MDL BF-BF conflict on ALTER and INSERT with multi-level foreign key parents #503
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…arents Issue: The Commit: 0584846 'Add TL_FIRST_WRITE in SQL layer for determining R/W' limits INSERT statements on write nodes to acquire MDL locks on it's all child tables and thereby wsrep certification keys added, but on applier nodes it does acquire MDL locks for all child tables. This can result into MDL BF-BF conflict on applier node when transactions referring to parent and child tables are executed concurrently. For example: Tables with foreign keys: t1<-t2<-t3<-t4 Conflicting transactions: INSERT t1 and DROP TABLE t4 Wsrep certification keys taken on write node: - for INSERT t1: t1 and t2 - for DROP TABLE t4: t4 On applier node MDL BF-BF conflict happened between two transaction because MDL locks on t1, t2, t3 and t4 were taken for INSERT t1, which conflicted with MDL lock on t4 taken by DROP TABLE t4. The Wsrep certification keys helps in resolving this MDL BF-BF conflict by prioritizing and scheduling concurrent transactions. But to generate Wsrep certification keys it needs to open and take MDL locks on all the child tables. The Commit: 0584846 change limits MDL lock to be taken on all child nodes for read-only FK checks (INSERT t1). But this doesn't works on applier nodes because Write_rows_log_event event logged for INSERT is also used to record update (check Write_rows_log_event::get_trg_event_map()), and therefore MDL locks is taken for all the child tables on applier node for update and insert event. Solution: Additional keys for the referenced/foreign table needs to be added to avoid potential MDL conflicts with concurrent update and DDLs.
sciascid
reviewed
Jun 6, 2025
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Is the following a regression?
10:35:28 galera.mysql-wsrep#31 w7 [ fail ]
....
10:35:28 mariadbd: /tmp/workspace/mariadb-10.6-pr-review-debug/sql/sql_base.cc:3658: bool open_and_process_routine(THD*, Query_tables_list*, Sroutine_hash_entry*, Prelocking_strategy*, bool, Open_table_context*, bool*, bool*): Assertion `0' failed.
This doesn't look like issue related to my code changes, as same failed on another jenkins run for #505 |
sciascid
reviewed
Jun 12, 2025
sciascid
approved these changes
Jun 16, 2025
|
Why we can't do as follows: In write node: for INSERT t1: t1 and t2 applying node: DROP TABLE t4: t1,t2,t3,t4 |
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Issue:
The Commit: 0584846 'Add TL_FIRST_WRITE in SQL layer for determining R/W' limits INSERT statements on write nodes to acquire MDL locks on it's all child tables and thereby wsrep certification keys added, but on applier nodes it does acquire MDL locks for all child tables. This can result into MDL BF-BF conflict on applier node when transactions referring to parent and child tables are executed concurrently. For example:
Tables with foreign keys: t1<-t2<-t3<-t4
Conflicting transactions: INSERT t1 and DROP TABLE t4
Wsrep certification keys taken on write node:
On applier node MDL BF-BF conflict happened between two transaction because MDL locks on t1, t2, t3 and t4 were taken for INSERT t1, which conflicted with MDL lock on t4 taken by DROP TABLE t4.
The Wsrep certification keys helps in resolving this MDL BF-BF conflict by prioritizing and scheduling concurrent transactions. But to generate Wsrep certification keys it needs to open and take MDL locks on all the child tables.
The Commit: 0584846 change limits MDL lock to be taken on all child nodes for read-only FK checks (INSERT t1). But this doesn't works on applier nodes because Write_rows_log_event event logged for INSERT is also used to record update (check Write_rows_log_event::get_trg_event_map()), and therefore MDL locks is taken for all the child tables on applier node for update and insert event.
Solution:
Additional keys for the referenced/foreign table needs to be added to avoid potential MDL conflicts with concurrent update and DDLs.