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

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

<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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> On 19 Feb 2018, at 5:54 pm, Rene Romero Benavides <rene(dot)romero(dot)b(at)gmail(dot)com> 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(at)inomial(dot)com <mailto: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 <tel:(407)%20872-4272>; blocked by process 7186.
> Process 7186 waits for ShareLock on transaction 4078724210 <tel:(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 <tel:(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 <tel:(408)%20978-3283> | 56/2306861 | 41715 | ExclusiveLock | | |
> (7 rows)
>
> TIA
>
> Cheers,
>
> -- David
>
>
> <PastedGraphic-4.tiff> <http://www.inomial.com/>
>
> <http://www.inomial.com/>David Wheeler • software engineer
> Inomial Pty Ltd • Automatic Billing <http://www.inomial.com/>
> p +61 3 9663 3554 <tel:+61%203%209663%203554>
>
>
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