Re: delta relations in AFTER triggers

> 2016-11-02 15:57 GMT+01:00 Kevin Grittner <kgrittn@gmail.com>:
>> On Sun, Oct 30, 2016 at 10:35 AM, Kevin Grittner <kgrittn@gmail.com> wrote:
>>
>>> SPI support would also
>>> allow us to consider using set logic for validating foreign keys,
>>> instead of the one-row-at-a-time approach currently used.
>>
>> Just as a proof of concept for this I used the attached test case
>> to create foreign keys using current techniques versus set-oriented
>> queries with the transition-tsr code.  These probably can be
>> improved, since this is a "first cut" off the top of my head.
>>
>> The delete of about one million rows from a "parent" table with no
>> matching rows in the "child" table, and no index on referencing
>> column in the child table, took 24:17.969 using current triggers
>> and 00:03.262 using the set-based triggers.  Yes, that reduces
>> current run time for that case by 99.78%

On Wed, Nov 2, 2016 at 11:07 AM, Pavel Stehule <pavel.stehule@gmail.com> wrote:
>
> this is great number

On Wed, Nov 2, 2016 at 11:41 AM, Adam Brusselback
<adambrusselback@gmail.com> wrote:
>
> That is really incredible.  Gets rid of the need for an index on referencing
> columns for a ton of use cases.

Keep in mind that this is just a quick proof of concept.  Unless
all participating transactions are at serializable isolation level
something would need to be done to handle race conditions, and that
might affect performance.  I do think that this approach is likely
to be better in enough circumstances, even after that is covered,
that it will be worth pursuing -- either as an option when
declaring a foreign key, or as the only implementation.  Until we
have a version that covers the race conditions and benchmark it in
a variety of workloads, it is hard to feel sure about the latter.
There may be some situations where crawling the indexes a row at a
time will perform better than this by enough to want to retain that
option.

A big plus of a single set-oriented statement is that it doesn't
suck unlimited RAM -- it will use work_mem to limit each tuplestore
and each query node, spilling to disk if needed.  The current FK
implementation sometimes runs for a very long time and can run
people out of memory.

--
Kevin Grittner
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