The row cache lock escalates into the row cache enqueue lock when multiple ddl statements are waiting to execute. The enqueue lock structures are allocated from the shared pool as needed but when these requests wait and time out, we see the row cache lock wait event. Demos, syntax, and example code of oracle table locks. Tx row lock contention wait event for an update statement. I was testing an application performance in 12c, and one job was constantly running slower than 11g. This update has filter columns which are part of unique index. High wait events only appear during the load test when the system is stressed.
All of a sudden, one of the databases started to work very slowly, and a quick look in ash data showed that it was spending over 70% of its time waiting on row cache lock not to be confused with latch. Row level security part 1 november 15, 2012 posted by mwidlake in database design, security. The latch events was bit by bit dominating the environment during the last months and turn on an attention alarm to us. A session must acquire a library cache lock on an object handle to prevent other. It could cache these blocks in various places, such as the default pool, a keep buffer pool, or a recycle buffer pool. The result cache feature introduced in oracle database 11g release 1 provides considerable benefits for operations where results sets do not change very often. This slowness is accounted towards row cache lock and dfs lock handle waits. When a row in oracle gets locked, the record itself is updated with the new value if any and, in addition, a lock which is essentially a pointer to transaction lock that resides in the rollback segment is placed right into the record. Easier way to resolve this problem is to increase extent size.
Oracle database cloud exadata service version na and later information in this document applies to any platform. Library cache locks and pins the library cache is not one cache, but many. The alert log file indicates waited too long for a row cache enqueue lock. I saw more often row cache lock issues on oracle 11. Ive been working a little on row level security rls recently and wanted to mention a few things, so first some groundwork. Oracle row cache lock wait event in rac environment. There are specialized middleware frameworks, such as oracle coherence, for caching data in java, php, and ruby, but what if there were a generic way to cache the data at the client level. In the past, oracle database cached database blocks. The other day, we found high occurrence of latch events in our principalcore environment 11. Caching has been a feature of oracle for as long as most of us can remember.
Library cache lock and library cache pin waits oracle. Lock in share mode sets a shared mode lock on any rows that are read. A quick look at the session waits during the issue showed around 100 sessions on each instance waiting for latch. To avoid library cache wait events to occur you can try two things, 1. Oracles locking concept is quite different from that of the other systems. We are all familiar with the buffer cache, keep cache, library cache, shared pool, pgauga and so on. In order for any ddl data definition language to execute in oracle database, it must acquire a row cache lock to lock the data dictionary information.
This feature helps store the final result sets in memory, providing highspeed access to the dataset. Hello, the database on one of 4 nodes started to hang. The origins are all different businesses channels that access objects through the use of vpd. The workload i will use is the hammerdb oracle oltp workload on a 4socket. Oracle rac hang waited too long for a row cache enqueue lock. The oracle row cache lock wait event is associated with a specific enqueue type on a data dictionary row. Deadlock and resulting waited too long for a row cache enqueue lock. It contains parse trees and execution plans selection from oracle internals. Developers and dbas get help from oracle experts on. Sql result cache is nice addition to deterministic functions. It was a oracle bug, caused by posting simultaneous messages many at a time will make the queue to a dead lock and it cant be used again and it was patched now. Oracle database 11g provides client query result cache for that purpose. On day 2 of aioug, i discussed on library cache related latches and some causes of the contention on these latches. The enqueue lock structures are allocated from the shared pool as.
As it often happens, it all started with customers complaining about the database being slow. Over the many years and versions, oracle has continually expanded its caching mechanisms. Each row cache lock will be on a specific data dictionary object. Truncate will suffer from slowness in object level checkpoint and further extent cleanup in the dictionary.
Before reading this article i suggest to read article about deterministic functions. Sql result cache 11g dbaora oracle expert presents. Hanganalyze is always the best tool for finding row cache lock holder as following. 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. The environment was a 4node rac of 64bit oracle 10. If the lock wait exceed a predefined threshold, the ddl will abort and cause a trace file to be generated. The sql they were executing was similar, but not same due to literal usage. Its quite possible its application related, but im just trying to determine the actual cause now. Problem with sequence and row cache lock 07112002 11. This view lists the locks currently held by the oracle database and outstanding requests for a. Database hangs periodically with many sessions being blocked by each other, deadlocks can be seen as well, with different waits such as cursor. The unavailability of the library cache object in the library cache is due to the undersized shared pool causing reloads often, too many hard parse because of unshared sqls. One such contention is for row cache object latches that protects the data dictionary loaded in the shared pool.
Row cache enqueue lock tips burleson oracle consulting. If you query data and then insert or update related data within the same transaction. The row cache or data dictionary cache is a memory area in the shared pool that holds data dictionary information to reduce physical io on the data dictionary tables. The waits on the library cache load lock is due to the objects not available in the memory. I hope the steps would be useful if you encounter similar issue. The row cache object latch can also indicate sql with excessive hard parsing or excessive reliance on data dictionary. The shared pool contains a cache of rows from the data dictionary that helps reduce physical io to the data dictionary tables and allows locking of individual data dictionary rows.
So that was oracle database row cache lock concurrency troubleshooting using sql, ash view and systemstate dump. Library cache lock and pin waits can hang instance, and in few cases, whole clusters of rac instances can be hung due to library cache lock and pin waits. You can avoid row lock contention by making sure that the row is available for update beforehand with a select for update and either wait x or nowait, e. Oracle database tips by donald burlesonseptember 5, 2015. I must admit this type of situation does not appear often but its very dangerous since it can considerably slow down a database instance or even freeze it for.
Oracle needs to hold the row cache lock in 5srx mode when calling nextvalue on the nocached sequence, which. We cover about oracle, mysql, postgresql, big data, hadoop, linux, sybase, teradata, goldengate, rac, replication, backup and recovery. Us contention and row cache lock waits were between 0 4. The first peaks on the following graphs corresponds to. Its is based on a real case of a database hang that i worked on recently. Using oracle database 11g release 2 result cache in an. This metric is used to wait for a lock on a data dictionary cache specified by cache id.
Although little known, this is an extremely useful view for systematic latch contention troubleshooting as it provides one important bit of information which no other oracle view or trace provides the session id of the latch holder. Oracle database 11g raises the bar on data caching. Excessive sql parsing on objects public synonyms may cause this latch contention and that library cache latches always precede row cache objects latch contention and a latch wait on a row cache object often means that there is a point of contention within the data dictionary. A rather odd performance issue happened at work a few days ago. This post will help to analyze oracle database instance slowdown that can happen due to considerable row cache lock enqueue wait events. I encountered few customer issues centered around library cache lock and library cache pin waits. Does anyone have some insights as to the root cause and what to do for a troubleshooting. The locks on the data dictionary rows are called row cache enqueue locks. Problem in an one hour period, over 90% of the db time spent on waiting for library cache lock.
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