Re: Deadlock with 2 processes waiting on transactionids and locking unrelated tables

[David Wheeler <david@xxxxxxxxxxx>]

Oh I see. Yeah it’s kind of obvious now that you point it out! 

Btw, do the transactions use explicit locking?

We occasionally use for update so that could be involved. We’ll have a closer look at the code involved. 

I’m still curious about why the locks are both transaction locks rather than one of them being a row lock. 

Thanks for your help!

Cheers, 

-- David

David Wheeler • software engineer

Inomial Pty Ltd • Automatic Billing

p +61 3 9663 3554

On 19 Feb 2018, at 5:54 pm, Rene Romero Benavides <rene.romero.b@xxxxxxxxx> wrote:

My guess is that the transaction doing:

 

update "planscheduleitem" set "planschedule"=$1 where "psi"=$2

updates ticket before reaching that point

And

update ticket set unread = true where ticketid = $1

updates planscheduleitem before that

Does it make sense to you? Btw, do the transactions use explicit locking?

2018-02-18 23:28 GMT-06:00 David Wheeler <david@xxxxxxxxxxx>:

Hi,

We’re seeing deadlock semi-regularly (0-2 per day) that I’m really having trouble getting to the bottom of. 

Process 7172 waits for ShareLock on transaction 4078724272; blocked by process 7186.

Process 7186 waits for ShareLock on transaction 4078724210; blocked by process 7172.

The two queries in question are updates on unrelated tables. Running the queries on their own shows no overlapping entries in pg_locks. 

Process 7172: update ticket set unread = true where ticketid = $1

Process 7186: update "planscheduleitem" set "planschedule"=$1 where "psi"=$2

How can I work out why Postgres has decided that the two processes are in deadlock? Is there an explainer somewhere on transaction level locks? I can’t see anything in the docs besides that they exist. 

Details below

select version();

                                                              version

-----------------------------------------------------------------------------------------------------------------------------------

 PostgreSQL 10.2 (Ubuntu 10.2-1.pgdg14.04+1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4, 64-bit

(1 row)

-------------------------------------------------------------------------------------------------------------------------------------------------------

after running update "planscheduleitem" set "planschedule"=$1 where "psi"=$2

SELECT locktype, virtualxid, transactionid, virtualtransaction, pid, mode, relname, page, tuple

FROM pg_locks LEFT JOIN pg_class ON (relation=oid) where pid =41715;

   locktype    | virtualxid | transactionid | virtualtransaction |  pid  |       mode       |         relname         | page | tuple

---------------+------------+---------------+--------------------+-------+------------------+-------------------------+------+-------

 relation      |            |               | 56/2306863         | 41715 | RowExclusiveLock | ticket_parentticketid   |      |

 relation      |            |               | 56/2306863         | 41715 | RowExclusiveLock | ticket_originalticketid |      |

 relation      |            |               | 56/2306863         | 41715 | RowExclusiveLock | ticket_tickettypeid_idx |      |

 relation      |            |               | 56/2306863         | 41715 | RowExclusiveLock | ticket_subject_idx      |      |

 relation      |            |               | 56/2306863         | 41715 | RowExclusiveLock | ticket_closedtime_idx   |      |

 relation      |            |               | 56/2306863         | 41715 | RowExclusiveLock | ticket_assignedto_idx   |      |

 relation      |            |               | 56/2306863         | 41715 | RowExclusiveLock | ticket_serviceuid_idx   |      |

 relation      |            |               | 56/2306863         | 41715 | RowExclusiveLock | ticket_parentuid_idx    |      |

 relation      |            |               | 56/2306863         | 41715 | RowExclusiveLock | ticket_createdtime_idx  |      |

 relation      |            |               | 56/2306863         | 41715 | RowExclusiveLock | ticket_txid             |      |

 relation      |            |               | 56/2306863         | 41715 | RowExclusiveLock | ticket_tickettype       |      |

 relation      |            |               | 56/2306863         | 41715 | RowExclusiveLock | ticket_ticketpriority   |      |

 relation      |            |               | 56/2306863         | 41715 | RowExclusiveLock | ticket_idx_0            |      |

 relation      |            |               | 56/2306863         | 41715 | RowExclusiveLock | ticket_pkey             |      |

 relation      |            |               | 56/2306863         | 41715 | RowExclusiveLock | number_constraint       |      |

 relation      |            |               | 56/2306863         | 41715 | RowExclusiveLock | ticket                  |      |

 virtualxid    | 56/2306863 |               | 56/2306863         | 41715 | ExclusiveLock    |                         |      |

 transactionid |            |    4089785154 | 56/2306863         | 41715 | ExclusiveLock    |                         |      |

 relation      |            |               | 56/2306863         | 41715 | RowExclusiveLock | ticket_fromuid_idx      |      |

(19 rows)

--------------------------------------------------------------------------------------------------------------------------------------------------------

after running update ticket set unread = true where ticketid = $1

SELECT locktype, virtualxid, transactionid, virtualtransaction, pid, mode, relname, page, tuple

FROM pg_locks LEFT JOIN pg_class ON (relation=oid) where pid =41715;

   locktype    | virtualxid | transactionid | virtualtransaction |  pid  |       mode       |               relname                | page | tuple

---------------+------------+---------------+--------------------+-------+------------------+--------------------------------------+------+-------

 relation      |            |               | 56/2306861         | 41715 | RowExclusiveLock | planscheduleitem_plan_company_idx    |      |

 relation      |            |               | 56/2306861         | 41715 | RowExclusiveLock | psi_uid_startdate                    |      |

 relation      |            |               | 56/2306861         | 41715 | RowExclusiveLock | psi_planschedule_startdate_starttime |      |

 relation      |            |               | 56/2306861         | 41715 | RowExclusiveLock | planscheduleitem_pkey                |      |

 relation      |            |               | 56/2306861         | 41715 | RowExclusiveLock | planscheduleitem                     |      |

 virtualxid    | 56/2306861 |               | 56/2306861         | 41715 | ExclusiveLock    |                                      |      |

 transactionid |            |    4089783283 | 56/2306861         | 41715 | ExclusiveLock    |                                      |      |

(7 rows)

TIA

Cheers, 

-- David

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David Wheeler • software engineer

Inomial Pty Ltd • Automatic Billing

p +61 3 9663 3554

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