Unveiling InnoDB’s Transaction Struct: 63 Variables Explained

InnoDB is MySQL’s transactional storage engine that provides non‑blocking consistent reads, redo/undo logs, dead‑lock detection and a rich set of locks.

Examining the source of InnoDB reveals that opening a transaction requires handling 63 distinct variables, each with its own type and semantics.

Source file

storage/innobase/include/trx0trx.h

magic_n – type ulint (defined as unsigned long).

mutex – type ib_mutex_t. Used to protect the fields state and lock (except some lock fields protected by lock_sys‑>mutex).

state – type trx_state_t. Transaction states include TRX_STATE_NOT_STARTED, TRX_STATE_ACTIVE, TRX_STATE_PREPARED, TRX_STATE_COMMITTED_IN_MEMORY (alias COMMITTED).

lock – type trx_lock_t. Holds information about transaction locks and state; protected by trx‑>mutex, lock_sys‑>mutex or both.

is_recovered – type ulint. 0 = normal transaction, 1 = recovered and must be rolled back; protected by trx_sys‑>mutex when the transaction is in the in_rw_trx_list.

op_info – type const char*. Holds operation‑specific information.

isolation_level – type ulint. Values: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, SERIALIZABLE.

is_registered:1 – type unsigned. Set to 1 after the transaction registers with the XA coordinator via trans_register_ha(), cleared on commit or rollback.

check_unique_secondary – type ulint. Usually TRUE; set to FALSE when the user disables secondary‑index unique‑key checks to speed up inserts.

support_xa – type ulint. When FALSE, the XA two‑phase commit steps are skipped, saving CPU time and about 150 bytes of undo‑log space.

flush_log_later – type ulint. In two‑phase commit, the log flush is deferred until after the prepare_commit mutex is released.

must_flush_log_later – type ulint. Set to TRUE in trx_commit() if flush_log_later was true and the transaction made modifications; forces a log flush in trx_commit_complete_for_mysql().

duplicates – type ulint. Flags such as TRX_DUP_IGNORE or TRX_DUP_REPLACE.

has_search_latch – type ulint. TRUE if the transaction holds the search system latch in S‑mode.

search_latch_timeout – type ulint. Controls how many new searches are waited for in row0sel.cc before re‑acquiring the latch, reducing contention.

dict_operation – type trx_dict_op_t. Indicates the operation type: 0 = no table‑structure change, 1 = alter table, 2 = modify index.

declared_to_be_inside_innodb – type ulint. TRUE if the transaction has been declared inside InnoDB via srv_conc_enter_innodb.

n_tickets_to_enter_innodb – type ulint. Counts tickets for entering InnoDB; decremented when srv_conc_innodb_enter is called.

dict_operation_lock_mode – type ulint. Lock mode for dict_operation_lock: 0, RW_S_LATCH or RW_X_LATCH.

no – type trx_id_t. Transaction serialization number, set just before the transaction reaches COMMITTED_IN_MEMORY; protected by trx_sys‑>mutex.

start_time – type time_t. Timestamp when the transaction last entered TRX_STATE_ACTIVE.

id – type trx_id_t. The transaction identifier.

XID – type XID. X/Open XA transaction identifier for branch tracking.

commit_lsn – type lsn_t ( typedef ib_uint64_t lsn_t). Log sequence number at commit time.

table_id – type table_id_t. Table to drop when dict_operation == TRX_DICT_OP_TABLE, otherwise 0.

mysql_thd – type THD*. Pointer to the MySQL thread handling this transaction (or NULL).

mysql_log_file_name – type const char*. Name of the latest binlog file when binary logging is enabled.

mysql_log_offset – type ib_int64_t. End offset of the binlog entry.

n_mysql_tables_in_use – type ulint. Number of InnoDB tables used by the current SQL statement.

mysql_n_tables_locked – type ulint. Number of tables locked by the current statement, excluding those locked for consistent read.

trx_list – type UT_LIST_NODE_T(trx_t). List of all transactions, protected by trx_sys‑>mutex.

mysql_trx_list – type UT_LIST_NODE_T(trx_t). List of transactions created for MySQL, also protected by trx_sys‑>mutex.

error_state – type dberr_t. 0 if no error; otherwise holds the error number (set only by the owning thread).

error_info – type const dict_index_t*. Points to the problematic index when a duplicate‑key error occurs.

error_key_num – type ulint. MySQL key number of the index that caused a duplicate‑key error.

sess – type sess_t*. Session associated with the transaction (or NULL).

graph – type que_t*. Current query running in the session; may survive a commit if it belongs to a stored procedure.

global_read_view_heap – type mem_heap_t*. Memory heap for the global read view.

global_read_view – type read_view_t*. Consistent read view linked to the transaction (or NULL).

read_view – type read_view_t*. Consistent read view used by the transaction or cursor.

trx_savepoints – type UT_LIST_BASE_NODE_T(trx_named_savept_t). List of savepoints created with SAVEPOINT, ordered oldest first.

undo_mutex – type ib_mutex_t. Protects fields in the undo section (except last_sql_stat_start).

undo_no – type undo_no_t ( typedef ib_id_t undo_no_t). Sequential number for undo log records, representing the number of rows modified/inserted.

last_sql_stat_start – type trx_savept_t. Undo number when the last SQL statement started; used to roll back on error.

rseg – type trx_rseg_t*. Rollback segment assigned to the transaction (or NULL).

insert_undo – type trx_undo_t*. Pointer to the insert undo log (or NULL if no inserts).

update_undo – type trx_undo_t*. Pointer to the update undo log (or NULL if no updates).

roll_limit – type undo_no_t. Minimum undo number to process during a rollback.

pages_undone – type ulint. Number of undo‑log pages undone since the last truncation.

undo_no_arr – type trx_undo_arr_t*. Array of undo numbers currently being processed by a rollback.

n_autoinc_rows – type ulint. Number of AUTO‑INC rows required by the statement, useful for multi‑row INSERTs.

autoinc_locks – type ib_vector_t*. AUTOINC locks held by the transaction; also appear in trx_locks list and must be freed when the transaction is destroyed.

read_only – type ibool. TRUE if the transaction is read‑only; such transactions are not assigned an UNDO log.

auto_commit – type ibool. TRUE if the transaction runs in autocommit mode.

will_lock – type ulint. Counter incremented each time the MySQL layer determines a lock will be acquired.

ddl – type bool. TRUE if the transaction is started for a DDL operation.

fts_trx – type fts_trx_t*. Full‑text search information; NULL if the transaction has not modified FTS‑indexed tables.

fts_next_doc_id – type doc_id_t. Document ID used for updates in FTS.

flush_tables – type ulint. Count of tables being flushed when a FLUSH TABLES statement is covered.

api_trx – type bool. TRUE if the transaction was started via the InnoDB API.

api_auto_commit – type bool. Indicates automatic commit when using the API.

read_write – type bool. Indicates whether the transaction performs read‑write operations.

detailed_error[256] – type char. Buffer for a detailed error message of the last error.