Shared lock and exclusive lock in DBMS
When a statement modifies data, its transaction holds an exclusive lock on data that prevents other transactions from accessing the data. Show This lock remains in place until the transaction holding the lock issues a commit or rollback. Table-level locking lowers concurrency in a multi-user system. 15.7.1 InnoDB Locking This section describes lock types used by
Shared and Exclusive Locks
If transaction
If a transaction Intention Locks
For example, The intention locking protocol is as follows:
Table-level lock type compatibility is summarized in the following matrix.
A lock is granted to a requesting transaction if it is compatible with existing locks, but not if it conflicts with existing locks. A transaction waits until the conflicting existing lock is released. If a lock request conflicts with an existing lock and cannot be granted because it would cause deadlock, an error occurs. Intention locks do
not block anything except full table requests (for example, Transaction data for an intention lock appears similar to the following in
Record Locks A record lock is a lock on an index record. For example, Record locks always lock index records, even if a table is defined with no indexes. For such cases, Transaction data for a record lock appears similar to the following in
Gap Locks A gap lock is a lock on a gap between index records, or a lock on the gap before the first or after the last index record. For example, A gap might span a single index value, multiple index values, or even be empty. Gap locks are part of the tradeoff between performance and concurrency, and are used in some transaction isolation levels and not others. Gap locking is not needed for statements that lock rows using a unique index to search for a unique row. (This does not include the case that the search condition includes only some columns of a multiple-column unique index; in that case, gap locking does occur.) For example, if the
If It is also worth noting here that conflicting locks can be held on a gap by different transactions. For example, transaction A can hold a shared gap lock (gap S-lock) on a gap while transaction B holds an exclusive gap lock (gap X-lock) on the same gap. The reason conflicting gap locks are allowed is that if a record is purged from an index, the gap locks held on the record by different transactions must be merged. Gap locks in Gap locking can be disabled explicitly. This occurs if you change the transaction isolation level to There are also other effects of using the Next-Key LocksA next-key lock is a combination of a record lock on the index record and a gap lock on the gap before the index record. Suppose that an index contains the values 10, 11, 13, and 20. The possible next-key locks for this index cover the following intervals, where a round bracket denotes exclusion of the interval endpoint and a square bracket denotes inclusion of the endpoint:
For the last interval, the next-key lock locks the gap above the largest value in the index and the “supremum” pseudo-record having a value higher than any value actually in the index. The supremum is not a real index record, so, in effect, this next-key lock locks only the gap following the largest index value. By default, Transaction data for a next-key lock appears similar to the following in
Insert Intention Locks An insert intention lock is a type of gap lock set by The following example demonstrates a transaction taking an insert intention lock prior to obtaining an exclusive lock on the inserted record. The example involves two clients, A and B. Client A creates a table containing two index records (90 and 102) and then starts a transaction that places an exclusive lock on index records with an ID greater than 100. The exclusive lock includes a gap lock before record 102:
Client B begins a transaction to insert a record into the gap. The transaction takes an insert intention lock while it waits to obtain an exclusive lock.
Transaction data for an insert intention lock appears similar to the following in
AUTO-INC Locks An The For more information, see Section 15.6.1.6, “AUTO_INCREMENT Handling in InnoDB”. Predicate Locks for Spatial Indexes To handle locking for operations involving To enable support of isolation levels for tables with What is shared and exclusive lock in DBMS?The two types are exclusive and shared locks. Exclusive locks can be active or retained; shared locks can only be active (see Active and retained states for locks ). Note that there are no delete locks in RLS mode.
What is shared lock in DBMS?Shared lock is also called read lock, used for reading data items only. Shared locks support read integrity. They ensure that a record is not in process of being updated during a read-only request. Shared locks can also be used to prevent any kind of updates of record.
What is exclusive lock in DBMS?When a statement modifies data, its transaction holds an exclusive lock on data that prevents other transactions from accessing the data. This lock remains in place until the transaction holding the lock issues a commit or rollback.
What are the two types of lock in database?Two types of Database lock are present: Shared Lock. Exclusive Lock.
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