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MySQL 8.0.39
Source Code Documentation
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MySQL 8.0.39
Source Code Documentation
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The transaction handle. More...
#include <trx0trx.h>
Public Member Functions | |
| bool | is_empty () |
| bool | is_insert_only () |
| bool | is_update () |
Public Attributes | |
| trx_rseg_t * | rseg |
| rollback segment assigned to the transaction, or NULL if not assigned yet More... | |
| trx_undo_t * | insert_undo |
| pointer to the insert undo log, or NULL if no inserts performed yet More... | |
| trx_undo_t * | update_undo |
| pointer to the update undo log, or NULL if no update performed yet More... | |
The transaction handle.
Normally, there is a 1:1 relationship between a transaction handle (trx) and a session (client connection). One session is associated with exactly one user transaction. There are some exceptions to this:
For DDL operations, a subtransaction is allocated that modifies the data dictionary tables. Lock waits and deadlocks are prevented by acquiring the dict_operation_lock before starting the subtransaction and releasing it after committing the subtransaction.
The purge system uses a special transaction that is not associated with any session.
If the system crashed or it was quickly shut down while there were transactions in the ACTIVE or PREPARED state, these transactions would no longer be associated with a session when the server is restarted.
A session may be served by at most one thread at a time. The serving thread of a session might change in some MySQL implementations. Therefore we do not have std::this_thread::get_id() assertions in the code.
Normally, only the thread that is currently associated with a running transaction may access (read and modify) the trx object, and it may do so without holding any mutex. The following are exceptions to this:
trx_rollback_resurrected() may access resurrected (connectionless) transactions while the system is already processing new user transactions. The trx_sys->mutex prevents a race condition between it and lock_trx_release_locks() [invoked by trx_commit()].
Print of transactions may access transactions not associated with the current thread. The caller must be holding trx_sys->mutex and exclusive global lock_sys latch.
When a transaction handle is in the trx_sys->mysql_trx_list or trx_sys->trx_list, some of its fields must not be modified without holding trx_sys->mutex exclusively.
The locking code (in particular, deadlock checking and implicit to explicit conversion) will access transactions associated to other connections. The locks of transactions are protected by lock_sys latches and sometimes by trx->mutex.
Killing of asynchronous transactions. Represents an instance of rollback segment along with its state variables.
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| trx_undo_t* trx_undo_ptr_t::insert_undo |
pointer to the insert undo log, or NULL if no inserts performed yet
| trx_rseg_t* trx_undo_ptr_t::rseg |
rollback segment assigned to the transaction, or NULL if not assigned yet
| trx_undo_t* trx_undo_ptr_t::update_undo |
pointer to the update undo log, or NULL if no update performed yet
1.9.2