transaction begins; it changes consistency semantics, not merely performance.

default_transaction_isolation is a USER-context setting and can be assigned per role, database, or session; its value is copied when a new transaction starts, so it does not rewrite a transaction already in progress.

The default_* variables seed the corresponding transaction_* state. Isolation, read-only status, deferrability, retries, and snapshot lifetime must be designed together.

Tuning advice

Tip

Advice. These are workload-specific starting points and must be validated with measurements.

Workload	Guidance
OLTP	Choose default_transaction_isolation for correctness semantics first. Keep the common OLTP path explicit, then override only transactions whose consistency contract justifies different blocking or retry behavior.
OLAP	For reporting, consider a read-only transaction and an isolation choice that matches snapshot requirements; use deferrability only with read-only serializable work.
Small nodes	Do not change default_transaction_isolation as a generic speed tweak. Higher isolation or long snapshots can amplify contention and vacuum pressure on a small node.

Pigsty

Values use the fixed 8-vCPU, 32-GiB, 100-GiB SSD fixture and render the current Pigsty templates for PG19 Beta 3; this does not assert current Pigsty support for that historical or beta release.

Template	Effective value	Versus upstream boot	Source expression
OLTP	Unmodified	—	—
OLAP	Unmodified	—	—
CRIT	Unmodified	—	—
TINY	Unmodified	—	—

Caution

Advice — pending human review. Fact from the current Pigsty template projection: OLTP: PG9.0–19 Beta 3 unmodified; OLAP: PG9.0–19 Beta 3 unmodified; CRIT: PG9.0–19 Beta 3 unmodified; TINY: PG9.0–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

Common pitfalls

Changing default_transaction_isolation in one session and assuming role defaults, database defaults, or other pooled sessions changed with it.
Changing transaction semantics as a performance experiment and silently weakening an application’s consistency contract.
Letting a pooled session retain transaction-related state because checkout or rollback/reset handling is incomplete.
Changing default_transaction_isolation globally without a rollback plan and a client or operational compatibility test.

transaction_isolation · default_transaction_read_only · transaction_read_only · default_transaction_deferrable · transaction_deferrable