Thread: Performance issues on FK Triggers after replacing a primary column

On 3/27/22 09:30, Per Kaminsky wrote:
> Hi there,
> 
> i recently stumbled upon a performance issue which i can't 
> really understand.
> The issue occured when i (roughly) did the following without a commit in 
> between:
> 
>   * Replace the PK column of a table A which has a referencing table B -
>     I have removed the FK from the referencing tables B and have
>     recreated them afterwards
>   * Now following i am working in one of the referencing tables B,
>     updating columns. This takes an extremely large amount of time. This
>     means, e.g. updating 1000 rows would now need 35-40 seconds.
>   * The "explain" tells, that the Foreign Key trigger in B referencing A
>     causes this mishap.

Post the query and the explain.

Also have you run vacuum and/or analyze on the tables involved?

>   * Re-creating the Index in B for the column referencing A does not
>     cause any performance gain.
>   * If i again remove the FK to A from B this again shrinks back to some
>     milliseconds.
> 
> The question is, what does cause the FK trigger to be less performant 
> than recreating the FK constraint? If executed on 100k or even 1m rows 
> the operation takes hours or even days.
> 
> Thank you very much.
> Sincerely, Per Kaminsky
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com

('Update on B (cost=0.00..71.50 rows=1000 width=244) (actual time=18.015..18.015 rows=0 loops=1)',)

('  Buffers: shared hit=26141 read=21 dirtied=77 written=54',)

('  ->  Seq Scan on B (cost=0.00..71.50 rows=1000 width=244) (actual time=0.102..0.536 rows=1000 loops=1)',)

('        Filter: ((type IS NULL) AND (firmid = 1))',)

('        Buffers: shared hit=59',)

('Planning Time: 0.430 ms',)

# ('Trigger for constraint B_firmid_fkey: time=25.592 calls=1000',)

# ('Trigger for constraint B_type_fkey: time=14.899 calls=1000',)

# ('Trigger for constraint B_userid_fkey: time=16.146 calls=1000',)

# ('Trigger for constraint B_version_fkey: time=11.285 calls=1000',)

# ('Trigger for constraint B_common_fkey: time=7.913 calls=1000',)

# ('Trigger for constraint B_shiftinstanceid_fkey: time=8543.369 calls=1000',)

('Trigger for constraint B_to_A_fkey: time=27246.413 calls=1000',)

('Execution Time: 35884.978 ms',)

Per Kaminsky <per.kaminsky@hawk-intech.com> writes:
> # ('Trigger for constraint B_firmid_fkey: time=25.592 calls=1000',)
> # ('Trigger for constraint B_type_fkey: time=14.899 calls=1000',)
> # ('Trigger for constraint B_userid_fkey: time=16.146 calls=1000',)
> # ('Trigger for constraint B_version_fkey: time=11.285 calls=1000',)
> # ('Trigger for constraint B_common_fkey: time=7.913 calls=1000',)
> # ('Trigger for constraint B_shiftinstanceid_fkey: time=8543.369 calls=1000',)
> ('Trigger for constraint B_to_A_fkey: time=27246.413 calls=1000',)

Sure looks like B_shiftinstanceid_fkey and B_to_A_fkey are running
seq scans for lack of any matching index.

> The tables have Index on each other. The vacuum can not be called, since all those table modifications are part of
onebig transaction to be able to make a rollback on any problem without causing an abnormal data state regarding the
program.

Hmm, are you dropping and re-adding the reference-side indexes as part of
the same transaction that is doing this?  I'm too short of caffeine to
recall details, but I think there are cases where a newly-made index
cannot be used right away.

            regards, tom lane

On 3/28/22 00:22, Per Kaminsky wrote:
> Sorry, i forgot to add the following:
> 
> Explain / Analyze for the last "update type on B" call, normally there 
> the table has million of rows but i removed most of them since otherwise 
> it would not finish sometime soon:
> 
>     ('Update on B (cost=0.00..71.50 rows=1000 width=244) (actual
>     time=18.015..18.015 rows=0 loops=1)',)
>     ('  Buffers: shared hit=26141 read=21 dirtied=77 written=54',)
>     ('  ->  Seq Scan on B (cost=0.00..71.50 rows=1000 width=244) (actual
>     time=0.102..0.536 rows=1000 loops=1)',)
>     ('        Filter: ((type IS NULL) AND (firmid = 1))',)
>     ('        Buffers: shared hit=59',)
>     ('Planning Time: 0.430 ms',)
>     # ('Trigger for constraint B_firmid_fkey: time=25.592 calls=1000',)
>     # ('Trigger for constraint B_type_fkey: time=14.899 calls=1000',)
>     # ('Trigger for constraint B_userid_fkey: time=16.146 calls=1000',)
>     # ('Trigger for constraint B_version_fkey: time=11.285 calls=1000',)
>     # ('Trigger for constraint B_common_fkey: time=7.913 calls=1000',)
>     # ('Trigger for constraint B_shiftinstanceid_fkey: time=8543.369
>     calls=1000',)
>     ('Trigger for constraint B_to_A_fkey: time=27246.413 calls=1000',)
>     ('Execution Time: 35884.978 ms',)
> 
> The tables have Index on each other. The vacuum can not be called, since 
> all those table modifications are part of one big transaction to be able 

Analyze can be run by itself in the transaction. As Tom said I am not 
seeing any information about indexes on the tables(s). Also, which one 
of the tables you showed is the temporary one or was that not shown?


> to make a rollback on any problem without causing an abnormal data state 
> regarding the program.
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com

On 3/28/22 08:47, Per Kaminsky wrote:
> The tables have Index to each other on each foreign key. The index 
> itself was not touched though, and a remove/recreate did not help. Could 
> it be possible, that when the PK and FK values are replaced the Index is 
> not (immediately) updated and thus cannot be used?

Have you tried an ANALYZE on "A" AND "B" after?:

UPDATE "A" SET id = id_temp;

As to the index not immediately updating, I don't know.


> 
> The temporary table is not shown. It is created to insert the new values 
> from file, then used to update the correct table with the new values, 
> and then removed, it has no connection (FK or something else) to any 
> other table.
> 

So that is the '// fill id_temp with new IDs' part?


-- 
Adrian Klaver
adrian.klaver@aklaver.com

On 3/28/22 09:39, Per Kaminsky wrote:
> Do you mean a simple "ANALYZE VERBOSE A"? Or something different?
> Following the thought that maybe the index got stale, i just tried to 
> add a "REINDEX TABLE B". This did not help as well, which might be the 
> case, if an index (re)build is always deferred until the end of the 
> transaction (which i don't know if that is the case).

The ANALYZE would be more about telling the planner that the data in 
tables changed and what they changed to, then anything to do with the 
index. For this you really don't need the VERBOSE.

> 
> ------------------------------------------------------------------------
> *From:* Adrian Klaver <adrian.klaver@aklaver.com>
> *Sent:* Monday, March 28, 2022 17:59
> *To:* Per Kaminsky <per.kaminsky@hawk-intech.com>; 
> pgsql-general@postgresql.org <pgsql-general@postgresql.org>; Tom Lane 
> <tgl@sss.pgh.pa.us>
> *Subject:* Re: Performance issues on FK Triggers after replacing a 
> primary column
> On 3/28/22 08:47, Per Kaminsky wrote:
>> The tables have Index to each other on each foreign key. The index 
>> itself was not touched though, and a remove/recreate did not help. Could 
>> it be possible, that when the PK and FK values are replaced the Index is 
>> not (immediately) updated and thus cannot be used?
> 
> Have you tried an ANALYZE on "A" AND "B" after?:
> 
> UPDATE "A" SET id = id_temp;
> 
> As to the index not immediately updating, I don't know.
> 
> 
>> 
>> The temporary table is not shown. It is created to insert the new values 
>> from file, then used to update the correct table with the new values, 
>> and then removed, it has no connection (FK or something else) to any 
>> other table.
>> 
> 
> So that is the '// fill id_temp with new IDs' part?
> 
> 
> -- 
> Adrian Klaver
> adrian.klaver@aklaver.com


-- 
Adrian Klaver
adrian.klaver@aklaver.com

On 3/27/22 23:53, Per Kaminsky wrote:
> The table structure looks (roughly) like this:
> 
>   * Table "Base": (id, created, deleted, origin, ...) ~3m rows
>   * Table "A": (id as FK on "Base", ...) ~400k rows
>   * Table "B": (id, ref_a as FK on "A", type, ...) ~2m rows
> 
> Swapping the PK of "A" happens as following, the FK is dropped during 
> the process since otherwise the performance issues also happen here when 
> updating the PK. The update calls do normally utilize a file based 
> import into a temporary table from which i do the actual update:
> 
> ALTER TABLE "B" DROP CONSTRAINT "B_to_A_fkey";
> ALTER TABLE "A" ADD COLUMN id_temp BIGINT;
> // fill id_temp with new IDs
> UPDATE "B" SET ref_a = "A".id_temp WHERE "B".ref_a= "A".id;
> UPDATE "A" SET id = id_temp;
> ALTER TABLE "B" ADD CONSTRAINT "B_to_A_fkey" FOREIGN KEY (ref_a) 
> REFERENCES A(id);

Now that the morning coffee has taken effect, I'm wondering why the 
above is necessary at all?

If "B_to_A_fkey" where to have ON UPDATE CASCADE then you could 
eliminate the dropping/adding back of the FK. The process would be:

ALTER TABLE "A" ADD COLUMN id_temp BIGINT;
// fill id_temp with new IDs
UPDATE "A" SET id = id_temp;
UPDATE "B" SET type = 2 WHERE type ISNULL;

It might even be possible to further simplify depending on what '// fill 
id_temp with new IDs' actually does?

> 
> And then the new occuring step, in the same transaction, which then also 
> has shown the performance issues described if i would not remove the FK 
> temporarily:
> 
> ALTER TABLE "B" DROP CONSTRAINT "B_to_A_fkey";
> UPDATE "B" SET type = 2 WHERE type ISNULL;
> ALTER TABLE "B" ADD CONSTRAINT "B_to_A_fkey" FOREIGN KEY (ref_a) 
> REFERENCES A(id);
> 
> 
> 
> **
> 
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com

On 3/28/22 10:49 AM, Per Kaminsky wrote:
> The usage of "on update cascade" certainly sounds plausible. Since the 
> update does only happen rarely if there is a special need to completely 
> change the backend structure it is not added to the FK at the moment.
> Just ran the whole thing again with the "ANALYZE A; ANALYZE B" but with 
> no gain unfortunately.
> Just looking at the output i am also quite puzzled why an update to a 
> field in B which has no connection itself to A would trigger all the 
> foreign keys, except if this is a deferred call from updating A only now 
> triggered by touching data in B.
> 

An UPDATE in Postgres is essentially a DELETE of the old row version and 
an INSERT of the new row version. I'm going to guess the INSERT of the 
new row version fires the FK triggers on B.


-- 
Adrian Klaver
adrian.klaver@aklaver.com