oracle：
Summary of Locking Behavior
The database maintains several different types of locks, depending on the operation that acquired the lock.
In general, the database uses two types of locks: exclusive locks and share locks. Only one exclusive lock can be obtained on a resource such as a row or a table, but many share locks can be obtained on a single resource.
Locks affect the interaction of readers and writers. A reader is a query of a resource, whereas a writer is a statement modifying a resource. The following rules summarize the locking behavior of Oracle Database for readers and writers:
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A row is locked only when modified by a writer.
When a statement updates one row, the transaction acquires a lock for this row only. By locking table data at the row level, the database minimizes contention for the same data. Under normal circumstances[Foot 1](#Foot 1) the database does not escalate a row lock to the block or table level.
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A writer of a row blocks a concurrent writer of the same row.
If one transaction is modifying a row, then a row lock prevents a different transaction from modifying the same row simultaneously.
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A reader never blocks a writer.
Because a reader of a row does not lock it, a writer can modify this row. The only exception is a
SELECT ... FOR UPDATEstatement, which is a special type ofSELECTstatement that does lock the row that it is reading. -
A writer never blocks a reader.
When a row is being changed by a writer, the database uses undo data to provide readers with a consistent view of the row.
SQL SERVER:
Shared Locks
Shared locks are acquired automatically by SQL Server when data is read. Shared locks can be
held on a table, a page, an index key, or an individual row. Many processes can hold shared
locks on the same data, but no process can acquire an exclusive lock on data that has a shared
lock on it (unless the process requesting the exclusive lock is the same process as the one
holding the shared lock). Normally, shared locks are released as soon as the data has been
read, but you can change this by using query hints or a different transaction isolation level.