The Oracle “log file sync” wait event is triggered when a user session issues a commit (or a rollback). The user session will signal or post the LGWR to write the log buffer to the redo log file. When the LGWR has finished writing, it will post the user session. The wait is entirely dependent … Read more Oracle Log File Sync Wait Event
About the direct path read temp wait event When a session reads buffers from disk directly into the PGA, the wait is on direct path read temp. This is closely related to the direct path read wait. If the I/O subsystem doesn’t support asynchronous I/Os, then each wait corresponds to a physical read request. If … Read more Oracle Wait Event Explained: Direct Path Read Temp
The db file sequential read wait event has three parameters: file#, first block#, and block count. In Oracle Database 10g, this wait event falls under the User I/O wait class. Keep the following key thoughts in mind when dealing with the db file sequential read wait event. Common Causes, Diagnosis, and Actions The db file … Read more Understanding the Oracle db file sequential read Wait Event
In order for DDL (Data Definition Language) to execute, it must acquire a row cache lock to lock the data dictionary information. The shared pool contains a cache of rows from the data dictionary that helps reduce physical I/O to the data dictionary tables and allows locking of individual data dictionary rows. The locks on the … Read more Oracle Row Cache Lock Wait Event Explained
When a session waits on the “read by other session” event, it indicates a wait for another session to read the data from disk into the Oracle buffer cache. If this happens too often the performance of the query or the entire database can suffer. Typically this is caused by contention for “hot” blocks or … Read more Oracle Read By Other Session Wait Event
The fn_dblog() function (formerly known as the DBCC command) is one of several undocumented functions for SQL Server; it allows you to view the transaction log records in the active part of the transaction log file for the current database. Note that use of the fn_dblog() function (and all other undocumented commands) against a production … Read more SQL Server fn_dblog() Function Details and Example
A user process acquires the CACHE BUFFERS CHAINS latch to scan the System Global Area (SGA) for database cache buffers. Blocks in the buffer cache are placed on linked lists (cache buffer chains). Blocks are put on the hash chain according to their DBA (data block address) and CLASS of the block. Each hash chain … Read more Oracle Cache Buffers Chain Latch
Recently, I was assisting one of our customers for Oracle trying to diagnose sessions waiting on the “enq: TM – contention” event. The blocked sessions were executing simple INSERT statements similar to: INSERT INTO supplier VALUES (:1, :2, :3); Waits on enq: TM – contention indicate there are unindexed foreign key constraints. Reviewing the SUPPLIER … Read more Solving Oracle enq: TM Contention Wait Events
The log buffer space wait event occurs when server processes write data into the log buffer faster than the LGWR process can write it out. The LGWR process begins writing entries to the online redo log file if any of the following conditions are true: The log buffer reaches the _log_io_size threshold. By default, this … Read more Oracle Log Buffer Space Wait Event
The PL/SQL lock timer wait event represents the amount of time a user or application has “slept” through the USER_LOCK.SLEEP or DBMS_LOCK.SLEEP procedures. The ‘idle’ event “PL/SQL lock timer” is worth watching because it can indicate issues with application response, throughput and possibly coding. The PL/SQL lock timer wait event is, as the command that issues … Read more Oracle PL/SQL Lock Timer Wait Event