Jonathan Lewis dissects a deadlock.

From Jonathan Lewis’ Oracle Scratchpad blog.

Here’s a deadlock graph that appeared on Oracle-L and OTN a couple of days ago:

It’s a little unusual because instead of the common TX mode 6 (eXclusive) crossover we have one TX and one TM lock, the TX wait is for mode 4 (S) and the TM wait is for a conversion from 3 (SX) to 5 (SSX).

The modes and types give us some clues about what’s going on: TX/4 is typically about indexes involved in referential integrity (though there are a couple of more exotic reasons such as wait for ITLs, Freelists or tablespace status change); conversion of a TM lock from mode 3 to mode 5 is only possible (as far as I know) in the context of missing foreign key indexes when you delete a parent row.

Here’s a simple data set to help demonstrate the type of thing that could have caused this deadlock:

Note that I have defined the primary key on the child the “wrong way round”, so that the foreign key doesn’t have a supporting index. Note also that the foreign key constraint is defined as ‘on delete cascade’ – this isn’t a necessity, but it means I won’t have to delete child rows explicitly in my demo.

To find out Jonathan’s next steps, read the full post at: