db file sequential read: Wait: single-block physical read, typically an index or table lookup

What does it mean?

A session waits for a single block to be read from disk. Usually seen with index access or single-block reads by ROWID; normally an expected event.

Why does it happen?

Blocks not in the buffer cache being read from disk, non-selective or wrong index usage, an insufficient buffer cache, or a slow storage layer.

How to fix it

If high, find the SQL statements doing the most reads (AWR Top SQL, v$sql). Improve queries with appropriate indexes and correct access paths, and reduce unnecessary nested loops. Evaluate the buffer cache size (DB_CACHE_SIZE). Check storage I/O latency. This event is often normal; if abnormally high, focus on SQL tuning.