If you performed lot of DMLs (specially Deletes) on a table, you may have lots of empty leaf blocks in the associated indexes on that table. These empty leaf blocks are left in the b-tree structure AND attached to the freelist. On subsequent inserts, the process might need to split a block so it takes…
Oracle acquires a table lock on a child table if it’s performing modifications on the primary key column in the parent table that’s referenced by the foreign key of the child table. And these locks on the child table are full table locks (TM) i.e. these locks aren’t restricted to any row but to the…
The TKProf facility accepts as input a SQL trace file and produces a formatted output file. TKPROF filename_source filename_output EXPLAIN=[username/password] SYS=[YES/No] TABLE=[tablename] filename_source: The trace file that needs to be formatted.filename_output: The output file where the formatted file will be written.EXPLAIN: This option causes TKProf to generate an execution plan for each SQL statement in…
Event 10046 is used to gather extended SQL_TRACE information for Oracle sessions. This article describes how to enable event 10046 to get extended information to troubleshoot a performance issue. To gather 10046 trace at the session level: alter session set tracefile_identifier=’10046′; alter session set timed_statistics = true;alter session set statistics_level=all;alter session set max_dump_file_size = unlimited;alter…
This post is to provide a set of queries to monitor any user session on an Oracle Instance more efficiently. Below query (Lets name it Query1 for the sake of discussion) gives you an overall status of what all sessions are active, what wait event they are waiting on and what are they executing. set…
The Row Cache or Data Dictionary Cache is a memory area in the shared pool that holds data dictionary information to reduce physical I/O on the data dictionary tables. The row cache lock is used primarily to serialize changes to the data dictionary and is waited on when a lock on a data dictionary cache…
When a user session commits, the session’s redo information needs to be flushed from memory to the redo logfile to make it permanent. At the time of commit, the user session will post the LGWR to write the log buffer (containing the current unwritten redo, including this session’s redo information) to the redo log file.…
log file switch (archiving needed) Waiting for a log switch because the log that the LGWR will be switching into has not been archived yet. Check the alert file to make sure that archiving has not stopped due to a failed archive write. To speed archiving, consider adding more archive processes or putting the archive…
Library cache pins are used to manage library cache concurrency. Pinning an object causes the heaps to be loaded into memory (if not already loaded). PINS can be acquired in NULL, SHARE or EXCLUSIVE modes and can be considered like a special form of lock. A wait for a “library cache pin” implies some other…
The library cache lock controls the concurrency between clients of the library cache by acquiring a lock on the object handle so that either: One client can prevent other clients from accessing the same object The client can maintain a dependency for a long time (no other client can change the object). This lock is…
Oracle Database Internal Mechanism 