explain ITL (Interested Transaction List) wait events

 ITL (Interested Transaction List) wait events occur when a transaction is waiting for a row that is currently locked by another transaction. When a transaction modifies a row in an Oracle database, it acquires a lock on that row to prevent other transactions from modifying it at the same time. The number of transactions that can lock a row is determined by the INITRANS storage parameter for the table or index.

If a transaction attempts to acquire a lock on a row that is already locked by another transaction, it will be added to the Interested Transaction List (ITL) of the block containing the locked row. The ITL is a list of transactions that have shown an interest in a particular data block, either because they hold a lock on a row in that block or because they have performed a read operation on a row in that block.

When a transaction is waiting for a row that is currently locked by another transaction, it will be in the ITL wait state. The time spent in ITL wait state can contribute to increased database response times, as other transactions may also be waiting for the same row or block.

ITL wait events in Oracle SQL can be identified using performance monitoring tools such as Oracle Enterprise Manager or by querying the V$SESSION_WAIT view. To mitigate ITL wait events, database administrators can adjust the INITRANS storage parameter for tables or indexes to allow for more concurrent transactions to access the same data block. They can also optimize SQL queries to minimize the number of rows locked at the same time or to reduce the time spent in ITL wait state.

eg:

ALTER TABLE Sales INITRANS 4;

INITRANS parameter does not determine the number of records that will be locked at a time.

Instead, it determines the initial number of transaction slots that are allocated in each data block for managing concurrent transactions. Each transaction that modifies a row acquires a transaction slot in the corresponding data block's ITL (Interested Transaction List).

 setting the INITRANS parameter on a table or index can help mitigate ITL issues, it does not guarantee that there will be no row lock contention. Other factors such as the number of concurrent transactions accessing the same rows, the frequency of updates on the rows, and the size of the ITL slot can also contribute to row lock contention.

Therefore, it is important to carefully monitor the performance of your database and adjust parameters as necessary to optimize performance and minimize contention. Additionally, it is a good practice to design your application and database schema in a way that minimizes the likelihood of row lock contention, such as by using proper transaction isolation levels and designing tables and indexes to minimize overlapping updates.