Posgresql - deadlock on non related queries

Question

I am using Postgresql 11.
I have found an error similar to following in my logs:

org.postgresql.util.PSQLException: ERROR: deadlock detected
  Detail: Process 332091 waits for ShareLock on transaction 99896257; blocked by process 332093.
Process 332093 waits for ShareLock on transaction 99923910; blocked by process 332091.
  Hint: See server log for query details.
  Where: while locking tuple (884208,33) in relation "tab_1"
SQL statement "SELECT 1 FROM ONLY "sch"."tab_1" x WHERE "tab_1_key" OPERATOR(pg_catalog.=) $1 FOR KEY SHARE OF x"
SQL statement "INSERT INTO sch.tab_2 (col1, col2, ..., coln)
   SELECT
      col1,
      col2,
      ...
      coln
   FROM
      sch.tab_3 
   LEFT JOIN sch.tab_4 
      ON tab_4.id = tab_3.id
   LEFT JOIN sch.tab_5 
      ON tab_5.id = tab_3.id

So basically the first query looks like some DB internal query, nothing from my code.
And it seems like it is making deadlock with my query. But they are not related by any table.
Any ideas about how to track the reason for the deadlock in such a case?

Answer 1

These queries are generated internally by the triggers that implement foreign key constraints.

For example, adding a row to a table with a foreign key constraint will add such a key share lock on the corresponding row in the referenced table, so that nobody can delete it or modify the primary key until the inserting transaction is done.

This can cause deadlocks between transactions that modify both tables.

When searching for the statements at fault, don't forget that it was not necessarily the last statement in the transaction (that is shown in the log message) that took the problematic lock.

One way to proceed is to set log_min_duration_statement to 0, and when the deadlock strikes again, examine all statements run by the affected transactions. The other way is to review the application code to find out which statements were executed in the transactions.