debug_parallel_query

Definition PG 18 manual

Allows the use of parallel queries for testing purposes even in cases where no performance benefit is expected. The allowed values of debug_parallel_query are off (use parallel mode only when it is expected to improve performance), on (force parallel query for all queries for which it is thought to be safe), and regress (like on, but with additional behavior changes as explained below).

More specifically, setting this value to on will add a Gather node to the top of any query plan for which this appears to be safe, so that the query runs inside of a parallel worker. Even when a parallel worker is not available or cannot be used, operations such as starting a subtransaction that would be prohibited in a parallel query context will be prohibited unless the planner believes that this will cause the query to fail. If failures or unexpected results occur when this option is set, some functions used by the query may need to be marked PARALLEL UNSAFE (or, possibly, PARALLEL RESTRICTED).

Setting this value to regress has all of the same effects as setting it to on plus some additional effects that are intended to facilitate automated regression testing. Normally, messages from a parallel worker include a context line indicating that, but a setting of regress suppresses this line so that the output is the same as in non-parallel execution. Also, the Gather nodes added to plans by this setting are hidden in EXPLAIN output so that the output matches what would be obtained if this setting were turned off.

How it works

debug_parallel_query forces safe queries through parallel mode even when no performance benefit is expected. on adds a top-level Gather where safe; regress also hides testing-only output differences.

Parallel-safety restrictions still apply. The forced context can expose functions incorrectly marked parallel safe and can prohibit operations such as subtransactions even when no worker is ultimately available.

It replaced the earlier force_parallel_mode name in PG16. Its purpose is regression testing, not making production queries faster or overriding all parallel cost decisions. Its user context permits session- or transaction-local changes; newly performed or newly planned work sees the value.

Operational considerations

Leaving debug_parallel_query enabled after the bounded diagnostic or recovery task.

Running the experiment on the only copy of production data.

Underestimating log, core-file, temporary-file, WAL, CPU, or connection-slot amplification.

Treating a server that merely starts or completes a query as proof that data and behavior are correct.

Workload guidance

OLAP: Long analytical runs can amplify debug_parallel_query's debug overhead and artifacts. Prefer standard EXPLAIN and statistics first, and isolate any developer experiment from normal users.

OLTP: Do not tune production OLTP with debug_parallel_query. Enable it only for a bounded reproduction with an owner, log/disk budget, rollback condition, and evidence-capture plan; restore the default immediately afterward.

SMALL: Keep debug_parallel_query at its upstream default. A small host has less spare CPU, disk, connection, and log capacity for developer instrumentation.