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

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(at)inomial(dot)com>:

> 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
> <(407)%20872-4272>; blocked by process 7186.
> Process 7186 waits for ShareLock on transaction 4078724210
> <(407)%20872-4210>; 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 <(408)%20978-5154> |
> 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 <(408)%20978-3283> |
> 56/2306861 | 41715 | ExclusiveLock |
> | |
> (7 rows)
>
> TIA
>
> Cheers,
>
> -- David
>
>
> <http://www.inomial.com/>
>
> <http://www.inomial.com/>*David Wheeler* • software engineer
> *Inomial Pty Ltd* • Automatic Billing <http://www.inomial.com/>
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>
>
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