mirror of
https://github.com/MariaDB/server.git
synced 2025-07-07 06:01:31 +03:00
a6f258e47f
Backported from 10.520e9e804c1and5948d7602e. sel_restore_position_for_mysql() moves forward persistent cursor position after btr_pcur_restore_position() call if cursor relative position is BTR_PCUR_ON and the cursor points to the record with NOT the same field values as in a stored record(and some other not important for this case conditions). It was done because btr_pcur_restore_position() sets page_cur_mode_t mode to PAGE_CUR_LE for cursor->rel_pos == BTR_PCUR_ON before opening cursor. So we are searching for the record less or equal to stored one. And if the found record is not equal to stored one, then it is less and we need to move cursor forward. But there can be a situation when the stored record was purged, but the new one with the same key but different value was inserted while row_search_mvcc() was suspended. In this case, when the thread is awaken, it will invoke sel_restore_position_for_mysql(), which, in turns, invoke btr_pcur_restore_position(), which will return false because found record don't match stored record, and sel_restore_position_for_mysql() will move forward cursor position. The above can lead to the case when awaken row_search_mvcc() do not see records inserted by other transactions while it slept. The mtr test case shows the example how it can be. The fix is to return special value from persistent cursor restoring function which would notify its caller that uniq fields of restored record and stored record are the same, and in this case sel_restore_position_for_mysql() don't move cursor forward. Delete-marked records are correctly processed in row_search_mvcc(). Non-unique secondary indexes are "uniquified" by adding the PK, the index->n_uniq should then be index->n_fields. So there is no need in additional checks in the fix. If transaction's readview can't see the changes made in secondary index record, it requests clustered index record in row_search_mvcc() to check its transaction id and get the correspondent record version. After this row_search_mvcc() commits mtr to preserve clustered index latching order, and starts mtr. Between those mtr commit and start secondary index pages are unlatched, and purge has the ability to remove stored in the cursor record, what causes rows duplication in result set for non-locking reads, as cursor position is restored to the previously visited record. To solve this the changes are just switched off for non-locking reads, it's quite simple solution, besides the changes don't make sense for non-locking reads. The more complex and effective from performance perspective solution is to create mtr savepoint before clustered record requesting and rolling back to that savepoint after that. See MDEV-27557. One more solution is to have per-record transaction id for secondary indexes. See MDEV-17598. If any of those is implemented, just remove select_lock_type argument in sel_restore_position_for_mysql().
36 lines
1.4 KiB
Plaintext
36 lines
1.4 KiB
Plaintext
CREATE TABLE t (a int PRIMARY KEY, b int NOT NULL UNIQUE) engine = InnoDB;
|
|
connect prevent_purge,localhost,root,,;
|
|
start transaction with consistent snapshot;
|
|
connect con_del_1,localhost,root,,;
|
|
INSERT INTO t VALUES (20,20);
|
|
SET DEBUG_SYNC = 'innodb_row_search_for_mysql_exit SIGNAL first_del_row_search_mvcc_finished WAIT_FOR first_del_cont';
|
|
DELETE FROM t WHERE b = 20;
|
|
connect con_ins_1,localhost,root,,;
|
|
SET DEBUG_SYNC = 'now WAIT_FOR first_del_row_search_mvcc_finished';
|
|
SET DEBUG_SYNC = 'lock_wait_suspend_thread_enter SIGNAL first_ins_locked';
|
|
SET DEBUG_SYNC = 'ib_after_row_insert SIGNAL first_ins_row_inserted WAIT_FOR first_ins_cont';
|
|
INSERT INTO t VALUES(10, 20);
|
|
connect con_del_2,localhost,root,,;
|
|
SET DEBUG_SYNC = 'now WAIT_FOR first_ins_locked';
|
|
SET DEBUG_SYNC = 'lock_wait_suspend_thread_enter SIGNAL second_del_locked';
|
|
DELETE FROM t WHERE b = 20;
|
|
connection default;
|
|
SET DEBUG_SYNC = 'now WAIT_FOR second_del_locked';
|
|
SET DEBUG_SYNC = 'now SIGNAL first_del_cont';
|
|
SET DEBUG_SYNC = 'now WAIT_FOR first_ins_row_inserted';
|
|
connection con_del_1;
|
|
connection default;
|
|
disconnect prevent_purge;
|
|
InnoDB 0 transactions not purged
|
|
SET DEBUG_SYNC = 'now SIGNAL first_ins_cont';
|
|
connection con_del_2;
|
|
connection con_ins_1;
|
|
connection default;
|
|
INSERT INTO t VALUES(30, 20);
|
|
disconnect con_ins_1;
|
|
disconnect con_del_1;
|
|
disconnect con_del_2;
|
|
connection default;
|
|
SET DEBUG_SYNC = 'RESET';
|
|
DROP TABLE t;
|