Summary
SyncScan is a PostgreSQL LWLock wait event. The official server documentation describes it as: “Waiting to select the starting location of a synchronized table scan.” (verbatim from PostgreSQL’s wait_event_names.txt catalog).
Classification
- wait_event:
SyncScan - wait_event_type:
LWLock - Reported in:
pg_stat_activity
What the LWLock class indicates
(Paraphrased explanation.) The process is waiting for a lightweight lock that protects a particular shared-memory data structure. Contention here usually reflects an internal hot spot.
How to observe it
(Illustrative query — not from the catalog.) You can see which sessions are currently reporting this wait event in the cumulative statistics view:
SELECT pid, state, wait_event_type, wait_event, query
FROM pg_stat_activity
WHERE wait_event_type = 'LWLock'
AND wait_event = 'SyncScan';Version applicability
Confirmed present in PostgreSQL major version(s): 12, 13, 14, 15, 16, 17, 18, 19 (verified against each release’s server source).
This is a long-standing wait event: it is present at least as far back as PostgreSQL 12 (the oldest release checked here) and very likely predates it.
The machine-readable wait_event_names.txt catalog exists from PostgreSQL 17 onward; presence in PostgreSQL 12–16 was verified directly from the wait-event, lock, and lightweight-lock definitions in those releases’ source code.
References
- PostgreSQL documentation — Wait Events
- PostgreSQL source —
src/backend/utils/activity/wait_event_names.txt