Will Oracle lock the whole table while performing a DML statement or just the row

Question

When i try to insert/update something in a db table, will Oracle lock the whole table or only the row being inserted/updated?

Is this something that can be controlled t

Answer 1

When a regular DML is executed (UPDATE/DELETE/INSERT,MERGE, and SELECT ... FOR UPDATE) oracle obtains 2 locks. Row level Lock (TX) - This obtains a lock on the particular row being touched and any other transaction attempting to modify the same row gets blocked, till the one already owning it finishes. Table Level Lock (TM) - When Row lock (TX) is obtained an additional Table lock is also obtained to prevent any DDL operations to occur while a DML is in progress.

What matters is though in what mode the Table lock is obtained. A row share lock (RS), also called a subshare table lock (SS), indicates that the transaction holding the lock on the table has locked rows in the table and intends to update them. An SS lock is the least restrictive mode of table lock, offering the highest degree of concurrency for a table.

A row exclusive lock (RX), also called a subexclusive table lock (SX), indicates that the transaction holding the lock has updated table rows or issued SELECT ... FOR UPDATE. An SX lock allows other transactions to query, insert, update, delete, or lock rows concurrently in the same table. Therefore, SX locks allow multiple transactions to obtain simultaneous SX and SS locks for the same table.

A share table lock (S) held by one transaction allows other transactions to query the table (without using SELECT ... FOR UPDATE) but allows updates only if a single transaction holds the share table lock. Multiple transactions may hold a share table lock concurrently, so holding this lock is not sufficient to ensure that a transaction can modify the table.

A share row exclusive table lock (SRX), also called a share-subexclusive table lock (SSX), is more restrictive than a share table lock. Only one transaction at a time can acquire an SSX lock on a given table. An SSX lock held by a transaction allows other transactions to query the table (except for SELECT ... FOR UPDATE) but not to update the table.

An exclusive table lock (X) is the most restrictive mode of table lock, allowing the transaction that holds the lock exclusive write access to the table. Only one transaction can obtain an X lock for a table.