The causes and fix recommendations for frequently observed wait events in Oracle databases
March 26, 2026
| Wait Event | Cause / Problem | Solution / Recommendation |
| db file scattered read | The session is waiting for a multiblock I/O operation (typically a full table scan or fast full index scan) to complete. Data is read into non-contiguous (scattered) memory in the buffer cache. | – Add indexes / perform query tuning – Increase the buffer cache size – Implement partitioning |
| db file sequential read | The session is waiting for a single-block I/O operation (typically an index fetch or row lookup by ROWID) to complete. Data is read into contiguous memory. | – Perform index tuning – Increase the buffer cache size |
| latch: cache buffers chains | A session is waiting for a latch to access a chain of buffers in the buffer cache. This usually indicates contention on a “hot block” being accessed by many sessions concurrently. | – Reduce contention on hot blocks (e.g., via partitioning or reverse key indexes) – Optimize application logic to reduce concurrent access to the same block |
| latch: cache buffers lru chain | A session is waiting for a latch to manage the Least Recently Used (LRU) list in the buffer cache. This often occurs when the buffer cache is insufficient or too many sessions are trying to modify the LRU chain simultaneously. | – Increase the buffer cache size – Reduce LRU access (e.g., by tuning queries that perform large scans) |
| latch: library cache | A session is waiting for a latch to manage the library cache (part of the shared pool). This is typically caused by excessive SQL parsing or insufficient shared pool memory. | – Increase the shared pool size – Optimize cursor reuse (e.g., by using bind variables) |
| enq: TX – row lock contention | A session is waiting for a Transaction (TX) lock, usually because it is trying to update or insert a row that is currently locked by an uncommitted transaction from another session. | – Optimize row distribution (e.g., to avoid concurrent modifications on the same row) – Reduce lock contention by tuning application logic (e.g., shorter transactions) |
| log file sync | A session (typically the foreground process) is waiting for the log writer (LGWR) to write the redo log buffer to the online redo log file. This occurs when a user issues a COMMIT. | – Batch commits (reduce the frequency of commits) – Tune redo log configuration (e.g., reduce disk I/O latency, increase log buffer size) |
| direct path read / write | The session is waiting for I/O operations that bypass the buffer cache. This commonly occurs during parallel execution, large sorts, or when reading from temporary tablespaces. | – Optimize temporary tablespace (e.g., use faster storage, distribute across multiple files) – Reduce sort operations (e.g., via SQL tuning or increasing memory parameters like PGA) |
| cursor: pin S wait on X | A session is waiting to acquire a shared (S) pin on a cursor, but another session currently holds an exclusive (X) pin. This indicates contention on a specific cursor, often during heavy parsing, recompilation, or invalidation of a frequently executed SQL statement. | – Avoid frequent DDL operations on objects used by active SQL – Use bind variables to reduce parsing overhead – Increase the shared pool size – Investigate why cursors are being invalidated (e.g., statistics gathering, object changes) |
| cursor: pin X wait on S | A session is waiting to acquire an exclusive (X) pin on a cursor, but one or more other sessions currently hold shared (S) pins. This occurs when a session needs to modify a cursor (e.g., for recompilation or invalidation) while other sessions are still using it. | – Avoid frequent DDL operations on objects used by active SQL – Reduce high-frequency parsing – Coordinate application changes and deployments during low-activity periods – Use bind variables to minimize cursor contention |
